IGCSE Physics 0625 · Common mistakes
Common mistakes in IGCSE Physics forces and motion
Updated · by the Fahmazing team
Motion and forces is the biggest topic area on the paper, so small slips here cost a lot. Each mistake below comes with a wrong answer and a corrected one.
Speed upside down
A runner covers 200 m in 25 s. Find the speed.
✗ Wrong: speed = 25 ÷ 200 = 0.125 m/s
✓ Right: speed = distance ÷ time = 200 ÷ 25 = 8.0 m/s
Tip: Speed is metres per second, so distance goes on top.
Free lesson: Motion
Using final speed instead of change in velocity
A car speeds up from 10 m/s to 30 m/s in 5.0 s. Find its acceleration.
✗ Wrong: a = 30 ÷ 5.0 = 6.0 m/s²
✓ Right: a = Δv ÷ Δt = (30 − 10) ÷ 5.0 = 4.0 m/s²
Tip: If the speed goes down, Δv and the acceleration are negative. Keep the minus sign.
Free lesson: Speed–time graphs and acceleration
Mixing up mass and weight
Find the weight of a 500 g apple. (g = 9.8 N/kg)
✗ Wrong: W = 500 × 9.8 = 4900 N
✓ Right: W = mg = 0.500 kg × 9.8 N/kg = 4.9 N
Tip: Mass (kg) never changes from place to place. Weight is a force in newtons.
Free lesson: Mass and weight
Thinking movement needs a resultant force
A car moves at a steady 20 m/s along a straight road. What is the resultant force?
✗ Wrong: There must be a forward resultant force to keep it moving.
✓ Right: Zero. The driving force balances friction and air resistance, so the velocity does not change.
Tip: Forces in opposite directions subtract. A resultant force changes speed or direction.
Free lesson: Resultant force, friction and circular motion
Moments with the distance in centimetres
A 40 N force acts 25 cm from a pivot. Find the moment in N m.
✗ Wrong: moment = 40 × 25 = 1000 N m
✓ Right: moment = force × perpendicular distance = 40 × 0.25 = 10 N m
Free lesson: Turning effect of forces and centre of gravity
Forgetting that velocity has direction (Extended)
A 0.20 kg ball hits a wall at 5.0 m/s and bounces back at 5.0 m/s. Find the change in momentum.
✗ Wrong: Δp = 0.20 × (5.0 − 5.0) = 0
✓ Right: Δp = 0.20 × (5.0 − (−5.0)) = 2.0 kg m/s. The speed is the same, but the velocity has reversed.
Free lesson: Momentum
Work done and power
A motor does 3600 J of work in 1 minute. Find its power.
✗ Wrong: P = 3600 ÷ 1 = 3600 W
✓ Right: P = W ÷ t = 3600 ÷ 60 = 60 W
Tip: Work done is force multiplied by distance, never divided.
Free lesson: Work and power