Chapter 3 of ICSE Class 9 Physics explores forces — what they are, how they change motion, and the universal rules (Newton's three laws) that govern every push, pull, collision, and orbit in the universe. Explore each idea below through animated, interactive simulations.
A force is a push or pull on an object. It can change the state of rest or motion of a body, and it can change the shape or size of a body. Forces are classified by how they act on a body — through direct touch (contact forces) or from a distance, without touching (non-contact forces).
(1) It can change the state of rest or motion of a body — i.e. it can produce or change motion. (2) It can change the shape or size of a body — e.g. squeezing a sponge, stretching a spring, hammering a piece of metal.
Produced only when two bodies are physically touching each other.
Opposes the relative motion (or tendency of motion) between two surfaces in contact.
The force exerted by a surface on a body resting on it, acting perpendicular (normal) to the surface.
The force developed in a stretched string, rope, or cable — equal and opposite at both ends, directed inward (toward the body).
A stretched or compressed spring exerts a restoring force trying to bring it back to its natural length.
Act on a body even when the source of the force is not touching it — they act "at a distance."
Every particle of matter attracts every other particle in the universe. This is why objects fall toward the Earth.
The force between two charged bodies. Like charges repel; unlike charges attract — e.g. a charged comb attracting bits of paper.
The force exerted by a magnet on magnetic materials (or other magnets) — like poles repel, unlike poles attract.
Click a scenario card to see whether the force involved is a contact or non-contact force, and watch the force arrows animate.
When a spring is stretched, it pulls its ends inward (toward its natural length); when compressed, it pushes its ends outward. In both cases the spring exerts an equal and opposite force on whatever is pulling/pushing it. This is called the restoring force and underlies oscillatory motion.
A body continues to remain in its state of rest, or of uniform motion in a straight line, unless an external force is applied on it to change that state.
This law is also called Galileo's law of inertia. It tells us two things: (i) if a body is at rest, it remains at rest unless a force acts on it, and (ii) if a body is moving, it continues to move with the same speed and direction unless a force acts on it.
Inertia is the inherent property of every body to resist a change in its state of rest or of uniform motion. The greater the mass of a body, the greater is its inertia — that's why mass is a measure of inertia.
| Type | Meaning | Everyday Example |
|---|---|---|
| Inertia of Rest | Tendency of a body to continue in its state of rest | A coin on a card flicked off a tumbler falls straight into the glass (the coin "stays behind") |
| Inertia of Motion | Tendency of a moving body to continue moving | A passenger leans forward when a moving bus suddenly stops |
| Inertia of Direction | Tendency of a body to continue moving in the same direction | Mud flies off a spinning bicycle wheel tangentially |
A coin rests on a card placed on a tumbler. Flick the card sharply — the coin's inertia of rest keeps it (almost) in place, and it drops straight into the tumbler.
A passenger standing in a moving bus — when the bus brakes suddenly, the passenger's body (due to inertia of motion) continues moving forward and tends to fall forward.
Shaking a tree: when you shake the branches, the leaves/fruits (which were at rest) tend to remain at rest due to inertia, while the branch moves — so they get separated and fall. A ball rolling on a smooth floor travels much farther than on a rough floor, because friction (an external force) is smaller — this shows that an object continues in motion unless something stops it.
It is common experience that a more massive object, or a faster-moving object, is harder to stop. This combined effect of mass and velocity is captured by the quantity called momentum.
Adjust the mass and velocity of two trolleys and watch how their momenta compare. Heavier or faster objects carry more momentum and are harder to stop.
| Kind | Statement |
|---|---|
| Inertia of Rest | A body at rest will remain at rest unless an external force acts on it. |
| Inertia of Motion | A body moving with a constant velocity will remain moving with the same velocity unless an external force acts on it. |
| Inertia of Direction | A body moving in a particular direction tends to continue moving in that same direction unless an external force changes it. |
Δp = m·Δv = m(v − u). When a force F acts on a body of mass m for time t, changing its velocity from u to v:
Newton's Second Law states that the rate of change of momentum of a body is directly proportional to the applied external force, and takes place in the direction of the force.
1 newton is the force which produces an acceleration of 1 m s⁻² in a body of mass 1 kg.
1 dyne is the force which produces an acceleration of 1 cm s⁻² in a body of mass 1 g.
Since 1 kg = 1000 g and 1 m s⁻² = 100 cm s⁻²:
1 N = 1000 g × 100 cm s⁻² = 10⁵ g·cm·s⁻² = 10⁵ dyne
Push the block with different forces and masses. Watch the acceleration animate in real time, and see the graphs of a vs F (mass constant) and a vs m (force constant).
A force of 16 N acts on a body of mass 0·8 kg, initially at rest. Acceleration a = F/m = 16 ÷ 0·8 = 20 m s⁻². Using v = u + at, after 1 s the velocity becomes v = 0 + 20 × 1 = 20 m s⁻¹.
Newton's Second Law is F = ma. If F = 0, then a = 0, meaning the body's velocity does not change — it remains at rest or moves with uniform velocity. This is exactly Newton's First Law, so the first law can be derived from the second.
To every action there is an equal and opposite reaction. Action and reaction act on two different bodies, are equal in magnitude, opposite in direction, and act simultaneously.
Choose a real-life situation and watch the equal-and-opposite force pair animate, with both forces shown as labelled arrows acting on different bodies.
The book pushes down on the table with its weight W (action). The table pushes up on the book with an equal normal reaction R (reaction). Since R = W, the book stays in equilibrium — at rest.
When you walk, your foot pushes the ground backward (action). The ground pushes your foot forward with an equal and opposite reaction — this reaction propels you forward.
A swimmer pushes water backward with their hands and feet (action). The water pushes the swimmer forward with an equal and opposite reaction — propelling them through the water.
The gun exerts a forward force on the bullet (action). The bullet exerts an equal and opposite backward force (reaction) on the gun, causing it to recoil.
Hot gases are expelled downward/backward from the rocket's nozzle at high speed (action). The gases exert an equal and opposite force on the rocket, pushing it upward/forward (reaction) — this is how rockets move in the vacuum of space.
When stepping off a boat onto the shore, you push the boat backward with your foot (action). The boat pushes you forward toward the shore (reaction) — and the boat drifts away from the shore.
Action and reaction never cancel each other out, even though they are equal and opposite — because they act on two different bodies. Forces cancel only when they act on the same body in opposite directions.
Every particle of matter in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The force acts along the line joining the two particles, and is always attractive.
When a body falls freely under gravity alone (no air resistance), it experiences a constant acceleration called g, directed toward the centre of the Earth.
Here M = mass of Earth, R = radius of Earth. The value of g decreases with altitude and depth, and varies slightly with latitude (slightly less at the equator than at the poles).
Drop the same ball on Earth, the Moon, and Jupiter to see how different values of g affect the rate of falling.
When a body falls freely under gravity (ignoring air resistance), its motion follows the same equations as uniformly accelerated motion, with acceleration a replaced by g.
If the body starts from rest, u = 0.
At the highest point, v = 0. Maximum height: hmax = u²/2g. Time to reach max height: t = u/g. Time of flight (up + down) = 2u/g.
Choose to drop the ball or throw it upward with an initial velocity, and watch its height-vs-time and velocity-vs-time graphs build live.
A stone is dropped from a height of 78·4 m (g = 9·8 m s⁻²). Using h = ½gt² with u = 0: 78·4 = ½ × 9·8 × t² → t² = 16 → t = 4 s. Final velocity: v = gt = 9·8 × 4 = 39·2 m s⁻¹.
Test what you've learned. Click an option — correct answers turn green, wrong answers turn red.