Force and Newton's Laws of Motion
Force and Newton's Laws of Motion
Force is the most heavily examined topic in the physics block, worth 2 to 3 questions. Newton's three laws are asked by number, by statement and by everyday example, so all three forms must be ready. The examples matter as much as the laws themselves.
What a Force Does
A force is a push or a pull. It can change the state of rest or motion, the speed, the direction or the shape of a body. Any of those four counts as an effect, and a question listing them is common.
| Type | Needs contact | Examples |
|---|---|---|
| Contact force | Yes | Muscular force, friction |
| Non-contact force | No | Gravitational, magnetic, electrostatic |
The SI unit of force is the newton. One newton is the force that gives a mass of one kilogram an acceleration of one metre per second squared.
The Three Laws
- First law — a body stays at rest or in uniform motion unless an unbalanced external force acts on it. This is the law of inertia.
- Second law — the rate of change of momentum is proportional to the applied force, giving F = ma.
- Third law — for every action there is an equal and opposite reaction.
F = m × a | Momentum p = m × v
Note the qualifier in the first law: the force must be unbalanced. Balanced forces do not change motion at all, which is why a book on a table stays put despite gravity acting on it.
Inertia and Everyday Examples
Inertia is the resistance of a body to a change in its state, and it depends on mass alone. A heavier body has greater inertia.
| Situation | Law involved |
|---|---|
| Passenger lurches forward when a bus brakes | First law |
| Dust falls out when a carpet is beaten | First law |
| A cricketer pulls his hands back while catching | Second law |
| Recoil of a gun when fired | Third law |
| A swimmer pushes water backward to move forward | Third law |
| A rocket rising as gases are expelled downward | Third law |
The cricketer example is the one most often misassigned. Pulling the hands back increases the time over which the ball's momentum changes, which reduces the force — that is the second law, not the first.
✗ A cricketer pulling his hands back while catching illustrates the first law | ✓ It illustrates the second law — more time means less force for the same momentum change
Action and Reaction
Action and reaction are equal in magnitude and opposite in direction, but they act on different bodies, which is why they never cancel each other out. A student who says the two forces cancel has misunderstood the law, and this is asked as a reasoning question.
The third law also explains why a gun recoils, why a balloon flies when released, and why walking is possible at all — you push the ground backward and it pushes you forward.
क्रिया और प्रतिक्रिया अलग-अलग वस्तुओं पर लगती हैं, इसीलिए वे रद्द नहीं होतीं।
TRE pointer: Papers set this topic by giving an everyday situation and asking which law explains it, which is why the example table above is worth more than the statements alone. The action and reaction act on different bodies point is the single most repeated reasoning question, because a teacher is expected to be able to correct that misconception in a class 8 student. With −1/3 negative marking, read whether the question asks for the law or for the effect.
60-Second Recap
- A force can change the state, speed, direction or shape of a body.
- Contact forces need touching; gravitational, magnetic and electrostatic do not.
- First law is inertia and needs an unbalanced force; balanced forces change nothing.
- Second law gives F = ma; a cricketer pulling his hands back illustrates it.
- Third law: action and reaction act on different bodies, so they never cancel.
- Inertia depends only on mass; the SI unit of force is the newton.
Frequently Asked Questions
Why do action and reaction not cancel each other?
Because they act on two different bodies. When you push a wall, your force acts on the wall and the wall's force acts on you. Forces only cancel when they act on the same body, which is the case with balanced forces, not with an action-reaction pair.
Which law explains a cricketer pulling his hands back while catching?
The second law. Drawing the hands back stretches the time over which the ball's momentum falls to zero, and since force equals rate of change of momentum, a longer time means a smaller force on the hands. It is commonly but wrongly attributed to inertia.
What does inertia depend on?
On mass alone. A heavier body resists a change in its state of rest or motion more strongly than a lighter one. Inertia does not depend on speed, shape or the material, and questions sometimes offer those as options to test the definition.
What is the difference between balanced and unbalanced forces?
Balanced forces are equal and opposite on the same body, so they produce no change in its motion, as with a book resting on a table. Unbalanced forces do not cancel and therefore cause acceleration. The first law applies only when the forces are unbalanced.