Chapter 2 Physical Science 1. Motion Objectives 1. Distinguish between distance and displacement. 2....

Chapter 2Physical Science

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Motion Objectives

1. Distinguish between distance and displacement.2. Distinguish between speed and velocity.3. Interpret motion graphs.4. Identify how acceleration, time and velocity are

related.5. Explain how positive and negative acceleration

affect motion.6. Solve problems involving time, distance, velocity,

and acceleration.

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How do we know an object is moving?

An object moves if its position changes against somebackground that stays the same.

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-this unchanging background is called the

reference frame or reference point-examples of reference points: trees, posts, buildings

motion: when an object changes position over time when

compared with a reference point

In describing motion, it is important to know how far an object has moved.

distance: total length of a path between two points

-does not have to be a straight line

-unit is the meter

You also need to know the direction you are going… 4

Start

Finish

displacement: direction from the starting point to the ending point in a straight line

-states how far (distance) away an object is from a given point and in what direction the object is from that point.

-example of a vector, a quantity that has magnitude (size, length, amount) and direction 5

Vectors

You can add or subtract displacements by vector addition or subtraction.

-vector addition: when going same direction -vector subtraction: when going opposite

directions

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Vector PracticeFor each of the following:a. draw a picture showing motionb. determine the total distance traveled c. calculate the displacement.

1.A sprinter runs 5 km north and 7 km south.2.A cyclist travels 200 m west and 100 m east.3.A truck travels 1 mi east, 2 mi south and 1 mi

west.

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Motion Quiz 1

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SpeedDo all objects move at the same speed?

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speed: distance an object moves to the amount of time the

object is in motion.

-measured in meters/second (m/s)

There are three ways to describe speed:1. instantaneous speed: speed measured at a

particular moment. -radar gun, speedometer

2. constant speed: speed that remains the same the entire trip.

-the object does not speed up or slow down -shown by a straight line on a distance-time

graph (we will look at these shortly)

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3 . average speed: speed computed for the entire trip

- average speed = total distance total time v = d

t

Example 1: If you traveled 200 miles in 3 hours, your

average speed was 66.7 mi/hr.

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Using the speed formula

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Example 2:If a student ran 5 kilometers in 2 hours, what would his/her speed be?

You MUST show the units in both the problem and the answer!!

Step 1: write the known variables

d = distance = 5 km

t = time = 2 hrs

Step 2: write the unknown variable

v = speed = ? km/hr


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Example 2:If a student ran 5 kilometers in 2 hours, what would his/her speed be?

Step 3: write the formula needed

v = d

tStep 4: plug in the numbers with their units

v = 5 km

2 hr



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Example 2:

If a student ran 5 kilometers in 2 hours, what would his/her speed be?

Step 5: Calculate answer showing all work. You

may need to show how you rearranged the

equation to show how you got the answer.

v = 2.5 km/hr



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Example 3:If a cheetah runs 20 miles per hour in 0.15 hours, how far does it travel?



v = speed = 20 mi/hr

t = time = 0.15 hrs


d = distance = ? mi


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Example 3:If a cheetah runs 20 miles per hour in 0.15 hours, how far does it travel?


v = d

tStep 4: plug in the numbers with their units

20 mi/hr = d

0.15 hr



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Example 3:

If a cheetah runs 20 miles per hour in 0.15 hours, how far does it travel??

Step 5: Calculate answer showing all work.

20 mi/hr = d

0.15 hr

(0.15hr) 20 mi/hr = d (0.15 hr)

0.15 hr

d = 3 miYou MUST show the units in both the problem and the answer!!

Speed Practice 1

You must show the given information, formula, units and work!!!

1. A car traveling at a constant speed covers a distance of

750m in 25s. What is the car’s speed?2. The cheetah, the fastest of land animals, can

run a distance of 274m in 8.65s at its top speed. What is that top speed?

3. A cat runs from a dog at a speed of 3 m/s. If the cat runs for 10 seconds, how far has it run?

4. An apple falls 2 meters from a tree at a speed of 1.5 m/s. How long did it take the apple to fall? 18

Motion Review 11. How do we know that motion has occurred?2. What is a vector? 3. Give two objects that can be used for a frame of

reference. (You cannot use the ones in your notes)4. What are the SI units for a. distance b. speed?5. What is the difference between distance and

displacement?6. How do we calculate speed?7. Contrast instantaneous speed, average speed and

constant speed.

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Homework 1Show all work & units!!

1. A plane flies 4000 miles in 5 hours. What is its speed?

2. A car travels from 20 meters to 60 meters in 10 seconds. What is its speed?

3. A car travels 60 m/s for 8 seconds. How far does it travel?

4. If it takes you 6.5 meters to stop when you are traveling 65 m/s, how long does it take you to stop?

5. A dog travels 3 blocks south, turns around and travels 2 blocks north. What is its displacement?

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Motion Quiz 2

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Distance-Time Graphs

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Calculate the slopefor each of the three graphs. Be sure to label each problem with the correct graph and show the units.

What is the dependent variable and the independent variable?

Distance-Time Graph Answers

Constant High Speed:slope = rise/run = 250m/10s = 25 m/s

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Constant Low Speed:

slope = rise/run = 125m/10s = 12.5 m/sVarying Speed:

slope = rise/run = 500m/20s = 25 m/s


What does the slope of a distance-time graph show?

slope = rise/run = d/t = speed (v)

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On a distance-time graph:A linear (straight) line shows constant

speed.

A flat line shows no speed (stopped) graph

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Velocity

Is knowing the speed of an object always enough?

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No. Sometimes you must know the speed and direction the object is traveling, which is known as velocity.

-velocity is positive in the direction of motion

Ex. speed: 55 mi/hr ; velocity: 55 mi/hr South

(a negative velocity means that the object is traveling in the opposite direction.)

Velocity

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constant velocity occurs when an object is traveling at the same speed and direction.

-if at any time the object changes direction, it changes the velocity.

Note: velocity is calculated using the same formula for speed (v = d/t), however be sure to state the direction when asked for velocity!

Motion Review 2Write the Q and A

1. What does a negative and positive velocity mean?

2. How does velocity differ from speed?3. What are the units for speed? For velocity?4. What causes an object to change velocity?

There are two causes.5. Is knowing the speed of an object always

enough?6. How do you know if a distance-time graph

shows if the speed is constant?

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Homework 2

Complete the worksheet handed out in class.

You may write on the worksheet.

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Motion Quiz 3

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Speed of a Cart LabCopy the following on a separate sheet of paper.

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Purpose: You will measure the stopping distance and the time required for your cart to come to a complete stop using various heights of a ramp.

Materials: 2 books, cart, whiteboard and marker, stopwatch, meter stick

Procedure:

1. Using a ruler, draw a line to mark a starting point for your cart.

2. Prop the whiteboard up with one book.

3. Placing the cart at the starting point, let it roll down the ramp and come to a complete stop. Do not push the cart.

4. As soon as the front tires hit the bottom of the ramp, start the stopwatch and stop it when the cart comes to a complete stop. Record the time in your data table.

Speed of a Cart LabCopy the following on a separate sheet of paper.

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5. Using a meter stick or tape measure, measure the distance from the bottom of the ramp to the front tires of your cart. This is your stopping distance. Record in the data table.

6. Repeat steps 3-5 five times, each time recording data into your data table.

7. Using the formula in your data table, calculate the velocity of the cart for each of the 5 trials. Leave room beneath your data table to show all work.

8. Calculate the average distance and average speed of the truck.

9. Repeat steps 2-7 with two textbooks.

10. Draw a graph using your average velocity and the average distance traveled. You will have 2 lines on the graph, so be sure to label each line (1 book or 2 books).

Speed of a Cart Lab

Data Table 1: 1 book

Trial d (cm) t (s) v (cm/s)

1

2

3

4

5

average

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Data Table: Use a ruler to copy this table two times (one for each set of books), leaving room to show calculations below each table.

Speed of a Cart Lab

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Analysis:

Explain the relationship between stopping distance and the speed the cart is going. Discuss how changing the height of the ramp affects both the speed and stopping distance. Use your data to explain. You should have a good 6-8 sentences.

(Be very detailed and specific)

Speed-Time Graphs

The speed an object travels can also be expressed on a graph, gives useful information about the object’s motion.

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-the slope of the line on a distance-time graph represents the change in distance (m) per the change in time (s); slope = d/t = v

-what does the slope of the line of a speed-time graph represent?

Speed-Time Graphs

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2. Describe the speed of the object shown on the graph.

3. How would I calculate the slope of this graph? Show the formula and how you determined the slope.

1. What is the dependent variable? The independent variable?

Speed-Time Graphs

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Slope = rise = speed (m)

run time (s)

What does the slope of speed/time show?

acceleration: rate at which velocity changes over time.

-can be described as a change in speed, direction or both

-is also a vector (like displacement and velocity)

-calculated in meters per second per second (m/s2)

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Acceleration

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A positive acceleration is an increase in velocity, as shown by a positive slope.

A negative acceleration is a decrease in velocity, as shown by a negative slope.

-the horizontal line represents constant velocity (speed) and zero acceleration.

Acceleration: Velocity-Time Graphs

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Straight lines in both graphs represent constant acceleration.The slope of a velocity-time graph gives acceleration.

Acceleration: Distance-Time Graph

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A straight line on a d-t graph shows constant motion.

What does the curved (nonlinear) line on a d-t graph show?

-accelerated motion.

-in this case, a positive slope means the object is speeding up

Motion Review 31. What does the slope of a distance-time graph give? A

velocity-time graph? 2. What do we mean by positive and negative

acceleration?3. For acceleration to occur, what two possible changes

must be made?4. What are the units for acceleration?5. Describe the object’s motion at each interval (A to B,

etc.)

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Homework 3



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Acceleration-Formula acceleration = final velocity – initial velocity time

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a = vf – vi

t

or

acceleration = change in velocity

time

a = v

t = “change”

Using the acceleration formula

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Example 1:A flowerpot falls from rest and hits the sidewalk 1.5 slater with a velocity of 14.7 m/s. What is the accelerationof the flowerpot?



t = time = 1.5 s

v = velocity = 14.7 m/s


a = acceleration = ? m/s2


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Example 1:A flowerpot falls from rest and hits the sidewalk 1.5 slater with a velocity of 14.7 m/s. What is theacceleration of the flowerpot?


a = v = vf-vi

t tStep 4: plug in the numbers with their units

a = 14.7 m/s – 0.0 m/s

1.5 sYou MUST show the units in both the problem and the answer!!


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Example 1: A flowerpot falls from rest and hits the sidewalk

1.5 s later with a velocity of 14.7 m/s. What is the acceleration of the flowerpot?


a = 14.7 m/s

1.5s

a = 9.8 m/s2



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Example 2:What is the time it took a plane to go from 200 m/s to 100

m/s if its acceleration is -20m/s2?



a = acceleration = -20m/s2

vf = 100 m/s

vi = 200 m/s


t = time = ? s


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Example 2:What is the time it took a plane to go from 200 m/s to

100 m/s if its acceleration is -20m/s2?


a = v = vf-vi

t tStep 4: plug in the numbers with their units

-20m/s2 = 100 m/s – 200 m/s

tYou MUST show the units in both the problem and the answer!!


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Example 2:What is the time it took a plane to go from 200 m/s

to 100 m/s if its acceleration is -20m/s2?


-20m/s2 = 100 m/s – 200 m/s = -100 m/s

t t

(t)-20m/s2 = -100 m/s (t)

t



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Example 2:What is the time it took a plane to go from 200 m/s

to 100 m/s if its acceleration is -20m/s2?


(t)-20m/s2 = -100 m/s

-20m/s2 -20m/s2

t = 5 s


Homework 4

1. A jetliner speeds up from 40m/s to 80m/s in 20s. What is the airliners acceleration?

2. An automobile manufacturer claims that its latest model can ‘go from 0 to 90’ in 7.5s. If the ’90’ refers to km/h, calculate the automobile’s acceleration.

3. A ball is dropped from a cliff and has an acceleration of 9.8m/s2. How long will it take the ball to reach a speed of 24.5m/s?

4. A sprinter leaves the starting blocks with an acceleration of 4.5m/s2. What is the sprinter’s speed 2s later?

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Motion Quiz 4

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Motion Review 41. Are the following statements true? Why or why

not?a. “You can determine your speed at the midpoint of a

trip by calculating the average speed for the entire

trip.” b. “If a car travels around a curve on a highway at

60km/hr, the velocity does not change.”2. What is the displacement of a cyclist who travels 1

mile north, 1 mile east, then 1 mile south?3. A horse on a carousel that is moving at a constant

speed is accelerating because________.4. In what ways can an object accelerate? 53

Homework 5



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Chapter 2 Physical Science 1. Motion Objectives 1. Distinguish between distance and displacement. 2....

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