Velocity of an object is equal to the gradient of a displacement-time graph.
A direction in which velocity and displacement are measured needs to be chosen.
A negative gradient on a displacement-time graph (sloping downwards) represents velocity in the opposite direction.
A negative value for displacement represents a position ‘behind’ the starting position.

Common misconception

Negative values on a displacement-time graph always represent motion that is backwards.

Provide opportunities for pupils to plot a range of displacement-time graphs in real time in order for them to understand how the graph represents different motions.

Keywords

Displacement - The displacement of an object is the distance and direction from a specific point. Displacement is a vector.
Displacement-time graph - A displacement-time graph shows the displacement of an object over a period of time.
Motion sensor - A motion sensor measures the distance between itself and an object. We can use the measurements to calculate and plot graphs of displacement, velocity and acceleration.

Allow pupils to plot displacement-time graphs in small groups using a motion sensor on their desk to detect the position of a dynamics trolley they move towards or away from the sensor. A piece of cardboard taped to the back of the trolley provides a reliable target.

Teacher tip

Content guidance

Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

Some swimmers compete in a 50m race. Their times are shown. Starting with the fastest, sort the swimmers in order of their speeds.

1 - 29.9 s

2 - 30.2 s

3 - 31.1 s

4 - 31.2 s

5 - 31.4 s

Q2.

Which of the following are vector quantities?

distance

Correct answer: displacement

speed

Correct answer: velocity

mass

Q3.

A rocket has a velocity of 200 m/s upwards. What distance will the rocket travel upwards in 30 s?

6.7 m

Correct answer: 6000 m

60 000 m

3000 m

600 m

Q4.

A jogger runs for 250 s and they end up 625 m west of their starting point. Calculate the average velocity of the jogger.

Correct answer: 2.5 m/s west

0.4 m/s west

375 m/s west

5.0 m/s west

Q5.

Two trains are travelling in the same direction. Train A has a speed of 6.5 m/s and train B has a speed of 5.5 m/s.

Correct Answer: 1.0 m/s, 1.0 metres per second, 1 m/s, 1 metres per second, 1.0 metres/second

Q6.

An small airplane travels 45 km north in 30 minutes. Calculate the velocity of the airplane in m/s.

1.5 m/s north

22 m/s north

Correct answer: 25 m/s north

1500 m/s north

6.6 m/s north

Exit quiz

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6 Questions

Q1.

Match the key word or phrase to the explanation.

Correct Answer:displacement,a vector which is the distance and direction from a point

a vector which is the distance and direction from a point

Correct Answer:velocity,a vector that gives speed and direction

a vector that gives speed and direction

Correct Answer:displacement–time graph,shows the displacement of an object over a period of time

shows the displacement of an object over a period of time

Correct Answer:motion sensor,measures the distance between an object and itself

measures the distance between an object and itself

Q2.

Which feature of a displacement-time graph shows the velocity?

the intercept on the displacement axis (y-axis)

the intercept on the displacement time (x-axis)

the area beneath the line

Correct answer: the gradient of the line

the maximum on the displacement axis

Q3.

An ultrasonic motion sensor is used to measure displacement of a ball. Put the stages in order to explain how the sensor works.

1 - the sensor sends out an ultrasound pulse

2 - the sound pulse travels to the ball

3 - the sound pulse reflects off the ball

4 - the sound pulse travels to the sensor

5 - the sensor detects the pulse

6 - the speed of sound and the time information use used to calculate displacement

Q4.

Which line on the graph show the highest velocity?

A (red)

B (blue)

Correct answer: C (black)

D (green)

Q5.

Select the options which describe the motion between 1 s and 5 s.

Correct answer: positive displacement

positive velocity

Correct answer: negative velocity

Correct answer: constant velocity

changing velocity

Q6.

A pupil has plotted a graph to show their movement as they run across the classroom. Why is this graph unrealistic?

Correct answer: The change in their velocity is too sudden.

Their velocity is too high for a human to run.

The velocity is too low for running.

It does not take into account the direction of travel.

They have not given the graph a title.