Velocity And Acceleration - Motion - OCR Gateway - GCSE Combined Science Revision - OCR Gateway - BBC Bitesize

In this guide

1. Revise
2. Audio
3. Test

Pages

1. Speed, distance and time
2. Average speed and distance
3. Distance-time graphs
4. Velocity and acceleration
5. Practical - investigating acceleration down a ramp
6. Velocity calculations
7. Velocity-time graphs

Velocity and acceleration

The velocity of an object is its speed in a particular direction. Velocity is a vector quantity because it has both a magnitude and an associated direction. To calculate velocity, displacement is used in calculations, rather than distance.

Unlike distance, which is a scalar quantity, displacement is a vector quantity. It includes:

• the distance travelled, measured in a straight line from start to finish
• the direction of the straight line

Learn more on acceleration in this podcast

Listen to the full series on BBC Sounds.

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Acceleration

Acceleration is the rate of change of velocity. It is the amount that velocity changes per unit time.

The change in velocity can be calculated using the equation:

change in velocity = final velocity - intial velocity

This is when:

• change in velocity is measured in metres per second (m/s)
• final velocity is measured in (m/s)
• initial velocity is measured in seconds (m/s)

The average acceleration of an object can be calculated using the equation:

\(acceleration = \frac{change~in~velocity}{time~taken}\)

This is when:

• acceleration is measured in metres per second squared (m/s2)
• change in velocity is measured in metres per second (m/s)
• time taken is measured in seconds (s)

If an object is slowing down, it is decelerating - in this case, its acceleration has a negative value.

Example

A car takes 8.0 s to accelerate from rest to 28 m/s. Calculate the average acceleration of the car.

First calculate change in velocity:

final velocity = 28 m/s

initial velocity = 0 m/s (because it was at rest – not moving)

change in velocity = (28 - 0) = 28 m/s

Then use the values to calculate average acceleration:

acceleration = 28 ÷ 8

acceleration = 3.5 m/s2

Question

A car takes 25 s to decelerate from 30 m/s to 20 m/s. Calculate the acceleration of the car.

Show answer

First calculate change in velocity:

final velocity = 20 m/s

initial velocity = 30 m/s

change in velocity = (20 - 30) = -10 m/s

Then use the values to calculate average acceleration:

acceleration = -10 ÷ 25

acceleration = -0.4 m/s2

The acceleration value is negative here because the car is slowing down or decelerating.

Next pagePractical - investigating acceleration down a rampPrevious pageDistance-time graphs

More guides on this topic

• Scalar and vector quantities - OCR Gateway
• Newton's laws - OCR Gateway
• Momentum, work and power - OCR Gateway
• Forces and elasticity - OCR Gateway
• Mass, weight and gravitational field strength - OCR Gateway

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