Describing MotionEdexcel GCSE Physics: Revision notes
Section 1
What is the difference between a scalar and a vector?
A scalar quantity has magnitude (size) only. A vector quantity has magnitude AND a direction.
- Scalar examples: distance, speed, mass, energy
- Vector examples: displacement, velocity, acceleration, force, weight, momentum
Velocity is simply speed in a stated direction — that direction is what makes it a vector, not the size of the number.
Students often say speed and velocity are the same thing — they have the same units but velocity always needs a direction to be complete.
Section 2
How do we calculate average speed and read distance-time graphs?
Average speed (m/s) = distance (m) ÷ time (s), so distance = average speed × time.
On a distance-time graph:
- The gradient (slope) of the line equals the speed
- A steeper gradient means a faster speed
- A flat (horizontal) section means the object is stationary
- A curved line means the speed is changing (acceleration)
When asked to find speed from a distance-time graph, examiners want to see: gradient = change in y divided by change in x, with correct units.
Section 3
How do we calculate acceleration?
Acceleration is the rate of change of velocity:
a = (v − u) ÷ t
where v is final velocity, u is initial velocity and t is time, giving acceleration in m/s².
For problems without time, use:
v² − u² = 2 × a × x
where x is the distance travelled. This is useful when time isn't given or asked for.
A car speeds up from 5 m/s to 25 m/s in 4 s: a = (25-5)/4 = 5 m/s².
Section 4
How do we interpret velocity-time graphs?
On a velocity-time graph:
- The gradient gives acceleration (steeper = greater acceleration)
- A horizontal line means constant velocity (zero acceleration)
- A negative gradient means deceleration
- The area between the line and the time axis gives the distance travelled (for uniform acceleration, split the area into triangles/rectangles)
Comparing two lines on the same graph, the one with the steeper gradient is accelerating faster.
A common error is finding distance from a velocity-time graph by reading the gradient instead of calculating the area — gradient always gives acceleration, area always gives distance, on this type of graph.
Section 5
How are speeds measured and estimated?
Light gates connected to a timer can measure the speed of an object as it passes, by dividing the length of the interrupting card by the time it blocks the beam.
Typical everyday speeds (approximate):
| Situation | Typical speed |
|---|---|
| Walking | 1.5 m/s |
| Running | 3 m/s |
| Cycling | 6 m/s |
| Car (urban) | 13 m/s |
| Sound in air | 330 m/s |
The acceleration due to gravity in free fall is g = 10 m/s². Everyday accelerations (e.g. a car pulling away) are typically a few m/s².
Must Know
- Scalars have magnitude only; vectors have magnitude and direction
- Average speed = distance ÷ time; distance = speed × time
- Gradient of a distance-time graph = speed
- a = (v − u) / t, and v² − u² = 2 × a × x
- Gradient of a velocity-time graph = acceleration; area under it = distance
- g = 10 m/s² for free fall on Earth
That's the notes covered.
Carry on to the next subtopic.