Newton's Laws of Motion
NCERT Class 9 • Science • Chapter "Force and Laws of Motion"
Newton's Laws of Motion
A force is a push or a pull that can change the state of motion, the speed, the direction or the shape of an object. The relationship between force and motion is captured in three laws, and everything from a seat belt to a rocket can be explained from them.
The First and Second Laws
- The first law states that an object stays at rest or in uniform motion in a straight line unless an external unbalanced force acts on it.
- The tendency to resist a change in the state of motion is called inertia, so the first law is also the law of inertia.
- Inertia depends on mass: the greater the mass, the greater the inertia.
- Inertia of rest explains why passengers fall backwards when a bus starts, and inertia of motion why they fall forwards when it stops.
- Momentum is the product of mass and velocity and is a vector quantity.
- The second law states that the rate of change of momentum is proportional to the applied force and happens in the direction of the force.
- From it comes F = ma, and the SI unit of force is the newton.
The Third Law and Conservation
- The third law states that to every action there is an equal and opposite reaction.
- Action and reaction act on two different bodies, which is why they never cancel each other.
- A gun recoils when fired, a swimmer moves forward by pushing water back, and a rocket rises by pushing gases downward.
- The law of conservation of momentum states that in the absence of an external force, the total momentum of a system stays constant.
- It follows directly from the second and third laws taken together.
- A cricketer draws the hands back while catching a ball to increase the time of impact, which reduces the force felt.
| Law | Statement in Short | Everyday Example |
|---|---|---|
| First law | No force, no change in motion | Passenger jerks when bus starts |
| Second law | Force equals rate of change of momentum | Harder push gives more acceleration |
| Third law | Action equals opposite reaction | Recoil of a gun |
Inertia — the tendency of a body to resist any change in its state of rest or of uniform motion.
Exam me kaise aata hai
- The first law of motion is also called the law of — inertia
- Inertia depends on which quantity — mass
- The SI unit of force is the — newton
- Recoil of a gun is explained by the — third law
UPSC/State PSC ke liye
- Action and reaction do not cancel because they act on different bodies; cancellation would require both to act on the same body.
- Conservation of momentum is more fundamental than F = ma, since it holds even where mass changes, as in a rocket burning fuel.
Yahan confuse hote hain
✗ Action and reaction cancel each other | They act on different bodies, so they do not cancel | ✓
✗ Inertia depends on velocity | Inertia depends on mass | ✓
✗ A moving body needs continuous force to keep moving | Without friction it keeps moving on its own | ✓
Ek nazar me
- First law: no unbalanced force means no change in state of motion.
- Inertia depends on mass; momentum is mass times velocity.
- Second law gives F = ma with force measured in newtons.
- Third law pairs act on different bodies; momentum is conserved without external force.