Forces questions are where a lot of O-Level Physics marks quietly disappear. Students know that F = ma, but they lose marks because they never wrote down which forces were acting, or they mixed up weight and reaction force, or they added forces that were pointing in opposite directions. The fix is almost always the same: draw a proper free-body diagram first, every single time.
Here is the method we teach at Dojo, built around how the Cambridge 6091 mark schemes actually award marks.
What a free-body diagram really is
A free-body diagram shows one object and only the forces acting on that object. Nothing else. No forces the object exerts on other things, no velocity arrows, no acceleration arrows mixed in with forces.
The four forces you will meet at O-Level are:
- Weight (W = mg), always vertically downwards from the centre of the object
- Normal contact force (R or N), perpendicular to the surface
- Tension (T), along a string or rope, always pulling away from the object
- Friction or air resistance (f), opposing relative motion
If a force in your diagram does not fall into one of these categories, ask yourself what is physically touching the object to produce it. If nothing is touching it and it is not gravity, the force probably does not exist.
Drawing it properly for marks
- Draw the object as a simple box or dot.
- Draw each force as an arrow starting on the object, pointing outwards.
- Label every arrow with a name or symbol and, where known, a value with units.
- Make the arrow lengths roughly proportional. If the object is accelerating upwards, the upward arrow should visibly be longer.
Examiners do check arrow direction and labelling. An unlabelled arrow usually scores nothing.
Applying Newton's Second Law
Once the diagram is drawn, the working is mechanical:
- Choose a positive direction, usually the direction of acceleration.
- Write resultant force = ma.
- Substitute: forces in the positive direction minus forces in the negative direction.
- Solve, then check whether the sign and size make physical sense.
Use g = 10 N/kg unless the paper states otherwise.
Worked example 1: the lift problem
A student of mass 60 kg stands in a lift accelerating upwards at 2 m/s². Find the force the floor exerts on the student.
Free-body diagram: weight 600 N downwards, normal contact force R upwards.
Taking upwards as positive:
R - W = ma
R - 600 = 60 × 2
R = 720 N
Notice that R is larger than the weight. That is exactly why you feel heavier when a lift starts moving up. If the lift accelerated downwards at 2 m/s², you would write W - R = ma and get R = 480 N.
Worked example 2: friction on a horizontal surface
A 10 kg crate is pulled along the floor by a horizontal force of 50 N. Friction on the crate is 20 N. Find the acceleration.
Free-body diagram: weight 100 N down, normal force 100 N up, applied force 50 N right, friction 20 N left.
Horizontally:
F - f = ma
50 - 20 = 10a
a = 3 m/s²
The vertical forces balance, so they contribute nothing to the horizontal acceleration. Say that explicitly in your answer if the question asks you to explain.
Worked example 3: terminal velocity
A skydiver falls and eventually reaches terminal velocity. Explain why.
Mark-scheme style answer:
- Initially, weight is greater than air resistance, so there is a resultant downward force and the skydiver accelerates.
- As speed increases, air resistance increases.
- Eventually air resistance equals weight, so the resultant force is zero.
- Acceleration is zero, so the skydiver falls at constant (terminal) velocity.
The phrase examiners look for is "resultant force is zero, therefore acceleration is zero", not "the forces cancel so it stops". Constant velocity is not zero velocity.
Five mistakes that cost marks
- Writing mass in newtons. A 5 kg object has weight 50 N. Never write "weight = 5 kg".
- Adding an extra "force of motion". A moving object does not need a forward force to keep moving. That is Newton's First Law.
- Drawing action and reaction on the same body. The pair in Newton's Third Law acts on two different objects, so they never appear in one free-body diagram.
- Forgetting to state the direction of acceleration. If you get a negative answer, say the object decelerates or accelerates in the opposite direction.
- Missing units. Forces in N, acceleration in m/s². Free marks lost otherwise.
A quick checklist before you move on
- Have I drawn one object only?
- Is weight always present and pointing down?
- Is every arrow labelled?
- Did I pick a positive direction and stick to it?
- Does my final answer have the right unit and a sensible size?
Work through past-year Paper 2 forces questions using this routine and you will find your marks become far more consistent, because the method is identical whether the object is a lift, a crate, a parachutist or a car braking on a wet road.
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