Welcome to Weight, Springs & Friction. This topic covers three forces you will meet on every CSCA physics paper: the weight of an object, the push or pull of a spring, and friction. Each of the four real papers from December 2025 to April 2026 asked about all three, about four questions a paper in total, and the December and April papers used them for Q2, Q3 and Q4. Most of these questions need one idea and one short step, so this topic gives a lot of marks for little study time.
All of those questions rest on a clear idea of what a force is, and that is what this page builds. We will start from things you can see and feel: a force always involves two objects; it is described by three things (its size, its direction and the point where it acts); and what it does is change how an object moves, or change its shape. Along the way you will learn to draw a force as an arrow, the picture every later page in this topic uses. The April 2026 paper asked two questions straight from this page, Q7 and Q17, and both are quick marks once these ideas are clear.
A force needs two objects
Think of a few forces from everyday life. You push a door open. You pull a bag off a shelf. A foot kicks a ball. Each time, one object does the pushing or pulling and a second object is pushed or pulled: the hand and the door, the hand and the bag, the foot and the ball.
Now try to picture a push with nothing to push, or a pull with nothing to pull. You cannot. A force is always the action of one object on another, which is why a force is called an interaction between two objects. The object that exerts the force is the agent; the English papers call it the exerting object. The object it acts on is the receiver, or receiving object. Every force has one of each. So a single object on its own cannot exert a force: there would be nothing for the force to act on.
The two objects do not have to touch. Let go of an apple and it falls, so something must be pulling it down, yet nothing touches it in mid-air. The agent is the Earth, pulling on the apple across the gap; the papers call this pull gravity. A magnet pulls an iron nail towards it from a few centimetres away in the same way. Pushes, the pull of a rope and friction need contact; gravity and magnetic forces act across a gap. Either way, there are still two objects.
Every force has an agent and a receiver. The hand touches the box; the Earth pulls the apple across a gap. In both pictures the arrow sits on the receiver, the object the force acts on.
Force as an interaction. A force is the action of one object on another. The object that exerts it is the agent (the exerting object), and the object it acts on is the receiver (the receiving object). Every force has both, whether the two objects touch or act across a gap, like the Earth and a falling apple. One object alone cannot exert a force.
The action runs both ways. When your hand pushes a door, the door pushes back on your hand: you can feel it on your palm. For the push on the door, the hand is the agent and the door is the receiver; for the push back on your hand, the roles swap. Forces between two objects always come in pairs like this, and you will meet these pairs again with Newton's laws. For now, the point is that every force has an agent and a receiver.
Worked example · naming the agent and the receiver
Name the agent and the receiver of each force: (1) the force that sends a football flying when it is kicked; (2) the force that pulls a falling apple down; (3) the force that pulls a sledge along when a child tows it with a rope.
(1) The foot acts on the ball, so the agent is the foot and the receiver is the ball.
(2) Nothing touches the apple, but the Earth pulls on it across the gap: the agent is the Earth and the receiver is the apple.
(3) The child pulls the rope, and the rope pulls the sledge. The object acting on the sledge is the rope, so the agent is the rope and the receiver is the sledge. The child's pull acts on the rope, not directly on the sledge.
To find the agent, ask which object is actually pushing or pulling on the receiver, by contact or across a gap.
Try it! · Check-in question
A magnet held above a paper clip lifts the clip off the table without touching it. For the upward force on the paper clip, which are the agent and the receiver?
The magnet pulls on the paper clip across the gap, so the magnet is the agent, and the paper clip, which the force acts on, is the receiver. So the answer is D. A swaps the roles: the clip does pull back on the magnet, but that is the other force of the pair, not the upward force on the clip. B assumes that a force needs contact; magnetic forces, like gravity, act across a gap. C names the agent of a different force: the Earth's gravity pulls the clip down, not up.
The three elements of a force
Knowing the agent and the receiver tells you where a force comes from, but not what it will do. For that, think about opening a door.
Push a door at its handle, straight into its face, and it swings open easily. Now push just as hard, in the same direction, but close to the hinges: the door barely turns. The size and the direction of the push were the same; only the place where it acted changed, and the effect changed with it. Pull the handle instead of pushing it, and the door swings the other way, so the direction matters as well. Push harder and the door swings open faster, so the size matters too.
A door seen from above. F₁ and F₂ have the same size and the same direction; only their points of application differ. F₁, at the handle, swings the door open easily; F₂, near the hinge, barely turns it.
So three things decide what a force does: how big it is, which way it acts, and where on the receiver it acts. These are the three elements of a force.
The three elements of a force. A force is fully described by its magnitude (its size), its direction (which way it acts) and its point of application (the point on the receiver where it acts). Change any one of them and the effect of the force can change.
Because a force has a direction as well as a size, it is a vector, like displacement and velocity. A mass or a time is complete as a number with a unit, such as 2 kg or 5 s; a force is not complete until you also say which way it acts, such as 10 N to the right or 30 N vertically down. The unit of force is the newton (N), and 1 N = 1 kg·m/s². For a sense of size, 1 N is roughly the pull of gravity on a small apple of about 100 g.
Same size does not mean same effect. Two forces can both be 10 N and still act very differently. A 10 N push to the left and a 10 N push to the right have opposite effects, and a 10 N push at a door's handle does far more than a 10 N push near its hinges. Before you call two forces the same, check all three elements.
Worked example · the three elements (April 2026, Q7)
The three elements of a force are magnitude, direction, and ( )
A. Nature B. Point of application C. Object of action D. Effect
The three elements are what you need to know to predict what a force will do: how big it is, which way it acts and where it acts. Magnitude and direction are already given, so the missing element is where the force acts, its point of application. The answer is B.
Each of the other options is a trap worth recognising.
Nature means the kind of force: gravity, a spring's pull, friction. It does not decide the effect: a 5 N pull from a rope and a 5 N pull from a spring, acting at the same point and in the same direction, do exactly the same thing.
Object of action is the object the force acts on, the receiver. Every force has one, but naming it tells you nothing about the force's size, direction or point of application, so it is not an element.
Effect is what the force does. The three elements decide the effect, so the effect cannot be one of them.
Try it! · Check-in question
A mechanic pushes on a spanner (wrench) close to a tight nut, and the nut will not turn. Pushing just as hard, in the same direction, at the far end of the handle loosens it easily. Which element of the force made the difference?
The two pushes are equally hard and in the same direction; only the place where the hand pushes on the spanner changed. That is the point of application, so the answer is C. A reads the bigger effect as a bigger force, but the pushes are equally hard: a bigger effect does not have to mean a bigger force. B would need the pushes to point different ways, but both are in the same direction. D, the nature of the force, is the same for both pushes (a hand pushing), and nature is not one of the three elements anyway: it was one of the traps in April 2026, Q7.
Drawing a force
Since three elements describe a force, a drawing of a force has to show all three. An arrow does exactly that, with each part of it carrying one element.
1
Start at the point of application. Draw the arrow on the receiver, with its tail at the point where the force acts. (Gravity acts on the whole object, so its arrow starts at the object's centre of gravity; the next page explains why.)
2
Point it the way the force acts. The arrowhead shows the direction.
3
Let the length show the size. A bigger force gets a longer arrow. For an accurate drawing, choose a scale, such as one unit of length for 10 N.
4
Label it. Write the force's symbol beside the arrow, with its size if you know it, such as F = 20 N.
Worked example · drawing the pull of a rope
A rope tied to the middle of the right-hand side of a box pulls the box horizontally to the right with a force of 20 N. Draw this force to a scale of one unit for 10 N.
The force acts on the box, so the arrow goes on the box.
The rope acts where it is tied, at the middle of the right-hand side. That is the point of application, so the tail goes there.
A rope pulls along its own length, here to the right, so the arrow points right.
At one unit for 10 N, a 20 N force is two units long. Label it F = 20 N.
The rope's pull drawn as a force arrow. The tail sits where the rope is tied, the arrowhead points the way the rope pulls, and the arrow is two scale units long, so F = 20 N.
Try it! · Check-in question
A hand pushes horizontally on the left-hand side of a trolley, towards the right, with a force of 15 N. Which description of the force arrow is correct?
The force acts on the trolley at the point where the hand touches it, the left-hand side, and it pushes towards the right. So the tail sits on the left-hand side of the trolley and the arrowhead points right: the answer is A. B draws the arrow on the agent: the hand exerts the force, but the force acts on the trolley, so the arrow belongs on the trolley. C points the arrow the wrong way: the hand pushes the trolley away from itself, to the right. D moves the tail to the right-hand side, a different point of application; that arrow would describe a pull on the far side, not this push.
What a force does
Watch what happens whenever a force acts on an object, and you will see one of two things, sometimes both. The object's motion changes: it starts moving, stops, speeds up, slows down or turns. Or its shape changes: it stretches, squashes, bends or twists.
Turning counts as a change of motion. An object's motion means its speed and its direction taken together, which the papers call its state of motion. So an object whose speed stays the same while its direction changes is still changing its motion, and something must be acting on it to make it turn.
What a force does. A force can change an object's motion (start or stop it, speed it up or slow it down, or change its direction), and it can change an object's shape (stretch, squash, bend or twist it), or both.
Worked example · reading the effect of a force
Say what each force changes: (1) a push from a hand starts a toy car rolling; (2) a pile of heavy books makes a shelf sag; (3) a string keeps a ball moving round in a circle at a steady speed.
(1) The car goes from still to moving: the force changes its motion.
(2) The shelf bends: the force changes its shape.
(3) The ball's speed does not change, but its direction does, at every moment. A change of direction is a change of motion, so the pull of the string changes the ball's motion.
Try it! · Check-in question
Which of these shows a force changing an object's motion?
A change of motion means a change of speed or of direction. The skater's speed is steady, but her direction keeps changing as she follows the curve, so her motion is changing, and a force must be causing it: the answer is B. A is the trap of thinking that forces acting means the motion is changing: gravity and the push of the road do act on the parked car, but it stays at rest, so its motion does not change. C confuses motion with a change of motion: the cyclist is moving, but her speed and her direction both stay the same. D mixes up the two kinds of effect: the sponge's shape changes, but its motion does not.
Balanced forces
Here is a puzzle. A lamp hangs from the ceiling on a cord, perfectly still. Gravity pulls it down, so a force certainly acts on it. Why does its motion not change?
Because a second force acts as well. The cord pulls up on the lamp, exactly as hard as gravity pulls down. The two forces are equal in size, opposite in direction and along the same line, so their effects on the motion cancel. Forces like this are balanced. A tug of war between two equally strong teams shows the same thing: both teams pull hard, and the rope stays where it is.
Two forces act on the hanging lamp: the cord pulls it up with 20 N, and gravity pulls it down with 20 N. They are equal in size, opposite in direction and along the same line, so they balance, and the lamp stays at rest.
Balanced forces. Two forces on the same object that are equal in size, opposite in direction and along the same line balance each other. Balanced forces do not change the object's motion: at rest, it stays at rest; moving, it keeps moving at the same speed in the same direction.
Balanced forces can still change a shape. Squeeze a sponge between your palms with two equal pushes: it goes nowhere, but it is squashed. Balanced forces leave the motion alone; they do not have to leave the shape alone.
Try it! · Check-in question
A book lies at rest on a table. Which statement about the forces on it is correct?
Gravity acts on the book whether it moves or not, and the table pushes up on it. The book's motion does not change, so these two forces must balance: equal in size and opposite in direction. The answer is D. A confuses "at rest" with "no forces": at rest means the forces balance, not that there are none. B treats the table as something that just gets in the way, but holding the book up takes a force, an upward push from the table. C makes the forces unbalanced, and unbalanced forces would change the book's motion: it would rise off the table.
A force changes motion; it does not keep it going
Everyday experience seems to say that motion needs a force: stop pushing a box across the floor and it soon stops. Look more closely, though, at what happens as the surface gets smoother. Give a puck the same push across a carpet, a wooden floor and a sheet of ice. On the carpet it stops almost at once; on the floor it slides further; on the ice it glides a long way, hardly slowing at all.
The same push on three surfaces. The weaker the friction, the further the puck slides before friction stops it. With no friction at all, nothing would stop it.
What stops the puck each time is friction, a force from the surface that acts against its motion. The weaker the friction, the further the puck goes. Follow that pattern to its end: with no friction and no air resistance at all, nothing would slow the puck, and it would keep moving at the same speed in the same straight line.
So the push was needed to start the puck moving, which is a change of motion, and friction is what stops it, another change of motion. Keeping it moving steadily in a straight line needs no force at all. A force is the cause of a change in motion, not the cause of motion itself. You will meet this idea again as Newton's first law.
That also explains the box. While you push it across the floor at a steady speed, your push is not what keeps it moving: it is balancing friction, so the motion does not change. Stop pushing and friction is left on its own, so the box slows down and stops. That slowing is a change of motion, and a force, friction, caused it.
Force and motion. A force causes a change in motion: a start, a stop, a change of speed or a change of direction. No force is needed to keep an object moving at a steady speed in a straight line. So if an object's motion is not changing, the forces on it are balanced, or there are none.
Trap: the force that travels with the ball. After a ball leaves a footballer's foot, no "force of the kick" travels with it. The kick acted only while the foot touched the ball. In flight, the only forces on the ball are gravity and air resistance. It keeps moving simply because a moving object keeps moving unless a force changes its motion.
Try it! · Check-in question
Far out in space, away from any planet or star, a spacecraft is coasting with its engines switched off, and no forces act on it. What happens to its motion?
With no force acting, nothing can change the spacecraft's motion, so it carries on at the same speed in the same straight line. The answer is A. B assumes that motion needs a force at every moment; but stopping is itself a change of motion, and a change of motion needs a force. C imagines a push that travels with the spacecraft and fades away; the engines pushed only while they were running, and slowing down is a change of motion that would need a force. D imagines that the push is stored and keeps acting; speeding up is a change of motion too, and there is no force to cause it.
Statement questions: putting it together
The exam tests this page with statement questions: four statements about force, and you pick the correct one, or the incorrect one. A statement can sound familiar and still be false, so test each one against four facts.
1
Two objects. Every force has an agent and a receiver, even when they do not touch. One object alone cannot exert a force.
2
Three elements. A force is a vector, described by its magnitude, direction and point of application. Equal size alone does not mean equal effect.
3
Effects. A force changes an object's motion, its shape, or both. Balanced forces leave the motion unchanged.
4
Motion. A force causes a change in motion. Steady motion in a straight line needs no force.
Before you test anything, read the stem again. Does it ask for the correct statement or the incorrect one? The CSCA uses both, and picking a true statement when the stem asks for the false one is an easy mark to lose.
Worked example · finding the incorrect statement (April 2026, Q17)
Which of the following statements about force is incorrect? ( )
A. Force is the cause of a change in an object's state of motion.
B. Force is a vector, having both magnitude and direction.
C. A force can have an exerting object but no receiving object.
D. Force is an interaction between two objects.
The stem asks for the incorrect statement, so three of the four are true and one is false.
A is fact 4: a force causes a change in motion. True. Students who half-remember that "motion needs no force" sometimes reject it, but the two ideas fit together: motion itself needs no force; a change in motion does.
B is fact 2: a force has a size and a direction, so it is a vector. True.
C breaks fact 1. The exerting object is the agent and the receiving object is the receiver, and every force has both: with nothing to receive it, there would be nothing for the force to act on. False.
D is fact 1 itself. True.
So the incorrect statement is C, and C is the answer.
Exam wording. The English papers call the agent the exerting object and the receiver the receiving object; the Chinese papers say 施力物体 and 受力物体. "The three elements of a force" is 力的三要素: magnitude (大小), direction (方向) and point of application (作用点). An object's "state of motion" (运动状态) means its speed and its direction taken together, so a change in either one is a change in its state of motion.
Try it! · Check-in question
Which of the following statements about force is correct?
B is fact 2: the three elements decide what a force does, so the answer is B. A breaks fact 1: it assumes every force needs contact, but gravity and magnetic forces act across a gap. C breaks fact 4: it is the idea that motion needs a force, but a puck gliding across ice keeps going with no force pushing it along. D confuses rest with no forces: a book at rest on a table has gravity and the table's push acting on it, and they balance.
Your turn! · Practice questions
Practice · Question 1
A swimmer pushes the water backwards with her hands and feet, and she moves forwards. What is the agent of the force that pushes the swimmer forwards?
Her hands push the water backwards, and the water pushes back on her, forwards. That forward push acts on the swimmer and is exerted by the water, so the agent is the water: the answer is C. A names the agent of the other force of the pair: the hands are the agent of the backward push on the water, not of the forward push on the swimmer. B names the agent of gravity, which pulls her down, not forwards. D breaks the two-object rule: a swimmer cannot push herself along, so the forward push must come from another object.
Practice · Question 2
A footballer kicks a ball. Which change to the kick would leave its effect on the ball exactly the same?
What a force does depends on its three elements: magnitude, direction and point of application. Kicking with the other foot changes the agent, but none of the three elements, so the effect stays the same: the answer is A. B changes the magnitude, so the ball flies off faster; it treats the size of a force as if it did not matter. C changes the point of application, so the ball spins and curves; it is the element most often forgotten, as with a push near a door's hinges. D changes the direction, so the ball goes a different way; it assumes that equal size alone means equal effect.
Practice · Question 3
A goalkeeper catches a fast ball. As it hits her gloves the ball squashes for a moment, and then it stops. What does the force from the gloves do to the ball?
The ball goes from moving fast to still, which is a change of motion, and it is squashed, which is a change of shape. So the force from the gloves does both, and the answer is D. A overlooks the squashing: a change of shape is an effect too. B overlooks the stopping: slowing to rest is a change of motion. C forgets that a force is an interaction: the ball pushes on the gloves, and the gloves push back on the ball, and that push is what stops it.
Practice · Question 4
A car travels along a straight, level road at a steady 20 m/s. Which statement about the forces on it is correct?
The car's speed and direction both stay the same, so its motion is not changing, and that means the forces on it balance: the force driving it forward equals the forces holding it back. The answer is C. A is the "motion needs a force" idea: a forward force bigger than the forces holding the car back would speed it up, not keep it steady. B confuses steady motion with no forces: gravity, the push of the road and air resistance all act on the car; they balance. D forgets that balanced forces leave the motion unchanged: forces acting on an object do not mean its motion is changing.
Practice · Question 5
Which of the following statements is incorrect?
The hand pushes the ball only while they are in contact. Once the ball leaves the hand, the only forces on it are gravity and air resistance, and both slow its rise; it keeps rising for a while because a moving object keeps moving unless a force changes its motion. So B is the incorrect statement, and the answer is B. A is true: gravity acts across a gap; it only looks false if you think every force needs contact. C is true: balanced pushes change the clay's shape but not its motion; it only looks false if you think every force must change the motion. D is true: it is this page's rule for drawing a force.
Things to remember
Concept
What to know
What a force is
An interaction: one object, the agent (exerting object), acts on another, the receiver (receiving object). Every force has both; one object alone cannot exert a force.
Contact or not
Pushes, the pull of a rope and friction need contact; gravity and magnetic forces act across a gap. Either way, two objects are involved.
Forces come in pairs
When one object pushes or pulls another, the second pushes or pulls back on the first. For that second force, the agent and the receiver swap roles.
Three elements
Magnitude, direction and point of application decide what a force does. "Nature", "object of action" and "effect" are not elements.
Vector and unit
A force has a size and a direction, so it is a vector. It is measured in newtons: 1 N = 1 kg·m/s².
Drawing a force
An arrow on the receiver: tail at the point of application, head showing the direction, length showing the size, and a label such as F = 20 N.
Effects of a force
A force changes an object's motion (its speed or its direction), its shape, or both. Turning at a steady speed is still a change of motion.
Balanced forces
Equal in size, opposite in direction, along one line: the motion does not change, though the shape still can.
Force and motion
A force causes a change in motion; steady motion in a straight line needs no force. No change in motion means the forces balance, or there are none.
Statement questions
First check whether the stem asks for the correct or the incorrect statement, then test each option against the four facts.
