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Guess Who…
Dynamics
Dynamics is the study of how forces act on objects and their effect on the motion.
Starter
� Discuss with your partner, what do you know about Forces and Sir Isaac Newton?
Force: Any push or pull.
The units of force are Newtons (N)
There are four fundamental forces that make up all of the forces in the universe:
1) Gravitational Force
2) Electromagnetic force
3) Strong Nuclear Force
4) Weak Nuclear Force
Force
� Force is a vector quantity. To describe it fully we must have size and direction. We therefore use arrows to denote the direction of a force.
15N
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What happens to the ball when a force is applied?
1. Speed
2. Direction3. Shape
Newton’s first law (law of inertia)
� Newton’s 1st law: an object at rest tends to stay at rest and an object in motion tends to stay in motion, unless another force acts.
� in other words, and object will always continue doing whatever it is doing – if it is at rest it will stay at rest, if it is moving it will stay moving, unless a force acts to change its motion.
Think and then discuss with your partner
Mr Marsella is testing his new sports car when his brakes fail. Unfortunately he isn’t wearing a seat belt. What happens to him upon impact with the wall? Why?
Explain this in terms of Newton’s 1st law.
Newton’s first law (law of inertia)
� Newton’s 1st law: an object at rest tends to stay at rest and an object in motion tends to stay in motion, unless another force acts.
� in other words, and object will always continue doing whatever it is doing – if it is at rest it will stay at rest, if it is moving it will stay moving, unless a force acts to change its motion.
Balanced Forces
Balanced forces are EQUAL FORCES which act in OPPOSITE DIRECTIONS. They CANCEL EACH OTHER OUT.
FF
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If balanced forces act on a STATIONARY OBJECT, it REMAINS STATIONARY.
FF
If balanced forces act on a MOVING OBJECT, it
continues moving in the same direction with CONSTANT VELOCITY.
What happens if the forces are not balanced?
Inertia
� Inertia: the resistance an object has to a change in its state of motion.
� What does this mean?
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Balanced or Not?� What will happen in each case?
What is the resultant force, direction and motion?
35Kg
50 N120N
50N
50N
Starter
� What are the net forces on each box?
CTransport 1996:6
Free Body Diagrams worksheet
Starter� At the London Olympics Ivan
Ukhov won Gold with a high jump of 2.38m.
� If Ivan went to the moon how might this affect his jump? Why?
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What is Gravity?
� Discuss with the person next to you?
Fg =
MASS WEIGHT
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Mass
� Amount of matter in an object
� Constant (relatively)
throughout the universe
Weight
� Gravitational attraction (weight)
� Changes depending on
where you are in a gravitational field
What is gravity?
What would happen if we could escape gravity?
https://www.youtube.com/watch?v=OZY8279b7BU
https://www.youtube.com/watch?v=0iDyhg0BPZA
Notes – complete the notes.
Force of Gravity: Any two objects will feel a force of attraction that depends on their masses and distance apart.
Mass (kg): The amount of matter in an object.
Weight (N): The amount of gravitational force felt by an object.
Mass is constant throughout the universe whereas weight changes depending on where you are.
The formula for force of gravity is:
Fg = mg
Where:
m = mass (kg)
g = acceleration due to gravity (m/s2)
= gravitational field strength (N/Kg)
g varies depending on the size of the planet and distance to the object
For Example:
� On Earth at sea level g = 9.80 m/s2
� On the moon’s surface g = 1.60 m/s2
� On Jupiter g = 24.5 m/s2
� The Sun g = 274 m/s2
Determine your weight (in Newtons) on Earth, the moon and Jupiter.
What would the weight of a 25.0kg bag of cement be on earth?
Fg = mg Fg = 25 x 9.80 = 245N
What would the weight of a 5.0kg bag of potatoes on earth?
Fg = mg Fg = 5 x 9.80 = 49N
What is the mass 2555 N Grizzly bear?
m = Fg / g Fg = 2555 / 9.80 = 261kg
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Jumping on the moon and the sun!
Assuming you wouldn’t burn up and die in a hot fiery mess before getting anywhere near the sun, how high could you jump? Workbook
� Read and answer questions 77-80
Starter
A 3.00 kg mass suspended by a rope is moving upwards with a constant velocity of 2.00 m/s.
3.00 kg
2 m/s
Calculate the tension (force) in the rope.
Constant velocity means forces are balanced, Fg = Ft
g mF =g
9.81 3×=N 29.4F =g
Tension in rope is 29.4 N(down )
Draw FBD’s for both questions!
What upward force must be supplied by the rotor blades when the helicopter is travelling at the constant speed?
• https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion-basics_en.html Net force
• Higher animation
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• NEWTON’S SECOND LAW states:
When an object is acted on by a constant UNBALANCED FORCE the body moves with constant acceleration in the direction of the unbalanced force.
Virtual Mechanics
Newton’s 2 nd Law “If a body is acted upon by an unbalanced force, it will accelerate.”
Stated as a formula:
Fnet = ma
Note that the units for force are N
Newton’s 2 nd Law “If a body is acted upon by an unbalanced force, it will accelerate.”
Stated as a formula:
Fnet = ma
Note that the units for force are N
Ex. A 5.0 kg block is pushed to the right along a frictionless track with a force of 10.0 N. What is its acceleration?
Ex. A 5.0 kg block is pushed to the right along a frictionless track with a force of 10.0 N. What is its acceleration?
Ex. A 650 kg car accelerates at 4.0 m/s2 south. What is the net force acting on it?
Ex. A 650 kg car accelerates at 4.0 m/s2 south. What is the net force acting on it?
Since Fnet can be determined using acceleration then you will sometimes be required to use kinematics in solving force problems.
Ex:
A 1500 kg ice cream truck accelerates from rest to a top speed of 45 km/h in 8.0 s. What was the net force acting on the truck?
Since Fnet can be determined using acceleration then you will sometimes be required to use kinematics in solving force problems.
Ex:
A 1500 kg ice cream truck accelerates from rest to a top speed of 45 km/h in 8.0 s. What was the net force acting on the truck?
Finding F net
To find F net when two forces work together simply add them.
Ex. Stan and Kyle are pushing a 75 kg Cartman along a frictionless ice rink. Stan pushes with 55 N and Kyle pushes with 45 N. Find Cartman’s acceleration.
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Complete worksheet What is the net force and acceleration?
35Kg
50 N 120N
50N
50N
Starter
A 3.00 kg mass suspended by a rope is moving upwards with a constant velocity of 2.00 m/s.
3.00 kg
2 m/s
Calculate the tension (force) in the rope.
Constant velocity means forces are balanced, Fg = Ft
g mF =g
9.81 3×=N 29.4F =g
Tension in rope is 29.4 N(down )
Draw FBD’s for both questions!
What upward force must be supplied by the rotor blades when the helicopter is travelling at the constant speed?
A 2 kg mass accelerates horizontally at 3 m/s2.
The mass is pulled by a force of 10 N.
Calculate the force of friction acting against the block.
kg 2m =-2ms 3a =
?Fnet =
a mFnet =3 2×=
right the to N 6Fnet =
2 kg10 NFriction
acceleration = 3 m/s2
6friction10 =−∴610friction −=
N 4friction =
Winner – Loser = Fnet
Starter
A 1000 kg car accelerates to the right at 4 m/s2. The force of friction acting on the car is 600 N. Calculate the force exerted by the car’s engine.
FENGINEFriction = 600 N
acceleration = 4 m/s2
a mFnet =kg 1000m =2m/s 4a =
?Fnet =4 1000 ×=
N 4000Fnet =
4000frictionF engine =−∴6004000Fengine +=
N 4600Fengine =
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What’s happening here?
CM1
http://www.bbc.co.uk/news/science-environment-21341485
• Why is felix accelerating?
• Why does he stop accelerating?
Terminal velocity
The air resistance acting on a moving object increases as it gets faster.
Terminal velocity is reached when the air-resistance (acting upwards) has increased to the same size as the force of gravity (fg) (acting downwards)
W = weight
Friction Ff(air resistance) = 0 N
time = 0s, velocity = 0 m/s, friction = 0 N
a = -9.81 m/s2
W = weight
Ff
a < -9.81 m/s2
v
Slide 55
CM1 Christopher Marsella, 12/6/2017
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W = weight
Ff
a = 0 m/s2
v
Equal & opposite forces=Zero Acceleration=Terminal velocity
Examples:1) You hit a baseball with a bat.The bat hits the ball - the ball hits the bat
2) A sprinter starts running.The sprinter pushes the ground - the ground
pushes the sprinter
3) A fish swims through water.The fish pushes the water backwards - the
water pushes the fish forwards
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Force of GUN on BULLET
Firing a gun
Force of BULLET on GUN
Force of RUNNER on BLOCKS
Starting a sprint
Force of BLOCKS on RUNNER
A Rocket
Force of ROCKET
pushing out Exhaust gas
Force of Exhaust gas on ROCKET
Imagine a bug hitting the windshield of a semi trailer.
What force pair occurs?
Which force is bigger?
Which object has a greater acceleration?
Hands up if you hate Newton!!!
Since, a = Fnetm
the truck has a much smaller acceleration because it is much, much more massive.
Bug hits truck –Truck hits bug
NEITHER!!!
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• ..\Physics 11 - Church\Dynamics\Other Resources\Zero-g.m4v