Cambridge IGCSE Physics 0625

1.2 Motion

Speed, velocity, motion graphs, acceleration and terminal velocity.

Core Extended

2026-2028 syllabus

Syllabus checklist

Core

Define speed as distance travelled per unit time and use v = s ÷ t.
Define velocity as speed in a given direction.
Use average speed = total distance travelled ÷ total time taken.
Sketch, plot and interpret distance–time and speed–time graphs.
Identify rest, constant speed, acceleration and deceleration from data or graph shape.
Calculate speed from the gradient of a straight-line section of a distance–time graph.
Calculate distance from the area under a speed–time graph for constant speed or constant acceleration.
State that free-fall acceleration near Earth is approximately constant at 9.8 m/s².

Supplement

Define acceleration as change in velocity per unit time and use a = Δv ÷ Δt.
Identify constant and changing acceleration from data or speed–time graph shape.
Calculate acceleration from the gradient of a speed–time graph.
Use deceleration as negative acceleration in calculations.
Describe motion during free fall with and without air or liquid resistance, including terminal velocity.

Key language

Definitions

Speed

Distance travelled per unit time.

Velocity

Speed in a given direction.

Average speed

Total distance travelled divided by total time taken.

Acceleration

Change in velocity per unit time.

Deceleration

Negative acceleration: velocity decreases with time.

Terminal velocity

The constant velocity reached when resistive force equals weight, so resultant force and acceleration are zero.

Core

Speed, velocity and graphs

Speed and velocity

Speed tells you how quickly distance is covered. It is a scalar quantity. Velocity includes direction, for example 12 m/s east, so it is a vector quantity.

Average speed uses the whole journey. A stop still adds to the total time, so it usually lowers the average speed.

Distance–time graphs

A distance–time graph shows how the total distance travelled changes with time. Its gradient is speed. A steeper line means a greater speed.

Worked distance-time graph divided into sections A to E — Fig. 1: A journey split into moving and stationary sections on a distance–time graph.

Graph shape	Motion
Horizontal line	At rest: distance does not change.
Straight rising line	Constant speed: constant gradient.
Curve becoming steeper	Accelerating: speed is increasing.
Curve becoming less steep	Decelerating: speed is decreasing.

Summary of distance-time graph shapes — Fig. 2: Comparing the main shapes of distance–time graphs.

Horizontal distance-time graph for an object at rest — Fig. 3: At rest.

Straight sloping distance-time graph for constant speed — Fig. 4: Constant speed.

Distance-time curve becoming steeper for acceleration — Fig. 5: Accelerating.

Distance-time curve becoming less steep for deceleration — Fig. 6: Decelerating.

Speed–time graphs

A speed–time graph shows how speed changes with time. A horizontal line above the time axis means constant speed. A line on the time axis means rest. A rising line means acceleration and a falling line means deceleration.

The area between the graph and the time axis gives the distance travelled. For a rectangular section, area = speed × time. For a triangular section, area = 12 × base × height.

Lorry speed-time graph with acceleration, constant speed and deceleration — Fig. 7: A lorry accelerates, travels at constant speed, then decelerates.

Area calculation beneath a speed-time graph — Fig. 8: Finding distance from areas under a speed–time graph.

Speed-time graph for an object at rest — Fig. 9: At rest.

Horizontal speed-time graph for constant speed — Fig. 10: Constant speed.

Rising speed-time graph for acceleration — Fig. 11: Accelerating.

Falling speed-time graph for deceleration — Fig. 12: Decelerating.

Free fall near Earth

When air resistance is negligible, an object near Earth accelerates downwards at an approximately constant 9.8 m/s². This means its velocity changes by about 9.8 m/s every second.

Supplement only

Acceleration and terminal velocity

Acceleration from speed–time graphs

Acceleration is the gradient of a speed–time graph. A straight sloping line has a constant gradient, so acceleration is constant. A curved line has a changing gradient, so acceleration changes.

Deceleration is negative acceleration. If the positive direction is fixed, a falling velocity gives a negative value of a.

Speed-time graph used to explain acceleration — Fig. 13: Acceleration is obtained from the gradient.

Speed-time graph with acceleration sections — Fig. 14: Interpreting acceleration on a speed–time graph.

Graph shapes for constant, increasing, decreasing and zero acceleration — Fig. 15: Constant, changing and zero acceleration.

Falling with resistance

In a vacuum, there is no air resistance, so an object falls with approximately constant acceleration g. In air or liquid, resistance acts opposite to the motion and increases as speed increases.

Just released: speed and resistance are small. Weight is greater than resistance, so the object accelerates downwards at nearly g.
Speed increasing: resistance increases. The resultant downward force becomes smaller, so acceleration decreases.
Terminal velocity: resistance equals weight. Resultant force is zero, acceleration is zero and velocity remains constant.

Terminal velocity graph and force stages for a falling object — Fig. 16: Falling through air until terminal velocity is reached.

Core and Extended

Equations

Speed

v = st

v: speed (m/s)
s: distance (m)
t: time (s)

Average speed

average speed = total distance travelledtotal time taken

Use the complete journey.
Keep units consistent.

Distance–time gradient

speed = change in distancechange in time

Use a straight-line section.
Unit: m/s

Speed–time area

distance = area under the graph

Rectangle: v × t
Triangle: 12 × base × height

Extended

Acceleration

a = ΔvΔt = v − ut

a: acceleration (m/s²)
u: initial velocity (m/s)
v: final velocity (m/s)

Extended

Speed–time gradient

acceleration = change in velocitychange in time

Positive gradient: positive acceleration.
Negative gradient: deceleration.

Cambridge-style practice

Practice questions

Core 1

Define velocity.

Velocity is speed in a given direction.

Core 2

A cyclist travels 450 m in 30 s. Calculate the cyclist's speed.

v = s ÷ t

v = 450 ÷ 30

v = 15 m/s

Core 3

A bus travels 12 km in 15 minutes, stops for 5 minutes, then travels 8 km in 10 minutes. Calculate its average speed in km/h.

Total distance = 12 + 8 = 20 km

Total time = 15 + 5 + 10 = 30 min = 0.5 h

average speed = 20 ÷ 0.5

average speed = 40 km/h

Core 4

A speed–time graph shows a constant speed of 6 m/s for 8 s. Calculate the distance travelled.

distance = area under graph

distance = 6 × 8

distance = 48 m

Core 5

State what a horizontal line on a distance–time graph shows.

The distance is not changing, so the object is at rest.

Extended 1

A car's velocity increases from 4 m/s to 16 m/s in 3.0 s. Calculate its acceleration.

a = (v − u) ÷ t

a = (16 − 4) ÷ 3.0

a = 4.0 m/s²

Extended 2

A vehicle slows from 20 m/s to 5 m/s in 6.0 s. Calculate its acceleration.

a = (v − u) ÷ t

a = (5 − 20) ÷ 6.0

a = −2.5 m/s²

The negative sign shows deceleration.

Extended 3

Explain why a falling object reaches terminal velocity in air.

As speed increases, air resistance increases.
Eventually air resistance equals weight.
The resultant force becomes zero.
Acceleration becomes zero, so the object continues at constant terminal velocity.

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Flashcards

Next topic 1.3 Mass and weight