How do you explain terminal velocity for full marks?

In sequence: as speed increases, air resistance increases; eventually air resistance equals weight; the resultant force is then zero; so acceleration is zero and the object falls at constant (terminal) velocity. Four linked statements, four marks.

What does the gradient of a distance-time graph give?

The speed. A steeper line means faster; a horizontal line means stationary; a curved line means the speed is changing.

How do you find distance from a speed-time graph?

Distance equals the area under the graph. Split the shape into triangles and rectangles, calculate each area, and add them.

← IGCSE Physics revision

IGCSE Physics revision · Mechanics

Motion

Motion opens almost every Paper 4, usually through a graph. The physics is simple; the marks are won and lost in reading gradients and areas correctly — and in knowing exactly what terminal velocity means.

Dr Desouky Physics Academy · Meet your physics tutor

What the syllabus demands

—Define speed, velocity and acceleration, and calculate each
—Read and sketch distance-time graphs: gradient = speed
—Read and sketch speed-time graphs: gradient = acceleration, area = distance
—Describe free fall, air resistance and terminal velocity, including the skydiver graph
—Know g = 9.8 m/s² (often approximated as 10 m/s²) for objects falling near Earth's surface

Definitions that earn marks

Clear definitions to practise — check your course mark scheme

Speed: The distance travelled per unit time.
Velocity: Speed in a given direction.
Acceleration: The change in velocity per unit time.
Terminal velocity: The constant maximum velocity reached by a falling object when the air resistance (upwards) has grown to equal the weight (downwards), so the resultant force and acceleration are zero.

The equations

Average speedv = s ÷ t · m/s

Accelerationa = Δv ÷ t · m/s²

Distance from a speed-time graphs = area under the graph · m

More equations to practise: the IGCSE formula sheet.

Where the marks die

Common mistakes to check

01
Confusing the two graphs. A horizontal line on a distance-time graph means stationary; on a speed-time graph it means constant speed. Label which graph you are reading before you write anything.
02
Describing terminal velocity as 'gravity equals air resistance'. Gravity is not a force name here — the mark scheme wants weight equals air resistance, resultant force zero, acceleration zero.
03
Finding distance from a speed-time graph by multiplying instead of taking the area. For changing speed, only the area under the graph earns the marks.
04
Dropping a minus sign — deceleration is negative acceleration, and 'the acceleration decreases' is not the same as 'the object decelerates'.

One worked example, done properly

Question

A car accelerates uniformly from rest to 24 m/s in 8.0 s, then travels at constant speed for 12 s. Calculate the total distance travelled.

Method

1.Phase 1 (triangle on the speed-time graph): s₁ = ½ × 8.0 × 24 = 96 m.
2.Phase 2 (rectangle): s₂ = 24 × 12 = 288 m.
3.Total distance = 96 + 288.

s = 384 m

Test yourself

10 questions · instant marking

Question 1 of 10

A car travels 300 m in 20 s. What is its average speed?

Question 2 of 10

On a distance-time graph, a horizontal line means the object is:

Question 3 of 10

The gradient of a speed-time graph gives the:

Question 4 of 10

A skydiver falls at terminal velocity. The resultant force on her is:

Question 5 of 10

A cyclist accelerates from 4 m/s to 10 m/s in 3 s. Her acceleration is:

Question 6 of 10

The area under a speed-time graph represents:

Question 7 of 10

An object in free fall near the Earth's surface (no air resistance) has:

Question 8 of 10

A train travels at 30 m/s for 2 minutes. How far does it go?

Question 9 of 10

Which quantity is a vector?

Question 10 of 10

As a falling object speeds up, air resistance:

Fit these topics into your free physics revision plan