The velocity of an object is its speed in a given direction.
The displacement of an object is the distance it travels in a particular direction.
Displacement and velocity are both vector quantities.
Velocity has a positive value in one direction and negative value in the opposite direction.
Motion in a circle involves a centripetal force which constantly changes the velocity of an object.

Common misconception

Pupils often have a tendency to believe that a velocity must have a positive value and have difficulty in associating a reverse in direction with a change in sign.

It is important to make sure pupils are secure in their knowledge of the vector–scalar distinction; use plenty of visual examples.

Keywords

Displacement - The displacement of an object is the distance travelled in a particular direction from a starting point.
Vector - A vector is a quantity with magnitude (size) and direction. Displacement and velocity are both vector quantities.
Velocity - The velocity of an object is its speed in a particular direction.
Centripetal force - A centripetal force acts towards the centre of a circle and causes object to travel in circular paths.

A common definition of velocity is ‘speed in a given direction’, but this is only true for instantaneous speed and velocity and very rarely true for average speed and velocity. This distinction is explained with examples in this lesson in the learning cycle 'average velocity'.

Teacher tip

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.

A pupil measures the time taken for different sports balls to travel 30 cm. Use the times to put the balls in order of decreasing speed, starting with the fastest.

1 - ball D: 0.25 s

2 - ball A: 0.28 s

3 - ball B: 0.31 s

4 - ball C: 0.34 s

5 - ball E: 0.36 s

Q2.

Which three of these equations show the correct relationship between average speed, distance travelled and time taken?

Correct answer: average speed = distance travelled ÷ time taken

average speed = distance travelled × time taken

Correct answer: distance travelled = average speed × time taken

Correct answer: time taken = distance travelled ÷ average speed

time taken = distance travelled ÷ average speed

Q3.

What should be done with a single anomalous result in a set of five readings?

Change the value of the anomalous result to match the others in the set.

Correct answer: Repeat the test for the anomalous result and replace it with the new value.

Repeat the experiment from the start using different equipment.

Correct answer: Cross out the result and ignore it in any calculations.

Use the result in the calculations but not in the graph.

Q4.

A pupil measures the time it takes a car to pass between two street lamps as 5.0 s. If the car is moving at 8.0 m/s how far apart are the lamps?

3.0 m

12.0 m

1.6 m

0.625 m

Correct answer: 40 m

Q5.

Which of these is a definition of a scalar quantity?

A quantity with a very large value.

A quantity with a very small value.

Correct answer: A quantity that has magnitude but no direction.

A quantity that has magnitude and direction.

A quantity that always has a zero value.

Q6.

A tennis ball is released from a height of 20 m and it falls to the ground, taking 2.0 s. Which statements about the movement are correct?

Correct answer: The average speed of the ball was 10 m/s.

The instantaneous speed of the ball was the same throughout the fall.

The highest instantaneous speed of the ball was 10 m/s

The highest instantaneous speed of the ball was less than 10 m/s

Correct answer: The highest instantaneous speed of the ball was more than 10 m/s

Exit quiz

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6 Questions

Q1.

Match the key word with the correct description.

Correct Answer:displacement,the distance travelled in a straight line from a starting point

the distance travelled in a straight line from a starting point

Correct Answer:vector quantity,a quantity with a magnitude (size) and a direction

a quantity with a magnitude (size) and a direction

Correct Answer:scalar quantity,a quantity with a magnitude (size) but not a direction

a quantity with a magnitude (size) but not a direction

Correct Answer:velocity,the speed of an object in a particular direction

the speed of an object in a particular direction

Correct Answer:centripetal force,a force that causes an object to travel in circle

a force that causes an object to travel in circle

Q2.

Which of these are examples of displacements?

Correct answer: 30 km north

Correct answer: 30 cm left

30 m

Correct answer: 3 m down

30 m/s

Q3.

A train takes 2 hours to travel from one station to another which is 150 km west of the starting point. Calculate the velocity of the train.

300 km/h west

150 km/h west

Correct answer: 75 km/h west

300 m/s west

75 m/s west

Q4.

A pupil walks 5 m north, 3 m south and then 2 m north. Which of these statements about the pupil's journey are correct?

The total distance travelled is 4 m.

Correct answer: The total distance travelled is 10 m.

Their final displacement is 10 m north.

Correct answer: Their final displacement is 4 m north.

Their final displacement is 2 m north.

Q5.

A ball on the end of a piece of string is spun in a circle so that it travels at 4 m/s. Which of these statements are correct?

The ball has a constant instantaneous velocity of 4 m/s.

Correct answer: The ball has a constant instantaneous speed of 4 m/s.

Correct answer: The average velocity for one complete revolution is 0 m/s.

A force is acting on the ball directed away from the centre of the circle.

Correct answer: A force is acting on the ball directed towards the centre of the circle.

Q6.

A pupil walks 10 m west in 10 s, then 5 m east in 5 s and then 15 m west in 25 s. Calculate the magnitude of the average velocity for the complete journey (do not give the direction).

Correct Answer: 0.5 m/s, 0.5 m/s west, 0.5 m/s w, 0.5 m/s W