A Force Always Needs Two Objects, Never One
Learn what a force is and which direction it acts, predict how it changes speed, direction or shape, see why every force needs two interacting objects, and sort forces into contact and non-contact kinds.
Can a single object exert a force by itself?
Never. A force is always the result of an interaction between two objects — one pushing or pulling, the other being pushed or pulled. Name a force and you can always name both.
That requirement is stricter than it sounds, and it is the idea this chapter is built on. This page covers everything in the CBSE Class 8 Science chapter's first part: what a force is, its effects, force as an interaction, and contact and non-contact forces.
That requirement is stricter than it sounds, and it is the idea this chapter is built on. This page covers everything in the CBSE Class 8 Science chapter's first part: what a force is, its effects, force as an interaction, and contact and non-contact forces.
What is a force and which way does it act?
A force is a push or a pull acting on an object. It has both a size and a direction, and its unit is the newton (N).
The direction matters as much as the size:
- Pushing a door away from you — force acts away
- Pulling a drawer towards you — force acts towards
- A bucket hanging from a rope — the rope pulls upward, gravity pulls downward
- Kicking a football — force acts in the direction of the kick
When two forces act in the same direction they add. Two people pushing a stalled car both push forward, so the total is their sum. When they act in opposite directions, the net force is the difference, in the direction of the larger one.
If the two are equal and opposite, they cancel. In a tug of war where neither team moves, both are pulling hard yet the net force is zero.
That balanced case is the one students misread. A stationary object is not necessarily free of forces — a book on a table has gravity pulling it down and the table pushing it up, equally. Nothing moves because the forces balance, not because there are none.
The direction matters as much as the size:
- Pushing a door away from you — force acts away
- Pulling a drawer towards you — force acts towards
- A bucket hanging from a rope — the rope pulls upward, gravity pulls downward
- Kicking a football — force acts in the direction of the kick
When two forces act in the same direction they add. Two people pushing a stalled car both push forward, so the total is their sum. When they act in opposite directions, the net force is the difference, in the direction of the larger one.
If the two are equal and opposite, they cancel. In a tug of war where neither team moves, both are pulling hard yet the net force is zero.
That balanced case is the one students misread. A stationary object is not necessarily free of forces — a book on a table has gravity pulling it down and the table pushing it up, equally. Nothing moves because the forces balance, not because there are none.
What can a force actually do to an object?
A force can change an object's speed, its direction of motion, or its shape — and often more than one at once.
Change of speed:
- A cyclist pedalling harder speeds up
- Applying the brakes slows the cycle down
- A football at rest starts moving when kicked
- A rolling ball on grass gradually stops, because friction acts against it
Change of direction:
- A batsman's stroke sends the ball back the way it came
- Turning the handlebar changes a cycle's direction without changing its speed
- A stone whirled on a string keeps changing direction to move in a circle
Change of shape:
- Pressing dough or clay flattens it
- Stretching a rubber band makes it longer
- Squeezing a sponge compresses it
- A car body dents in a collision
Making chapatis shows two effects together — the rolling pin changes the dough's shape, while the pin itself is being pushed along.
An object at rest needs a force to start moving, and a moving object needs a force to stop, speed up or turn. The point to be careful about is that a change of direction counts as a change of motion even when the speed stays the same — which is why a cycle going round a bend at a steady pace still has a force acting on it.
Change of speed:
- A cyclist pedalling harder speeds up
- Applying the brakes slows the cycle down
- A football at rest starts moving when kicked
- A rolling ball on grass gradually stops, because friction acts against it
Change of direction:
- A batsman's stroke sends the ball back the way it came
- Turning the handlebar changes a cycle's direction without changing its speed
- A stone whirled on a string keeps changing direction to move in a circle
Change of shape:
- Pressing dough or clay flattens it
- Stretching a rubber band makes it longer
- Squeezing a sponge compresses it
- A car body dents in a collision
Making chapatis shows two effects together — the rolling pin changes the dough's shape, while the pin itself is being pushed along.
An object at rest needs a force to start moving, and a moving object needs a force to stop, speed up or turn. The point to be careful about is that a change of direction counts as a change of motion even when the speed stays the same — which is why a cycle going round a bend at a steady pace still has a force acting on it.
Why does every force involve two objects?
Because a force is an interaction, and an interaction needs two participants. There is no such thing as a lone push.
Identify both in each case:
- A boy kicks a football — the boy's foot and the ball
- A book rests on a table — the book and the table
- A magnet attracts a pin — the magnet and the pin
- An apple falls from a tree — the apple and the Earth
- A bullock pulls a cart — the bullock and the cart
- A boat is rowed forward — the oar and the water
In every case the force can be described from either side: the foot pushes the ball, and the ball pushes back on the foot — which is why a hard kick can hurt.
Swimming makes it vivid. A swimmer pushes the water backwards and the water pushes the swimmer forwards. Neither could happen alone, and a swimmer in an empty pool goes nowhere.
This is what makes "the ball has force" a wrong statement, and it is worth saying precisely. A moving ball does not carry force; a force acted on it and may act on it again. Force describes what happens between two objects during an interaction, not a substance stored inside one of them.
Identify both in each case:
- A boy kicks a football — the boy's foot and the ball
- A book rests on a table — the book and the table
- A magnet attracts a pin — the magnet and the pin
- An apple falls from a tree — the apple and the Earth
- A bullock pulls a cart — the bullock and the cart
- A boat is rowed forward — the oar and the water
In every case the force can be described from either side: the foot pushes the ball, and the ball pushes back on the foot — which is why a hard kick can hurt.
Swimming makes it vivid. A swimmer pushes the water backwards and the water pushes the swimmer forwards. Neither could happen alone, and a swimmer in an empty pool goes nowhere.
This is what makes "the ball has force" a wrong statement, and it is worth saying precisely. A moving ball does not carry force; a force acted on it and may act on it again. Force describes what happens between two objects during an interaction, not a substance stored inside one of them.
What is the difference between contact and non-contact forces?
A contact force acts only when the two objects touch. A non-contact force acts even when they are apart, across a distance.
Contact forces:
- Muscular force — the force exerted by our muscles, used in lifting a bucket, pushing a door, chewing food or a bullock pulling a plough. Inside the body, muscles move the heart and the food along the intestine.
- Friction — the force between two surfaces in contact that opposes motion. It stops a rolling ball, lets us walk without slipping, and wears out shoe soles and brake pads. It always acts opposite to the direction of motion.
Non-contact forces:
- Gravitational force — the Earth pulls every object towards itself, which is why a dropped ball falls and why we stay on the ground. Every object attracts every other, but the Earth's pull is the one we notice.
- Magnetic force — a magnet attracts iron, cobalt and nickel without touching them, and two magnets attract or repel depending on their poles.
- Electrostatic force — a charged object attracts light objects. A plastic comb rubbed on dry hair picks up small bits of paper from a distance.
A magnet lifting a pin before they touch, and a comb bending a thin stream of water from a tap, are the everyday demonstrations.
The test to apply is whether the force would still act with a gap between the objects. Friction and muscular force need contact and stop the instant it is broken; gravity, magnetism and electrostatic force reach across empty space — which is exactly why the Moon stays in orbit with nothing between it and the Earth.
Contact forces:
- Muscular force — the force exerted by our muscles, used in lifting a bucket, pushing a door, chewing food or a bullock pulling a plough. Inside the body, muscles move the heart and the food along the intestine.
- Friction — the force between two surfaces in contact that opposes motion. It stops a rolling ball, lets us walk without slipping, and wears out shoe soles and brake pads. It always acts opposite to the direction of motion.
Non-contact forces:
- Gravitational force — the Earth pulls every object towards itself, which is why a dropped ball falls and why we stay on the ground. Every object attracts every other, but the Earth's pull is the one we notice.
- Magnetic force — a magnet attracts iron, cobalt and nickel without touching them, and two magnets attract or repel depending on their poles.
- Electrostatic force — a charged object attracts light objects. A plastic comb rubbed on dry hair picks up small bits of paper from a distance.
A magnet lifting a pin before they touch, and a comb bending a thin stream of water from a tap, are the everyday demonstrations.
The test to apply is whether the force would still act with a gap between the objects. Friction and muscular force need contact and stop the instant it is broken; gravity, magnetism and electrostatic force reach across empty space — which is exactly why the Moon stays in orbit with nothing between it and the Earth.
Exam tip
Exam tip: naming both objects in every interaction
Force questions are marked on precision, and three habits secure the marks.
When asked about a force, name both interacting objects — the bullock and the cart, the apple and the Earth. An answer naming only one is incomplete.
Give the direction with the force, and the unit as the newton (N).
For classification, state the test: magnetic force is non-contact because it acts without the magnet touching the pin. The reason is usually the mark, not the label.
When describing an effect, say which effect it is — change of speed, of direction, or of shape — and remember that a change of direction counts even at a constant speed.
And never write that a moving object "has force". A force acts on an object; it is not stored inside one.
When asked about a force, name both interacting objects — the bullock and the cart, the apple and the Earth. An answer naming only one is incomplete.
Give the direction with the force, and the unit as the newton (N).
For classification, state the test: magnetic force is non-contact because it acts without the magnet touching the pin. The reason is usually the mark, not the label.
When describing an effect, say which effect it is — change of speed, of direction, or of shape — and remember that a change of direction counts even at a constant speed.
And never write that a moving object "has force". A force acts on an object; it is not stored inside one.
Did you know
Why does a swimmer move forward by pushing water backwards?
Because a force always works in both directions between two objects.
When a swimmer's hands sweep the water backwards, they exert a force on the water. The water exerts an equal force back on the swimmer — forwards — and that is what moves them through the pool.
The same pairing explains a rowing boat, where the oars push water back, and walking, where your foot pushes the ground backwards while the ground pushes you forwards. It is also why walking on a slippery floor fails: without friction the ground cannot push back, so there is nothing to move you.
When a swimmer's hands sweep the water backwards, they exert a force on the water. The water exerts an equal force back on the swimmer — forwards — and that is what moves them through the pool.
The same pairing explains a rowing boat, where the oars push water back, and walking, where your foot pushes the ground backwards while the ground pushes you forwards. It is also why walking on a slippery floor fails: without friction the ground cannot push back, so there is nothing to move you.
Key takeaways
Forces: quick revision
- A force is a push or a pull with both size and direction, measured in newtons (N).
- Forces in the same direction add; opposite forces give a net force equal to their difference, and equal opposite forces balance to zero — so a stationary object may still have forces acting on it.
- A force can change an object's speed, its direction of motion or its shape — and a change of direction counts even at constant speed.
- Every force is an interaction between two objects, so both can always be named; force is never stored inside a moving object.
- Contact forces need touching: muscular force and friction, which always opposes motion.
- Non-contact forces act across a gap: gravitational, magnetic and electrostatic — the test is whether the force would still act with a space between the objects.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Forces in the same direction add; opposite forces give a net force equal to their difference, and equal opposite forces balance to zero — so a stationary object may still have forces acting on it.
- A force can change an object's speed, its direction of motion or its shape — and a change of direction counts even at constant speed.
- Every force is an interaction between two objects, so both can always be named; force is never stored inside a moving object.
- Contact forces need touching: muscular force and friction, which always opposes motion.
- Non-contact forces act across a gap: gravitational, magnetic and electrostatic — the test is whether the force would still act with a space between the objects.
You will remember all of this far better after answering five questions on it than after reading it twice.