IGCSE Physics revision · Mechanics
Forces
Forces is the largest mechanics topic and the one the six-mark questions love. Nearly every explanation on this topic follows one chain: find the resultant force, then state what it does to the motion.
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What the syllabus demands
- —Find the resultant of forces acting along a line
- —Know that a resultant force changes the velocity of an object (speed and/or direction)
- —Use F = ma for calculations
- —Describe the effects of friction and air resistance
- —Use Hooke's law (F = kx), the limit of proportionality, and load-extension graphs
- —Describe circular motion: a perpendicular force causes a change in direction at constant speed (Extended)
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Resultant force
- The single force that has the same effect as all the forces acting on an object combined.
- Hooke's law
- The extension of a spring is directly proportional to the load applied, provided the limit of proportionality is not exceeded.
- Friction
- The force that opposes the relative motion between two surfaces in contact, transferring kinetic energy to thermal energy.
- Limit of proportionality
- The point beyond which extension is no longer proportional to load, and the load-extension graph stops being a straight line.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Using the total length of the spring instead of the extension in Hooke's law. Extension = stretched length minus original length — every time.
- 02
Saying a resultant force is needed to keep an object moving. It is not: with zero resultant force an object continues at constant velocity. A resultant force is needed to change velocity.
- 03
In circular motion, saying the speed changes. At constant speed in a circle the velocity changes (direction), so there is an acceleration towards the centre — caused by a resultant force towards the centre.
- 04
Adding forces that act in opposite directions instead of subtracting. Draw the arrows before you calculate.
One worked example, done properly
Question
A 1200 kg car experiences a driving force of 3600 N and total resistive forces of 900 N. Calculate its acceleration.
Method
- 1.Resultant force = 3600 − 900 = 2700 N (forces oppose, so subtract).
- 2.a = F ÷ m = 2700 ÷ 1200.
a = 2.25 m/s²
Test yourself
10 questions · instant marking
Question 1 of 10
A resultant force of 20 N acts on a 4 kg mass. The acceleration is:
Question 2 of 10
An object moves at constant velocity. The resultant force on it is:
Question 3 of 10
A spring is 12 cm long unstretched and 17 cm with a load. Its extension is:
Question 4 of 10
Hooke's law is valid:
Question 5 of 10
Forces of 12 N left and 8 N right act on an object. The resultant is:
Question 6 of 10
An object moves in a circle at constant speed. Its velocity is:
Question 7 of 10
The weight of a 5 kg object on Earth (g = 9.8 N/kg) is:
Question 8 of 10
A spring with spring constant 200 N/m is stretched by 0.03 m. The force is:
Question 9 of 10
Friction acting on a sliding box transfers kinetic energy mainly to:
Question 10 of 10
A car of mass 900 kg accelerates at 2 m/s² with resistive forces of 300 N. The driving force is: