# Force and laws of motion class 9 notes

Topic covered : Force, Effect of force, Contact force, Non contact force, Newton’s laws of motion, Galileo’s observation, and much more….. Force and laws of motion class 9 notes

## FORCE

A force is defined as any influence that can produce a change in the velocity of an object. It is a vector quantity.

• Force is an external effort in the form of pushing, pulling, stretching, compressing etc., which may move a body at rest or stop a moving body or change the speed of a body or change the direction of a moving body or change the size and shape of a body.

Effects of force

• A force can make a stationary object move
• A force can make a moving object stop
• A force can change the speed of a moving object
• A force can change the direction of a moving object
• A force can change the shape of an object
• A force can change the size of an object

Forces can be classified into two categories

## Contact Forces

1. Applied force: Force which is applied to the object by another object.  A person pushing his car is an example of applied force. When the person pushes the car then there is applied force acting upon the car.
2. Frictional force:  Friction force is the result of two surfaces being pressed together closely.  The frictional forces depend upon the nature of the surfaces. The more rough the surface the more will be the frictional force.
3. Air Resistance Force:  The air resistance force acts upon objects as they travel through the air. This force opposes the motion of the object in the air.  Example: A skydiver diving in the sky.
4. Spring force: The spring force is the force exerted by a compressed or stretched spring.  Here the magnitude of the spring force is directly proportional to the amount of stretch or compression of the spring.
5. Tension Force:  The tension force is transmitted through a string, rope or cable. The tension force is directed along the length of the wire.  This force pulls equally, the objects that are placed at opposite ends of the wire.
6. Normal force:  This is also called the support force. If an object is resting upon a table then the table is exerting an upward force upon the object in order to support the weight of the object.  If a person leans against a wall, the wall pushes horizontally on the person.

## Non-contact Force

1. Gravitational force: Gravity is a non-contact force between of two objects. Such as:  An object thrown up is pulled back to the earth by the gravitational force of the earth
2. Electric forces:  Electric charges are able to jump between objects that are within a close enough range but do not have to actually be in physical contact.
3. Magnetic forces:  Magnetic forces act over a distance. Objects need not be in physical contact with each other for magnetic field to pass through them.

## Types of forces acting on a body

### Balanced forces

If the resultant of all the forces acting on a body is zero, the force is called balanced forces. Example: When we hold a suitcase steady at some height, the suitcase is under the action of two forces.

1. Weight mg of the suitcase due to gravitational pull of earth, acting vertically downwards.
2. Force applied P by our hand in the vertically upward direction. As mg = P, therefore, the forces are balanced, and the suitcase is steady.

### Unbalanced forces

If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces. Example: A ball set rolling on the ground stops after sometime. This is due to unbalanced force of friction between the ball and the ground.

Description of Balanced and Unbalanced Forces

Notice when the diver jumps down on a diving board. The board springs back and forces the diver into the air. The action force exerted on the board by the diver causes a reaction force by the board equal and opposite to the force exerted by the diving board. These interacting forces are unbalanced and differ from the balanced forces. No motion occurs when the forces are balanced. If the diver stands quietly on the diving board, all the forces on the diver and the board are balanced. Neither the diver not the board moves.

## GALILEO’S OBSERVATIONS

Galileo arrived at some important conclusion about motion of objects by observing their motion on an inclined plane.

Galileo observed that when a small marble ball rolls down an ideal, frictionless plane inclined on both sides.

Galileo concluded that an unbalanced external force is required to initiate the motion from state of rest, but no unbalanced net force is needed to sustain the uniform motion, objects move with a constant speed along a straight line when no force acts on them.

## Newton’s laws of motion

Sir Isaac Newton was one of the greatest scientists. He was born in England. He was born in the same year that Galileo died. Newton studied in Trinity College, Cambridge. Newton worked his way in college. His work laid the foundations for modern science. While Newton was in college, he had new ideas about motion and state’s three laws of motion.

Newton’s laws of motion are used to define the concept of the force and for describing the motion and also used as the basis of classical mechanics.

## First law

According to this law, a body continues to be in its state of rest or of uniform motion along a straight line, unless it is acted upon by some external force to change its state.

The law consists of three parts:

1. A body at rest continues to remain at rest until an external force is applied on the body to move it.
2. A body in uniform motion continues to move uniformly unless an external force is applied to change its speed.
3. A body moving along a straight line will continue to move along the same straight line unless an external force is applied on the body to change its direction of motion.

NEWTON’S FIRST LAW DEFINE FORCE

According to Newton’s first law of motion, a body continues to be in a state of rest or in a state of uniform motion along a straight line; unless an external force is applied on the body to change the state. This means that force applied on a body alone can change its state of rest or state of uniform motion along a straight line; we may define force as an external effort in the form of a push or pull which

• Newton’s first law of motion is also called as Galileo’s law of inertia. This law states that everybody has some inertia.
• Inertia is that property of body due to which it tries to remain in its initial state until the force is applied. Greater the inertia of body, greater will be the force required to change its state. Heavier object has more inertia than lighter objects. Such as: cricket ball has more inertia than rubber ball of same size. Thus, Newton’s first law of motion defines inertia and is rightly called the law of Inertia.