# Force and Laws of Motion For Class 9

The Force and Laws of Motion

What is Force?

It is an external agent capable of changing the state of rest or motion of a particular body. A push or pull on a body is known as force. In other words, force may be defined as an agency (a push or pull) that changes or tends to change the state of rest or motion or the direction of a body.

Note: (i) Force has a magnitude and a direction. It is a vector quantity.

(ii) The SI unit of force is Newton (N).

(iii) The SI base unit of force is kg.m/s 2

(iv) Other units of force: Dyne, pound-force, kilo-pond.

1 Newton = 105  dyne

(v) Formula of force, F = m x a

(vi) Dimensional formula of force = M L T-2

Contents

### Effect of force:

1. A force can move a stationary body.
2. A force can stop a moving body.
3. Force can change the speed of an object.
4. Force can change the direction of motion of an object.
5. A force can change the shape & size of an object.

### Types of Force

#### Balanced force

When two forces are equal in magnitude but opposite in direction acting on the same body, we get the net forces acting on a body equal to zero. Balanced forces do not change the state of rest or of uniform motion of an object.

#### Unbalanced force

When two forces are unequal in magnitude but in opposite directions acting on the same body, we get the net forces acting on a body unequal and non-zero. Unbalanced forces do bring change the state of rest or of uniform motion of an object.

## Newton’s laws of motion

Newton has given three laws to describe the motion of the objects. These three laws are called Newton’s laws of motion.

### Newton’s first law of motion

In other words, an object at rest remains at rest, or if in motion, remains in motion at a constant velocity that tends to retain its inertia until and unless acted on by a net external force. Newton’s first law of motion is also comes to be considered as the law of inertia.

(i) Newton’s first law is also called the law of inertia.

(ii) Newton’s first law explains the definition of force.

#### Inertia

The inherent property of a material body by virtue of which it cannot change, by itself, its state of rest or of uniform motion in a straight line is called inertia.

• Mass is a measure of the inertia of a body.
• Mass is directly proportional to inertia.
• Heavier objects have more inertia than lighter objects.
• To overcome the inertia and make a body move from rest, we apply an external force.

Example: 1. When a hanging carpet is beaten with a stick, the dust particles start coming out of it.

1. When a tree is shaken vigorously, its leaves and fruits fall down.

#### Momentum

The momentum of a particle is the product of two quantities (mass and velocity).

Momentum = mass x velocity

p = m x v

• The SI unit of momentum is kg.m/s
• Momentum is a vector quantity.
• Every moving body possesses momentum.
• If a body is at rest, hence its momentum is also zero.
• Example: A karate player can break a pile of tiles or slab of ice with a single blow of his hand because a karate player strikes the piles of tiles with his hand very, very fast. Since, the large momentum of the fast-moving hand is reduced to zero in a very, very short time. This exerts a very large force on the pile of tiles or the slab which is sufficient to break them apart.

### Newton’s second law of motion

The rate of change of linear momentum of a body is directly proportional to the applied force and the change takes place in the direction of the applied force. Newton’s second law gives us a relationship between the force applied to a body and the acceleration produced in the body.

(i) Newton’s second law is also called universal law/ Real law.

(ii) Newton’s second law explains the formula of force.

### Newton’s third law of motion

Newton’s 3rd law state that to every action there is an equal and opposite reaction.

(i) Newton’s third law is also called the action-reaction forces.

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