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Motion
Motion
The Plan
• Part One: Measuring speed and acceleration
• Part Two: Forces and Newton’s Laws
• Part Three: Collisions and Energy
Part One – Measuring speed and acceleration
• Goal: To find out – will the duck survive?
Units
• In Physics we use the SI (Système Internationale) system of units. This metric system of units uses the basic units of metre, kilogram, second and ampere (M.K.S.A.).
• There are seven basic quantities from which all others are derived.
Units
Quantity UnitUnit Symbol
MassLengthTimeElectric currentTemperatureLuminous intensityAmount of substance
second
kelvin
metre
candela
ampere
kg
mole
kilogrammsAKcdmol
Units
• Derived quantities are derived from the basic quantities by means of a defining equation.
Derived Units (figure out)
Quantity Unit Unit Symbol DerivationForceEnergyPowerPressureElectric chargeEMF (electric potential)Frequency VelocityAcceleration
Newton N kgms-2
Joule
kgm2s-2A-1
kgm-1s-2PPascalkgm2s-3WWattkgm2s-2J
VVoltAsCCoulomb
ms-2ms-1
Hz s-1Hertz
Derived Units
• The word per in physics means divided by. So instead of saying speed is the metres divided by the seconds, we say metres per second.
• It is written as m/s or or ms-1. • Also for simplification we use m3 instead of
saying cubic metres.
Prefixes1024 1021 1018 1015 1012 109 106 103 100
Yotta Zetta Exa Peta Tera Giga Mega Kilo
Y Z E P T G M k
10-3 10-6 10-9 10-12 10-15 10-18 10-21
milli micro nano pico femto atto zepto
m μ n p f a z
Converting between prefixes
• Want over got.• Convert 45nm to metres• We know 1 nm = 10-9 m
• So Got Want
Standard Form
• Read and highlight from notes.
Significant Figures
The following have 2 sig figs.86, 2.3, 0.56, 2.0, 0.00052, 1.7 x 10-3, 3.0 x 108
The following have 3 sig figs.816, 2.03, 0.560, 2.00, 0.000522, 1.71 x 10-
3, 3.03 x 108
Significant Figures
• When working with sig figs, you can only reliably quote your answer to the level of precision of the measurement with the least number of significant figures, used in your calculation.
• Eg 3.02 x 4.55012 = 13.7413624
= 13. 7• Which was rounded to 13.7, because we can only
quote 3 sig figs in our answer (because 3.02 has 3 sig figs.)
Questions
Question 1Write down the following quantities in standard form:a. 2230 m, the height of Mt Kosciusko above sea level
b. 120 000 000 m, the diameter of the planet Saturn
c. 0.000 84 m, the thickness of a certain piece of wire
d. 0.000 000 000 25 m, the diameter of a gold atom.
QuestionsQuestion 2State the number of significant figures in each of the followinga. 307 km, the distance from Albury to Melbourne
b. 5.0 days, the half-life of a radioactive isotope of Bismuth
c. 6.3 x 1017 m, the distance from the Earth to the star Gegulus
d. 0.000 902 m, the thickness of a particular sheet of paper
e. 60 seconds, the number of seconds in a minute.
Questions
Question 3Paying due attention to the number of significant figures in your answer, deduce how much faster Superman is at 1.4 km/sec than a speeding bullet at 0.57 km/sec.
Questions
Question 4Which of the following would you regard as stating sin 52.4O to the appropriate number of significant figures?
a) 0.7923 b) 0.792c) 0.79 d) 0.8
Questions
Question 5Complete the following table: a. 4 kN = Nb. 5 pF = Fc. 22 MΩ = Ωd. 12 ms = se. 0.7 µC = Cf. 365 nm = m
Extension
Question 6. Express in standard form: a. an area of 5 km2 in m2 m2
b. a volume of 2 cm3 in m3 m3
c. an area of 1.6 µm2 in m2 m2
d. a volume of 2.5 mm3 in m3 m3.
Vectors and Scalars
e.g. temperature, speed…..
• Scalar Only needs a number
• Vectore.g. velocity…..
Needs a number AND a direction
Place the following quantities into the correct column:
Example
VoltageEnergyTemperatureMass
Vector
VelocityForceAcceleration
Scalar
Voltage, Velocity, Energy, Force, Temperature, Mass, Acceleration
Vectors
• Vectors are represented by symbols in bold, or which have a line above or below them. Eg. v, v, or v .
• Sometimes it easier to draw the vector as an arrow. Eg, applying a 10N force to the right
10N
Adding Vectors
To add two vectors we use the rule “head to tail”. Suppose we have to add two vectors, v1 and v2 shown below
Adding vectors
• How do they add?
Questions
Question 7Add these vectors
Question 8Add these vectors
Questions
Question 9Add these vectors
Question 10Add these vectors
Distance and Displacement
e.g. total distance of travel
• Distance Length an object has travelled
• Displacemente.g. final position – initial position
Change in position of an object.
Scalar
Vector
How do you describe direction in 1D?
• Left Described with negative numbers
• Right Described with positive numbers.
01 2 3 4 5 6 7 8-8 -7 -6 -5 -4 -3 -2 -1
Examples
• Baker starts at A and ends at B. What is his distance travelled? What is his displacement?
Final Position – Initial Position-6 – 2 = -8
• Javed starts at A, and he then moves. His displacement is -4. What is his final position?
-2
01 2 3 4 5 6 7 8-8 -7 -6 -5 -4 -3 -2 -1
AB
QuestionQuestion 11: Thisali is an Olympic swimmer. She swims one length of the 50m pool.a. What is her distance travelled?
50mb. What is her displacement?
50m
Question 12 Thisali now swims the 50m back to complete one lap of the pool.a. What is her distance travelled?
100mb. What is her displacement?
0m
Questions
• Question 13a. Grace throws a ball directly up, but doesn’t catch it on the way up. At the top of the flight, what is the distance?
• b. What is the displacement?• c. As it hits the ground, what is
the distance?• d. What is the displacement?
5m
1.8m
Extension questions
Question 14. Krishna is following a treasure map. He moves 18m north, then 24m east.a. Draw two vectors for his movement. Add in a resultant/total vectorb. What is the distance he moves?c. What is his displacement?
Extension Questions
Question 15: Tim starts looking for a different treasure. He travels 20m North, then 12m west and then 11m south.a. Draw three vectors for his movement. Add in a resultant/total vectorb. What is the distance he moves?c. What is his displacement?
Speed and velocity
Speed: Defined in terms of distance. It’s a scalar quantity.
Velocity: Velocity is defined in terms of the displacement. It’s a vector quantity.
Speed and velocity are both measures of how fast something is going.
Instantaneous Speed and Velocity
• Can be measured using a radar gun
• Gives exact measure of speed or velocity at that precise time.
Average Speed and Velocity
Can be calculated by measuring the distance/displacement and the time taken.
Δ means “change in …” (final – initial)
Examples
A plane flies 3000km in 4 hours. What is the speed?
An athlete can run 400m in 47s. What is his speed?
Measure speed…
• Small Prac
To convert from kmh-1 to ms-1
To convert from kmh-1 to ms-1: To convert from ms-1 to kmh-1: ÷ 3.6
x 3.6
Questions
Question 16. Usain Bolt’s record for the 100m is 9.58s. What is his average speed in m/s?
QuestionsQuestion 17: The world’s longest downhill skiing race is held in Switzerland. It is 15.8 km long and the record winning time is 13 minutes 53 seconds. Calculate the average speed of the record holder:a. in metres per second b. in kilometres per hour.
Question 18: A flight from Auckland (NZ) to Melbourne takes 3.5hours. The plane has an average speed of 900km/h. What is the distance between the two cities?
Questions
Question 19: Which has the greater speed?. A bird that flies 200m in 22s or a dog that runs 50m in 8s?
Question 20: Kevin on a bike has a speed of 12ms-1. How many metres does he travel in 1s?In 2s?In 10s?In 20s?
Questions
A frog climbing a slippery wall first leaps 80 cm up the wall before slipping down 20 cm. It then climbs another 80 cm before slipping 30 cm. Finally it reaches the top of the wall by climbing another 40 cm.Question 21: How high is the wall? Question 22: What is the distance travelled by the frog? Question 23: If the frog took 30 s to complete the climb, calculate its average velocity.
Extension Questions
• Question 24: Which has the greater speed?. A car that travels 50km in 30 mins or a truck that travels 3000m in 150s?
• Question 25: Explain how Usain Bolt’s top speed in a 100m sprint (calculated in Q16) is larger than his average speed
Extension Questions
Question 26: A train leaves Melbourne at 9am, at an average speed of 65km/h. At 10am, a car leaves Melbourne at an average speed of 80km/h. a. At what time does the car overtake the train?b. At what distance from Melbourne does this occur?
Acceleration
• When speed changes, this is acceleration• Deceleration is slowing down• Negative acceleration could be slowing down,
OR accelerating in the backwards direction
AccelerationAcceleration: Change in velocity divided by
time taken
Change in Velocity: Final velocity – initial velocity
𝑎=∆𝑣∆ 𝑡
𝑎= ∆𝑣∆ 𝑡 =
𝑣𝑓 −𝑣 𝑖
∆ 𝑡 =𝑣−𝑢∆ 𝑡
Examples
Daniel is skateboarding at 6ms-1 and sees a dog up ahead. He slows down and stops. This slowing takes him 5s. What is his acceleration?
Questions:
Question 27: Stefan (in his Lamborghini) is trying to overtake Mr McGovern in his Prius. Stefan accelerates from 10ms-1 to 20ms-1. This acceleration takes 5s. What is his acceleration?Question 28: Goran is in a drag race. He accelerates from rest to 100kmh-1 in 3.4s. What is his acceleration? (Hint: Turn 100kmh-1 into ms-1 first)
Questions
Question 29: While driving, Pegah sees a duck on the road and slams on the brakes. She slows from 50kmh-1 to rest in 3.5s. What is the acceleration?
Extension
• Question 30: At another drag race Anushka accelerates from rest to 40ms-1 with an acceleration of 14ms-2. What time did this take?
• Question 31: A train is travelling along at a constant speed. Then it puts on its brakes and slows to rest over 6.7s with an average acceleration (deceleration) of -4ms-2. What was the initial speed?
Graphs in motion
• If we can calculate stuff, why would we bother graphing stuff?
• We never get people travelling at same speed for ever with out stopping.
• Life is far more interesting.• For example… Carla travelled 40m in 20s, she then
stopped for 10s, then travelled back to where she started from in a further 10 seconds
• Is there another way of representing this?
Displacement-Time GraphTime (s) 0 10 20 30 40 50 60 70 80Displacement (m)
0 20 40 60 60 60 30 0 -30
Displacement-Time Graph
The gradient of a graph is:
In this case we have:
𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡𝑡 𝑖𝑚𝑒 =𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦
Displacement-Time Graph
Question 32: Find the gradient to find the velocity between 10 and 20 seconds.Question 33: What is the velocity between 30 and 40 seconds?Question 34: What is the velocity between 60 and 70 seconds?
Displacement-Time Graph
1) 2) 3)
Data Loggers
• Use the data loggers to create the following four graphs.
• Write one sentence for how you created each one.
Velocity-Time GraphTime (s) 0 10 20 30 40 50 60 70 80Velocity (ms-1)
0 2 4 2 0 0 -2 -2 0
Velocity-Time Graph
Write a story to go with this velocity-time graph
Velocity-Time GraphMegatron started getting faster and faster, hitting his top speed after 20s. He started slowing down and stopped moving completely between 40s and 50s. He started going backwards, reaching a top speed of 2ms-1 and holding that speed for 10s. He slowed down and stopped at 80s.
Velocity-Time Graph
The gradient of a velocity-time graph is:
The area between the velocity-time graph and the axis is:
The displacement
Velocity-Time Graph
Question 35: Calculate the acceleration at 10sQuestion 36: Calculate the displacement during the entire 80s.
Velocity-Time Graph
Velocity-Time Graph
• Calculating the area between the graph and the axis
80m
-40m
Total = 80-40= 40m
Team Graphing Challenge
• You have been given a graph (it could be a displacement-time graph or a velocity-time graph).
• In your team you are to come up with a story and act it back to the class.
CUPS Activity
• 5 mins by yourself• 5mins with your table• 5mins whole class
• The whole class has to agree on an answer, and that answer must be correct for the whole class to move on…
CUPS Activity 1.
The Equations of motion…
When dealing with motion…
• If there is no acceleration… Use
• If there is acceleration…Use one of the “equations of motion”
Equations of motion
x= displacement, u=initial velocity, v=final velocity, a=acceleration, t=time
How to answer motion questions using these formulas.
1. Write down what information you have been given.
2. Put into correct units3. Make sure values are positive or negative
depending on direction4. Choose your formula based on what
information you have been given.5. Calculate answer.
Example:
1. A car, travelling at 30ms-1, accelerates to 40ms-1 in order to pass a slower car. This acceleration takes 20s. What distance does he travel during this acceleration?
Example
2) What acceleration does this car experience?
Questions
Question 37: Ushan, riding a horse, accelerates from rest at 3ms-2 over 5s. What is his final speed?Hint: What does “at rest” mean?
Questions
Question 38. Wenbo is driving his new Lamborghini. He is travelling at 30ms-1, and has to slam on his brakes. He comes to a halt 4s later. Over what distance did he stop?
Rearranging the equations of motion
• Example: Usain Bolt accelerates from rest to 10ms-1 in 20m. What is his acceleration?
We could use: Rearrange:
Questions
Question 39: Baker is jogging along, and the accelerates at 1ms-2, reaching a top speed in 5s. This acceleration takes 30m. What was his initial speed?
Questions
Question 40: [2011 Year 12 Exam]. A tractor and trailer accelerate from rest at 0.5ms-2.
What is the distance covered in the first 5s?
Extension
Question 41: Javed is running towards a bus-stop at a speed of 5.0 m s–1, but is still 60 m away from it when his bus passes him. The bus is travelling at 40 km h–1 but is decelerating uniformly as it passes the running Javed. Assuming that Javed keeps running at 5.0 m s–1, calculate the time that the bus must wait at the bus-stop for Javed to arrive.
Scenarios with constant speed AND acceleration
Question 42: A duck walks out 55m in front of a car which is travelling at 30ms-1. Krishna takes some time to react (during which he travels at a constant speed). Then slams on the brake and decelerates at 13ms-1. Does the duck survive?
Reaction Time.
• [Draw this scenario]• Measure the reaction times of a driver under
different scenarios.• How did we measure an average reaction
time? (In your own words)
Reaction Time1. Calculate the distance the car goes before
the brakes are applied. [Hint: constant velocity]
2. Given that a=-13ms-2 when the brakes are applied, calculate stopping distance [Hint: Use an equation of motion]
3. Calculate the total distance from incident to stop
Does the duck survive? Or does Krishna go to jail?
Vertical Motion under Gravity
• What does your instinct say?• If dropped out of a plane, which would fall
faster: A rhino or a mouse?
Aristotle
• Never did experiments• Thought that everything had a “natural place”.
A rock’s natural place is on the earth, so if you pick it up and drop it, it will return there.
• Thought that the speed of a falling object is related to its weight. Eg a rhino will fall faster than a mouse
Galileo
• Actually did experiments• Persecuted by the church• Found that two objects will accelerate at the
same rate under gravity, no matter what their weight
Gravity Prac• If I drop something, does it go faster?• Yes! So gravity accelerates things.• Your job is to find the acceleration of earth’s gravity. • Many, many ways of doing this. Lets talk about a few. • This is not marked on you getting the “right answer”. This is marked
on how well you did the science. Eg, how repeatable is your experiment. How clear are your results. Have you talked about error?
• Starting questions. Is it easy to measure acceleration? What measures acceleration? What other quantities in kinematics are more often measured? Can we find acceleration from the other quantities?
• Example prac, and write up.
Vertical Motion• All objects accelerate at the
same rate• a = 10ms-2 (downwards) on
earth. (9.81ms-2)• Air resistance may slow some
things down (paper, a feather).• Without air resistance a feather
and hammer will fall at the same rate.
• We ignore air resistance in our calculations.
Hex-NutsWhen I tie hex nuts to a string and
drop them, the time between “clangs” gets shorter and shorter.
• Tie 5 or 6 hex-nuts to your
string so that the time between “clangs” is approximately equal.
• Write a sentence to describe why you decided to tie them like you did, and if you were successful.
QuestionA sheep drops off the edge of a cliff.
Assume we have the data logger and sonic ranger positioned above where the sheep falls. Draw
what you think the position-time graph will look like on the data logger for the falling sheep.
ExampleA sheep drops off the edge of a cliff. Question 43a: Fill in the following table for the displacement, velocity, and acceleration of the sheep.[Hint, start with a, then v, then x]
b. Draw a displacement-time graph for the sheep.c. Draw an velocity-time graph for the sheep.d. Draw a acceleration-time graph for the sheep.e. Describe the shape of each of the graphs you have drawn.
ExampleThe sheep drops off the edge of a cliff.
Time 0s 1s 2s 3s 4s 5s
Displacement [m]
0 5 20 45 80 125
Velocity[ms-1]
0 10 20 30 40 50
Acceleration[ms-2]
10 10 10 10 10 10
Example
Questions
• Question 44: Tim is being mischievous and decides to climb a tall building so he can drop a water bomb on unsuspecting passers by. The building is 40m high. How long does it take for a water bomb to fall 40m?
Question
Question 45: Thisali throws a ball upwards at 30ms-1. Use an equation of motion to show:a. After what time does it reach the top of its flight?b. What height does it reach?c. After what time does it fall back into her hands?
Question
1) Use
2) Use
Question3) When the ball reaches my hand again, what do we know?Find v first, then t.
Now:
Extension (Algebra Question)
Question 46: Consider someone throwing something in the air vertically, at a speed vinitial, similar to the last questiona. What is the velocity at the top of the flight path?b. Come up with an equation for the time to reach the topc. How does the time to reach the top compare with the time to come back down again?d. What is the velocity when it reaches the place it started from
CUPS 2
Vertical Motion Summary
• (downwards) at ALL points in an objects vertical motion.
• At the top point of the motion • The time it takes to go up: is the same as the
time to go down again• The velocity has the same magnitude, but
opposite direction when it comes back down again.
Part One Summary
• Units, scalars and vectors. Q BLAH to BLAH• How to work out average and instantaneous
velocities• Graphical Representation of Motion: Q BLAH
to BLAH• Motion with acceleration described by the
equations of motion: Q BLAH to BLAH• Vertical Motion under gravity: Q BLAH to BLAH
A thought
• So we have constant velocity equation and the equations of motion to describe motion.
• Question is, what causes motion?• What causes a ball to be thrown up in the air?• What causes it to fall back to earth?• What causes a car to accelerate?• What causes it to brake?
The answer…
Forces!
Part Two: Forces and Newton’s Laws
Introduction
• You are all scientists!
• You all observe the world, and come up with theories to explain your observations
• You then use these theories to make predictions.
You are a scientist!
• For example: You have watched or played sports?
• In that, you have observed what happens to a ball that rolls along the ground?
• So you can make a prediction: What happens if I roll this ball along the ground?
You are a scientist!
• If prediction/hypothesis is correct, your theory is right.
• Your theory is correct until you meet an experiment where your prediction is wrong
• Then you need to come up with a new theory/explanation
You are a scientist
• My job is to show you enough different experiments so we can get rid of any wrong theories, and just have the correct theories!
Small experiment template
Lets get the ball rolling
• We rolled the ball. It stopped. That’s your observation
• I want you to come up with your own theory as to why
• Theory:
Let’s get the ball rolling
• What things could we change to test this theory in a number of different ways?
• Eg – Could we change the type of ball?
• …
How is this new theory different to our old theory?
• Most people think a ball will stop rolling because…
• It runs out of “power” or “force” or “something”
• We know a ball will only stop rolling if…• A force acts on it• Otherwise, it will keep rolling forever
Experiment two
Sum up so far
• A force is needed to start something moving
• But it will keep moving forever unless another force acts on it to slow it down.
So lets test our theories:
• Crash test teddies!
Keep on rolling
• If a ball is rolling along, how can we change its direction?
Riddle me this…
• How does a car change direction? (Don’t just say “with the steering wheel”. Use some physics to describe it!)
Sum up…
• Need a force to start something moving• Something will move forever in a straight line,
unless a force slows it down or changes its direction
So answer me this…
• How come, when Im slumped in a chair, I don’t move?
• Isnt there a force on me? Gravity?
Total Forces
• So when we sit on the chair, gravity acts on us downwards…
• But the chair provides a reaction force (normal force) upwards.
• It cancels out gravity.• Total force = zero
Gravity
Reaction Force
Lets revise what we know
• Sum up your findings from Pg ___, Pg ____ and Pg ___ into one or two sentences in your own words.
• This is similar to Newton’s First Law• But since you did the experiments, and came
up with the law: Name it after yourself!
Newton’s/Krishna’s Second Law
• We know that if an object has a total force on it, it will start moving (speed up) or slow down (if it’s already moving).
• Another word for speeding up or slowing down is…
• Acceleration• Krishna’s Second Law is about finding out how
much something accelerates
Prac!
• I want to you to investigate two things:• 1) How does changing the total force on an
object change it’s acceleration?• 2) How does changing the mass of an object
change its acceleration?
Prac!
• Working in groups, you will design this experiment yourself, carry it out, and present your results to the class in the MPSC Forces and Motion Physics Conference 2013. [Lunch provided free to delegates]
How do I get started!
• What questions are we asking?• 1) How does changing the total force on an
object change it’s acceleration?• 2) How does changing the mass of an object
change its acceleration?
How do I get started?
• Should we attempt to answer both of these questions at the same time?
• No!• Why not?• We could get confused about which is causing things
to change – the mass or the force. Lets investigate one at a time!
Brain storm some ideas
• What are some objects we could apply a force to?
• Can we easily change the size of the force
Recording results
• One of the most important thing is recording your results as they happen!
• I can guarantee you wont record enough
Recording results
• An example of recording results• [Choose one groups prac and show them how
to record results on board]• [Emphasise BEFORE each experiment you
write down values for each variable. Then do experiment, then record result]
• [Repeat for each experiment]
Krishna’s 2nd Law
• What you have found is Newton’s 2nd Law• If there is a total force on an object, it
accelerates. But its acceleration depends on its mass
• Mathematically…
Total Force
Newton’ 2nd Law
• What happens if total force = 0?
• Then acceleration = 0• Does this agree with our First Law?
Questions
• Question 47. Draw all the forces on the following objects• a. A book is at rest on a table top• b. An egg is free-falling from a nest in a tree. Ignore air
resistance• c. A plane flies at a constant velocity (Note: There will be
an applied force generated by the engines as well as a lift force provided by the wings).
• d. A rightward force is applied to a book in order to move to across the desk with a rightward acceleration. Ignore air resistance, but consider the desk friction.
Questions
• e. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional force, but ignore air resistance.
• f. A football is moving upwards towards its peak after having been booted directly upwards by the punter
• g. A car is rolling to the right and slowing down. The engine has been turned off.
Questions
Hint: All these questions can be answered by considering either Newton’s first or second lawQuestion 48: A 1kg book rests on a table.a. What is the weight force?b. What is the total force on the book?
Questions
Question 49. A 500g model rocket is being launched. The engine provides a 20N force upwards.a. Draw and calculate the TWO forces on the rocketb. What is the total force?c. Calculate the acceleration
Questions
Question 50: A 200kg car is driving along the road. The engine provides a 500N driving force, and the air friction and road friction total a 500N friction force. Is the car acceleration or travelling at a constant speed?
Questions
Question 51. At the start of a basketball game, the ref throws a 1kg basketball up. At the top of its flight…a. What is its velocity?b. Draw any forces on itc. What is the total force on the balld. What is the acceleration of the ball
Extension – 2012 Q3
• Question 52
Extension – 2010 Q4
• Question 53.
CUPS
CUPS
Rocket
• Send up the rocket in good weather
Newton’s Third Law
• Newtons 3rd Law• Tension (POE same weight, different heights)• Ramps - Prac
Want to teach Newtons Laws via inquiry
• Ideas• Forces and adding them. Total force• Newtons third law• The other laws (basically the same law). • Concept questions (as well as the unit 3 ones.
Throw a ball on a plane)• Pracs: Balloon one? • Prac: table cloth.
Summary of practical activities
• Mickey ears• Crash test dummies• Bottle rocket• Table cloth• Water race• Throwing a ball while running• Spinning a raw and hard boiled egg.
• Extension (two different balls roll off desk)
• Types of forces.• Friction• Tension (POE same weights, different heights)• Ramp prac. Then do the theory: (but not too
hard). Make them remember the concept.• Springs. Prac. Hookes Law, but no energy.
Last section. Momentum, energy and work.
• Momentum – extended practical investigation• Impulse. Impulse prac (eggs). • Energy and work. More than last time.
• Summary and questions. How questions can draw from all parts of motion. You can often answer a question in more than one way.
• SAC
Newton’s Laws Chapter 5• Newton connect the forces that act on a object with the motion that object
experiences.• Force intro: Something about push/pull. • How would we show a force on something-arrows! Shows the direction. How would
we show a bigger force?• Talked about forces on a ball im holding in my hand. Talked about adding the forces
to get the net/total/resultant/unbalanced force• Notes: The net force (or resulting/total force) can be found by adding the individual
forces acting on a body. Draw man with ball in hand.• Force is a vector: It has a direction. If we add forces together to find the net force, we
add the force vectors head to tail. The net force is found from the tail of the first to the head of the second. Eg:
• Up + down. Left Plus up. UP plus down. • Important to show direction of the resulting force – do for all above.• The units for the force are the Newton: N. Ch 5 Question 5.1 Pg 149 1-3, 6,7
Newtons 1st and 2nd Law• Forces recap: Add these forces(mag and direction): 10 up, 10 down.
10 up, 5 down. 1 left 1 up. 3 left, 4 up. 10 up, 10 down, 6 left, 4 right.• An object will continue in its motion (either stationary or constant
velocity) unless acted upon by a net (total) force. [Thinking of a reword: If the net force acting on an object it zero, then it will continue its motion (stationary or constant velocity)] eg Draw stationary book. And draw a constant moving car.
• Law 2: If there is a net force on an object, that object accelerates. This is the equation: F=ma. So to put these two together: If there is no net force: constant velocity; if there is a net force: think acceleration.
• Pg 155 5.2 Q3-5. Pg 163 5.3 Q1-2.
Weight
• Started with problem from radiation.• Place these into newtons first law or newtons
second law• Weight: How much do you weigh?• Weight is a specific example of Newtons 2nd Law.
W=mg. Everything experiences the same acceleration , although a different force.
• Weight on other planets. No air resistance.• Skydiving example.
More Newtons Laws
• See Newtons Second Law Prac Document. Has the questions and everything that we did.
Components of vectors and Newtons 3rd
• Do now: Is there a net force acting on these objects? [have graphs] (didn’t do)• Do now: Worksheet [called Forces.doc]• Leads to: What do all these problems have in common? Had no diagonals. What
if there was a diagonal force? Eg. Draw.• Problem: I want to go up to the top left. How do I describe that to a normal
person who doesn’t know about diagonals? Do example calculation. This is the vector components. 90% of the time, we want to find the vertical and horizontal components of a force. But we can turn a vector into any components if we want. Eg draw.
• Two questions for them to do to find the components. Get them to draw their answers as well (so they comprehend the meaning of their answers eg. So your force is the same is applying two forces in these directions…)
• Newton’s Third Law: For every action on Object A, there is an equal and opposite reaction to Object B. Draw three examples (push a box), kick a ball, fall to earth.
• Ramp prac
So what have we found…?
• Mass doesn’t affect the time…• Angle does affect the time…
Theory to prove it
• We can use forces to explain it (its hard!)
Newtons 3rd law and inclined planes
• Normal force: Ball on table – normal cancels out gravity. Ball on incline. Normal force doesn’t cancel out. Go to the extreme angles.[Demo]
• Notes: The normal force acts perp to the surface.• Draw the situation, draw components of gravity
and work out (mgsin30). • “The normal force cancels out the perpendicular
component of gravity.” Did homework question.
Give a summary
• 2 situations:
Tension
• Drew a monkey hanging from the tree, with the two forces. Then had a demo of the pulley with equal weights on either side.
• POE.• When the weights are different(3kg, 2kg): It
turns, accelerates at 2ms-2 (10N/5kg). So net force on the 3kg is 6N, gives a tension of 24N upwards.
Ch 6 Momentum (conservation of)• Jonah video. Why was jonah so hard to stop?• Rhino video. Why is a tram travelling at 3ms-1 harder to stop than a ping
pong ball?• Momentum is a measure of hard it is to stop something. P =mv (arrows
to name and unit) Two examples: Man running and car.• Air track. How can we make this twice as hard to stop? – Didn’t really
work.• The total momentum is always the same before and after a collision. This
is called the conservation of momentum. (This is also Newtons thrid law). P(total) before = P(total) after
• Example: Question 1.• Do 1. DRAW IT.• 6.2 2, 3, 4, 5, 6
Change in momentum. Impulse.• Start with Olympics question: How long does it take for divers
to reach the water from the 10m diving board?• Sheet and egg prac.• Impulse: Draw egg broken on ground and egg in sheet. “Why?
The ground and the sheet supply the same total force, but the sheet provided it over a longer time.”
• Draw a graph for both.• Impusle = I = Fxt. Impulse is also equal to the change in
momentum. Change in momentum =final-initial = Impulse• Change in momentum = Impulse = F x t• Pg. 165. Q2, 5, 10.
Energy and Work.• Conservation of energy says: “Energy is neither created nor destroyed, it is merely
changed from one form to another”• Types – Kinetic, Elastic, Potential, Chemical, electrical, gravitation, light, sound.• Picture/diagram of changing energy from sunlight to coal to power station to my
toaster• Kinetic Energy is the energy of movement. Ek = 1/2mv2• Gravitational energy is the energy obtained from lifting something = U = mgh• Girl on slide example: 3 questions: What’s her U, what is this converted to; what
is her speed• Q208 – 1-3• Work: Work is changing energy.• W = Fx• Picture of me pushing a box 10m, with 10N forward and 5N back• Picture of me carrying the box 10m. Did questions in the homework.
• Springs prac
Hookes Law and Springs• Talked about how a spring can store energy. And talked about
how if you pull it, it provides a force. • “Hookes Law: The force a spring applies gets bigger the more
you stretch it out. F = -kΔx.” Label force, extension, spring constant k
• Questions: Why is it negative?• If you had a spring that was hard to stretch does it have a large
or small k?The energy stored in a spring U = 1/2kx2.Hookes Law Prac.Next time: Energy stored in a spring prac.
Power and Energy Efficiency
• P = E/t (E is energy, t is time)• Eg a toaster is using 12kJ of energy in 1 minute.
What is the power … P=E/t=12000/60=200W• Energy efficiency=useful energy/total energy x100• Most waste energy is heat• Eg A car uses 500kJ of energy to provide 92kJ of
kinetic energy. What is the energy efficiency?...
SIMPLE ACCELERATION TIME GRAPHS
• START WITH A FLAT DISPLACMENT, velocity and time graph and ask them to describe.
Acceleration-Time Graph
• Take Megatron’s swim and graph as acceleration.
• First calculate his acceleration
Acceleration-Time Graph
• Take Megatron’s swim and graph as acceleration.
• First calculate his acceleration
0ms-2
0ms-2
0.2ms-2
-0.2ms-2
-0.2ms-20.2ms-2
Acceleration-Time Graph
