Motion Graphs

Motion Graphs 1

M. Poarch – 2003

http://science-class.net

Name___________________________

Motion Graphs

Describing the motion of an object is occasionally hard to do with words. Sometimes

graphs help make motion easier to picture, and therefore understand.

Remember:

• Motion is a change in position measured by distance and time.

• Speed tells us the rate at which an object moves.

• Velocity tells the speed and direction of a moving object.

• Acceleration tells us the rate speed or direction changes.

DISTANCE-TIME GRAPHS

Plotting distance against time can tell you a lot about motion. Let's look at the axes:

If an object is not moving, a horizontal line is shown on a distance-time graph.

Time is always plotted on the X-axis

(bottom of the graph). The further to

the right on the axis, the longer the

time from the start.

Distance is plotted on the Y-axis (side

of the graph). The higher up the

graph, the further from the start.

Time is increasing to the right, but its

distance does not change. It is not

moving. We say it is At Rest.

Motion Graphs 2

M. Poarch – 2003

http://science-class.net

If an object is moving at a constant speed, it means it has the same increase in

distance in a given time:

Let’s look at two moving objects:

Both of the lines in the graph show that each object moved the same distance, but the

steeper dashed line got there before the other one:

Graphs that show acceleration look different from those that show constant speed.

Time is increasing to the right, and distance

is increasing constantly with time. The

object moves at a constant speed.

Constant speed is shown by straight

lines on a graph

A steeper line indicates a larger distance

moved in a given time. In other words,

higher speed.

Both lines are straight, so both speeds are

constant.

The line on this graph is curving upwards. This

shows an increase in speed, since the line is

getting steeper:

In other words, in a given time, the distance the

object moves is change (getting larger). It is

acceleratin

Motion Graphs 3

M. Poarch – 2003

http://science-class.net

Summary:

A distance-time graph tells us how far an object has moved with time.

• The steeper the graph, the faster the motion.

• A horizontal line means the object is not changing its position - it is not moving,

it is at rest.

• A downward sloping line means the object is returning to the start.

(Graph from:

http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml

)

Motion Graphs 4

M. Poarch – 2003

http://science-class.net

Examine the graphs below.

Whic

h of

the

follo

wing

graph

indica

tes

that

one

of runners started 10 yards further ahead of the other?

Which of the graphs shows that one of runners started 10 yards further ahead of the

other? Explain your answer.

Motion Graphs 5

M. Poarch – 2003

http://science-class.net

In which of the following graphs below are both runners moving at the same speed?

Explain your answer.

Motion Graphs 6

M. Poarch – 2003

http://science-class.net

The distance-time graphs below represent the motion of a car. Match the descriptions

with the graphs. Explain your answers.

Descriptions:

1. The car is stopped.

2. The car is traveling at a constant speed.

3. The speed of the car is decreasing.

4. The car is coming back.

Graph A matches description _____ because __________________________________.

Graph B matches description _____ because __________________________________.

Graph C matches description _____ because __________________________________.

Graph D matches description _____ because __________________________________.

Motion Graphs 7

M. Poarch – 2003

http://science-class.net

SPEED-TIME GRAPHS

Speed-Time graphs are also called Velocity-Time graphs.

Speed-Time graphs look much like Distance-

Time graphs. Be sure to read the labels!!

Time is plotted on the X-axis. Speed or

velocity is plotted on the Y-axis.

A straight horizontal line on a speed-time

graph means that speed is constant. It is

not changing over time.

strai

ht line does not mean that the

object is not moving!

his

raph shows increasin

speed.

The moving object is accelerating.

This graph shows decreasing speed.

The moving object is decelerating.

Motion Graphs 8

M. Poarch – 2003

http://science-class.net

What about comparing two moving objects at the same time?

Summary:

A speed - time graph shows us how the speed of a moving object changes with time.

• The steeper the graph, the greater the acceleration.

• A horizontal line means the object is moving at a constant speed.

• A downward sloping line means the object is slowing down.

(Graph from:

http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml

)

Both the dashed and solid line show increasing

speed.

Both lines reach the same top speed, but the

solid one takes longer.

The dashed line shows a greater

acceleration.

Motion Graphs 9

M. Poarch – 2003

http://science-class.net

The speed-time graphs below represent the motion of a car. Match the descriptions

with the graphs. Explain your answers.

Descriptions:

5. The car is stopped.

6. The car is traveling at a constant speed.

7. The car is accelerating.

8. The car is slowing down.

Graph E matches description _____ because __________________________________.

Graph F matches description _____ because __________________________________.

Graph G matches description _____ because __________________________________.

Graph H matches description _____ because __________________________________.

Motion Graphs 10

M. Poarch – 2003

http://science-class.net

Questions:

( Some questions adapted from

http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml

)

Look at the graph above. It shows how three runners ran a 100-meter race.

Which runner won the race? Explain your answer.

Which runner stopped for a rest? Explain your answer.

How long was the stop? Explain your answer.

How long did Bob take to complete the race? Explain your answer.

Calculate Albert’s average speed. (Figure the distance and the time first!)

Motion Graphs 11

M. Poarch – 2003

http://science-class.net

The graph below shows how the speed of a bus changes during part of a journey

Choose the correct words from the following list to describe the motion during each

segment of the journey to fill in the blanks.

• accelerating

• decelerating

• constant speed

• at rest

Segment 0-A The bus is ______________________________. Its speed changes

from 0 to 10 m/s in 5 seconds.

Segment A-B The bus is moving at a __________________________________of 10

m/s for 5 seconds.

Segment B-C The bus is ____________________________________. It is slowing

down from 10 m/s to rest in 3 seconds.

Segment C-D The bus is ____________________________________________. It has

stopped.

Segment D-E The bus is ________________________________________________.

It is gradually increasing in speed.