IGCSE Physics: Forces Practice Questions
A resultant force changes the motion of an object. If the resultant force is zero the object stays at rest or continues at constant velocity, and if it is not zero the object accelerates in the direction of that force, following F = ma.
Sub-topic 1.5 of Cambridge IGCSE Physics 0625 carries more calculation marks than almost any other part of the course, and it feeds directly into momentum, energy and pressure. Nearly every mark is earned by writing the equation, substituting correctly with units, and giving the answer to a sensible number of significant figures. The questions below are structured the way Paper 3 and Paper 4 present them.
What you need to know for Forces
- Resultant forceThe single force that has the same effect as all the forces acting on an object. Forces in the same direction add, forces in opposite directions subtract.
- Newton's first lawAn object stays at rest or moves at constant velocity in a straight line unless a resultant force acts on it. This is why an unbelted passenger keeps moving when a car brakes.
- Newton's second lawResultant force = mass x acceleration, written F = ma, with force in newtons, mass in kilograms and acceleration in metres per second squared.
- Hooke's lawThe extension of a spring is directly proportional to the load applied, up to the limit of proportionality. F = kx, where k is the spring constant.
- Moment of a forceMoment = force x perpendicular distance from the pivot, measured in newton metres. It is the turning effect of a force.
- Principle of momentsFor an object in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that point.
IGCSE Physics Forces questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A car of mass 1200 kg accelerates at 2.5 m/s2 along a level road. The total resistive force acting on it is 800 N. Calculate the driving force produced by the engine.
Show the worked answer
- Find the resultant force using F = ma.
- F = 1200 x 2.5 = 3000 N.
- The resultant force is the driving force minus the resistive force, so driving force = resultant + resistive.
- Driving force = 3000 + 800 = 3800 N.
A uniform beam is balanced on a pivot at its centre. A weight of 4.0 N is placed 0.60 m from the pivot on the left. A weight of 3.0 N is placed on the right. Calculate the distance of the 3.0 N weight from the pivot for the beam to balance.
Show the worked answer
- Apply the principle of moments: clockwise moments equal anticlockwise moments.
- Anticlockwise moment = 4.0 x 0.60 = 2.4 N m.
- Clockwise moment = 3.0 x d.
- So 3.0d = 2.4, giving d = 2.4 / 3.0 = 0.80 m.
A spring extends by 4.0 cm when a load of 2.0 N is hung from it. Calculate the spring constant, then find the extension produced by a load of 5.0 N. Assume the spring stays within its limit of proportionality.
Show the worked answer
- Convert the extension to metres: 4.0 cm = 0.040 m.
- Rearrange F = kx to give k = F / x.
- k = 2.0 / 0.040 = 50 N/m.
- For 5.0 N: x = F / k = 5.0 / 50 = 0.10 m, which is 10 cm.
Explain, using Newton's first law, why a passenger not wearing a seat belt continues to move forward when a car brakes suddenly.
Show the worked answer
- The braking force acts on the car, not directly on the passenger.
- Newton's first law states that an object continues moving at constant velocity unless a resultant force acts on it.
- With no seat belt there is no significant backward force on the passenger, so they keep moving forward at the original speed until they hit something.
Common mistakes in this topic
- Adding forces that act in opposite directions instead of subtracting them.
- Forgetting to convert centimetres to metres or grams to kilograms before substituting.
- Using the total distance along a beam instead of the perpendicular distance from the pivot.
- Quoting a bare number with no unit. Unit marks are given and lost routinely in this topic.
- Treating weight and mass as the same thing. Weight in newtons is mass in kilograms multiplied by gravitational field strength.
Exam tips
- Write the equation, then the substitution, then the answer with a unit. Method marks are awarded even when the arithmetic goes wrong.
- Draw and label every force on a diagram before calculating. Most resultant force errors come from a missing arrow.
- For any balancing question, take moments about the pivot so that the pivot force has no moment.
- Check that your answer is physically sensible. A car accelerating at 250 m/s squared means a decimal point has moved.
- Round to the same number of significant figures as the data in the question, usually two or three.
Practise 20 more questions like this, free
vStudyWise marks every answer instantly, tracks the topics you keep dropping marks on and turns them into a weekly study plan.
Forces FAQs
What is the difference between mass and weight?
Mass is the amount of matter in an object, measured in kilograms, and it does not change with location. Weight is the force of gravity acting on that mass, measured in newtons, calculated as W = mg. On the Moon your mass is unchanged but your weight is roughly one sixth of its value on Earth.
How do I calculate a resultant force?
Add forces that act in the same direction and subtract forces that act in opposite directions. The resultant is the single force that would have the same effect. If the resultant is zero the object is in equilibrium, and if it is not zero the object accelerates in the direction of the resultant.
What is the limit of proportionality in Hooke's law?
The limit of proportionality is the point beyond which extension is no longer directly proportional to the load. Up to that point a load extension graph is a straight line through the origin. Beyond it the line curves, and the equation F = kx no longer applies.
What are the units of a moment?
Moments are measured in newton metres, written N m. Multiply the force in newtons by the perpendicular distance from the pivot in metres. If the distance is given in centimetres, convert to metres first unless the question asks for N cm.
Continue through the IGCSE Physics syllabus
Related IGCSE Physics topics
See all 24 IGCSE Physics practice topics ›
Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.