GCSE Physics · Physics
Forces: a clear study guide
Forces change motion, but the effect depends on the resultant force. Draw the forces first, then decide whether they balance before choosing an equation.
Balanced and unbalanced forces
The resultant force is the overall force after opposing forces are combined. If the resultant force is zero, an object remains at rest or continues at constant velocity. If it is non-zero, the object accelerates — its speed or direction changes.
A free-body diagram shows the forces acting on one object. Label the direction and, where useful, compare the sizes of opposing forces.
- Resultant force = mass × acceleration (F = ma).
- A larger resultant force gives a larger acceleration for the same mass.
- Acceleration can mean speeding up, slowing down or changing direction.
Mass, weight and gravity
Mass is the amount of matter in an object and is measured in kilograms. Weight is the force caused by gravity and is measured in newtons.
Use W = m g. Near Earth, g is approximately 9.8 N/kg, or 10 N/kg when the question allows a rounded value.
A reliable exam method
Write the equation, substitute values with units, and rearrange only after identifying the quantity you need. Check whether the answer is physically sensible: a heavier object has greater weight, but not necessarily greater acceleration.
Quick check
What happens when the resultant force is zero?
Answer: The object stays at rest or continues at constant velocity.
What is the weight of a 2 kg object using g = 10 N/kg?
Answer: 20 N.
Common questions
Is weight the same as mass?
No. Mass is measured in kilograms; weight is the gravitational force on that mass and is measured in newtons.
Can an object move when the forces are balanced?
Yes. Balanced forces mean constant velocity, which can be zero or non-zero.