Revision NotesPast Papers
GCSEScienceWJECScience (Double Award)PhysicsRevision NotesForces, Space & RadioactivityDistance, Speed & AccelerationDescribing Motion

Describing Motion (WJEC GCSE Science (Double Award)): Revision Note

Ann Howell

Last updated

WJEC Science (Double Award): PhysicsWJEC Physics

Describing Motion

Speed & Velocity

  • Speed is a measure of how fast or slow an object is moving

  • It is a scalar quantity

    • Because it only contains a magnitude (without a direction)

  • The velocity of a moving object is similar to its speed, except it also describes the object’s direction

  • Velocity is a vector quantity 

    • The velocity of an object contains both magnitude and direction

    • e.g. ‘15 m / s south’ or ‘250 mph on a bearing of 030°’

    Comparing Speed and Velocity

Speed & Velocity, downloadable IGCSE & GCSE Physics revision notes

The cars in the diagram above have the same speed (a scalar quantity) but different velocities (a vector quantity). Fear not, they are in different lanes!

Acceleration

  • Acceleration is defined as the rate of change of velocity

  • In other words, it describes how much an object's velocity changes every second

  • The acceleration of an object can be positive or negative, depending on whether the object is speeding up or slowing down

    • If an object is speeding up, its acceleration is positive

    • If an object is slowing down, its acceleration is negative (sometimes called deceleration)

Positive and Negative Acceleration

Acceleration Examples, downloadable IGCSE & GCSE Physics revision notes

A rocket speeding up (accelerating) and a car slowing down (decelerating)

Calculating Speed & Acceleration

Calculating Speed

  • For objects that are moving with a constant speed, use the equation below to calculate the speed:

Speed space equals space distance over time

  • Where:

    • Speed is measured in metres per second (m/s)

    • Distance travelled is measured in metres (m)

    • Time taken is measured in seconds (s)

The Speed of Different Objects

Person vs Bee, downloadable IGCSE & GCSE Physics revision notes

A hiker might have a speed of 2.0 m/s, whereas a particularly excited bumble bee can have a speed of up to 4.5 m/s

 

Calculating Acceleration

  • The equation below is used to calculate the average acceleration of an object:

 acceleration space equals space fraction numerator change space in space velocity over denominator change space in space time end fraction

a space equals space fraction numerator increment v over denominator increment t end fraction

  • Where:

    • a = acceleration in metres per second squared (m/s2)

    • Δv = change in velocity in metres per second (m/s)

    • Δt = time taken in seconds (s)

Worked Example

Planes fly at typical speeds of around 250 m/s. Calculate the total distance travelled by a plane moving at this average speed for 2 hours. 

Answer:

Step 1: List the known quantities

  • Average speed = 250 m/s

  • Time taken = 2 hours

Step 2: Write the relevant equation

Speed space equals space distance over time

Step 3: Rearrange for the total distance

total distance = average speed × time taken

Step 4: Convert any units

  • The time given in the question is not in standard units

  • Convert 2 hours into seconds:

2 hours = 2 × 60 × 60 = 7200 s

Step 5: Substitute the values for average speed and time taken

total distance = 250 × 7200

total distance = 1 800 000 m

Worked Example

A Japanese bullet train decelerates at a constant rate in a straight line. The velocity of the train decreases from 50 m/s to 42 m/s in 30 seconds.

(a) Calculate the change in velocity of the train.

(b) Calculate the deceleration of the train, and explain how your answer shows the train is slowing down.

Answer:

Part (a)

Step 1: List the known quantities

  • Initial velocity = 50 m/s

  • Final velocity = 42 m/s

Step 2: Write the relevant equation

change in velocity = final velocity − initial velocity

Step 3: Substitute values for final and initial velocity

change in velocity = 42 − 50

change in velocity = −8 m/s

Part (b)

Step 1: List the known quantities

  • Change in velocity, Δv = − 8 m/s

  • Time taken, t = 30 s

Step 2: Write the relevant equation

 a space equals space fraction numerator increment v over denominator increment t end fraction

Step 3: Substitute the values for change in velocity and time

a = −8 ÷ 30

a = −0.27 m/s

Step 4: Interpret the value for deceleration

  • The answer is negative, which indicates the train is slowing down

Examiner Tips and Tricks

Remember the units for acceleration are metres per second squared, m/s2. In other words, acceleration measures how much the velocity (in m/s) changes every second, m/s/s.

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Did this page help you?

    Previous:Communication SatellitesNext:Distance-Time Graphs
    Ann Howell

    Author: Ann Howell

    Expertise: Physics Content Creator

    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.