Edexcel GCSE Physics (1PH0) · Topic 9 — Forces and their effects
Mini-Lesson
Forces and their Effects
This mini-lesson walks you through the whole of Edexcel Topic 9: how forces interact, why some are vectors, how to add and resolve them with diagrams, and the turning effect of a force — moments, the principle of moments, and how levers and gears use it.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Screens marked HT are Higher Tier; Physics only screens are for separate Physics, not Combined Science. Press Start when you're ready.
9.1 · How forces arise
Forces are interactions
A force is always a push or pull on one object caused by another. There are two families:
Contact forces — the objects touch: normal contact force, friction, tension, air resistance.
Non-contact forces — they act at a distance through a field: gravitational, electrostatic and magnetic.
Forces come in pairs and have both a size and a direction — so we draw them as arrows (vectors).Quick check
Contact or field?
?A compass needle swings to line up near a bar magnet, even though nothing touches it. Which type of force is acting?
9.2 · Two kinds of quantity
Vectors vs scalars
Some quantities need only a size (magnitude); others need a size and a direction:
A vector is drawn as an arrow: its length shows the magnitude, the way it points shows the direction.
Watch out:weight is a force (a vector, in newtons) pulling down; mass is a scalar (in kilograms) — they are not the same thing.
Sort it
Vector or scalar?
Tap a quantity, then tap the box it belongs in.
➡️ Vector
🔢 Scalar
9.4 · Higher Tier
Free-body force diagrams
A free-body diagram shows a single object as a dot or box, with every force on it drawn as an arrow from that object. The arrow's length shows the size and the way it points shows the direction.
A box on a slope: weight acts vertically down, the normal contact force N acts at right angles to the surface, and friction acts up the slope.
Tip: draw only the forces acting on the object — never the forces it exerts on other things.
9.5 · Higher Tier
Resultant & balanced forces
When several forces act on an object you can replace them with a single resultant force — the overall push or pull. Along one line you simply add forces, taking one direction as positive.
resultant = sum of the forcesbalanced forces give a resultant of zero — but the object need not be still
When the resultant is zero the forces are balanced (equilibrium): a stationary object stays still, and a moving object keeps a constant velocity.
Worked example
A sledge is pulled forwards with 50 N while friction pulls back with 30 N.
Resultant = 50 − 30 = 20 N forwards — so it accelerates forwards.
Calculate · HT
Your turn — resultant force
1A swimmer pushes forwards with a 180 N thrust while water resistance pushes back with 110 N. What is the resultant force on the swimmer?
N
Hint: take forwards as positive — resultant = 180 − 110.
9.3 · Higher Tier
Adding forces with a vector diagram
When two forces act at an angle, you can't just add the numbers. Draw them tip-to-tail to scale — the resultant runs from the very start to the very end, closing the triangle. You then measure its length and angle.
Two perpendicular forces, 40 N east and 30 N north, give a 50 N resultant. The same triangle, read backwards, lets you resolve one force into two parts.
Equilibrium on a scale diagram shows up as a closed shape — the tip of the last arrow lands exactly back on the start, so the resultant is zero.
Quick check · HT
Reading the triangle
?On a scale vector diagram for three forces acting on an object, the arrows are drawn tip-to-tail and the last arrow's tip lands exactly on the very first arrow's tail. What does this tell you?
9.6 · Physics only
When a force makes things turn
So far forces have pushed objects in a straight line. But a force applied at a distance from a fixed point — a pivot — makes the object rotate instead. You meet this turning effect everywhere:
pushing a door open about its hinges,
turning a spanner on a nut,
a child on a see-saw, or a steering wheel and tap.
The size of this turning effect is called the moment of the force — that's the next screen.
9.7 · Physics only
Moment of a force
The moment measures the turning effect. It grows with the force and with how far from the pivot you push:
moment = F × dmoment (newton metre, N m) = force (N) × distance normal to the force (m)
A longer spanner (bigger d) gives a bigger moment for the same pull — so the nut turns more easily.
Two big misconceptions: (1) d is the distance perpendicular (at right angles) to the line of the force, not just any distance. (2) A force whose line passes through the pivot has d = 0, so its moment is zero — it can't turn the object.
Worked example
A force of 12 N pushes at right angles, 0.25 m from a pivot.
moment = 12 × 0.25 = 3 N m
Calculate · Physics only
Your turn — calculate a moment
2A mechanic pushes down with a force of 40 N on the end of a spanner, at right angles, 0.30 m from the centre of the bolt. Calculate the moment.
N m
Hint: moment = F × d = 40 × 0.30.
9.8 · Physics only
The principle of moments
An object is balanced (in rotational equilibrium) when the forces trying to turn it one way exactly match those turning it the other way:
Σ clockwise moments = Σ anticlockwise momentsabout the same pivot, for rotational equilibrium
Anticlockwise: 200 N × 1.5 m = 300 N m. Clockwise: 300 N × 1.0 m = 300 N m. They are equal, so the see-saw balances. ⚖️
Watch out: "balanced" means zero net moment (and zero resultant force) — it does not mean nothing is happening. A balanced see-saw could even be turning at a steady rate; it just isn't being made to turn faster either way.
Calculate · Physics only
Your turn — balance the beam
3On a balanced see-saw a child weighing 250 N sits 1.2 m to the left of the pivot. A second child weighing 300 N sits on the right. How far from the pivot must the second child sit so the see-saw balances?
m
Hint: anticlockwise = clockwise, so 250 × 1.2 = 300 × d. Solve for d = 300 ÷ 300.
9.9 · Physics only
Levers and gears
Both levers and gears use moments to transmit and change the turning effect of a force.
Levers as force multipliers: a long effort arm and a short load arm let a small effort balance a large load — because the small force acts at a bigger distance from the pivot.
A crowbar: a small effort, far from the pivot, gives a moment big enough to balance a large load close to it.
Gears: teeth on two wheels interlock, so the rim forces match. A small gear driving a large gear gives a bigger turning effect (moment) but a slower rotation; a large gear driving a small one trades turning effect for speed.
The gears turn in opposite directions. The larger gear turns more slowly but, because the force acts at a greater radius, it provides a larger moment.Quick check · Physics only
How gears trade off
?A small driver gear turns a much larger gear. Compared with the small gear, the large gear turns…
9.10 · Reducing waste
Reducing unwanted energy transfer
Whenever surfaces rub or gears mesh, friction dissipates energy by heating the surroundings — energy that is no longer useful. To waste less of it:
Lubrication — a thin film of oil or grease between moving parts lets them slide past each other, cutting friction and the heating it causes.
Less friction means smoother levers and gears, less wear, and more of the input ending up as useful output.
Quick check
Why lubricate?
?Oiling the chain and gears of a bike makes it more efficient. What is the main reason?
Recap
The key ideas to know
Forces (9.1–9.2): contact vs non-contact (field); forces are vectors (size + direction), scalars have size only.