Forces and momentum is the part of IGCSE Physics where you explain why motion changes. You draw the forces on an object, combine them into a resultant, link that resultant to acceleration with F = ma, and use momentum to predict what happens when objects collide or push apart.
Check the current Cambridge IGCSE Physics 0625 syllabus for the content points and the Core or Extended split in your exam year. Our Physics learning guide shows where this module sits among the others.
What should you already know?
You should be comfortable with speed and acceleration from motion and graphs, and with the difference between scalar and vector quantities from measurement and quantities. Weight and mass come in more detail in mass, weight and density.
An orienting example
All data are invented for practice. A car of mass 1200 kg is driven forwards with a force of 3000 N. Friction and air resistance together give 900 N backwards. Find the acceleration.
Step 1, forces: draw weight (down), normal force (up), driving force (forward, 3000 N) and resistance (backward, 900 N).
Step 2, resultant: forwards is positive. 3000 − 900 = 2100 N forwards.
Step 3, F = ma: a = 2100 ÷ 1200 = 1.75 m/s² forwards.
Check: 1200 × 1.75 = 2100 N, which matches the resultant.
Momentum link: after 4.0 s from rest, the speed is 1.75 × 4.0 = 7.0 m/s, so the momentum is 1200 × 7.0 = 8400 kg m/s. This equals force × time (2100 × 4.0 = 8400), which shows that a resultant force over time changes momentum.
That one example used the diagram, the resultant and F = ma. The lessons train each step separately.
In which order should you study it?
- Draw a force diagram for a stated situation: every later step depends on a diagram with the right arrows.
- Calculate a resultant force: adds signs and direction to the diagram.
- Relate acceleration to resultant force: brings in F = ma and the unit checks.
- Apply momentum conservation in a simple closed model: extends the ideas to interactions between two objects.
- Distinguish balanced forces from absence of forces: tightens the wording you will need in written answers.
Then work through the mixed practice set and keep a mistake log for the error types that return.
Which traps catch most students here?
- Adding opposing forces instead of subtracting them.
- Using one force in F = ma instead of the resultant.
- Drawing a “force of motion” arrow on a moving object.
- Forgetting signs in collisions, so two momenta that point opposite ways are added.
- Leaving mass in grams when the equation needs kilograms.
- Saying zero resultant means no forces, rather than forces that cancel.
How should you use the practice set?
Answer on paper first and write units and directions. Then compare your method with the worked answer, not just the final value. The table at the end of the practice set sends each kind of error back to its lesson.
For students who want help choosing a pace, our study route for Physics may help, and online one-to-one Physics tuition is there if you would like a teacher to look at your own working.