Class 9 Science

# Forces and Laws of Motion

• Effects of force
• Types of force
• Newton's first law of motion
• Mass and inertia

## Force

A push or a pull is called force.

This means whenever we push or pull an object, we apply force on that object. When you try to open the door, you need to either push it or pull it. When you need to open the drawer, you need to pull it. When you need to close the drawer, you need to push it.

### Effects of Force

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body. For example: when you hit a football, you set the football in motion, by applying force.

#### Force can stop a moving body

When the driver applies brakes, he applies force to stop the vehicle. When a fielder catches a ball, he stops the moving ball. While doing so, he applies a force.

#### Force can change direction of motion

Suppose you are riding your bike and want to change its direction. For this, you will apply force on the steering. When batsman hits a ball, he changes the direction of motion of the ball. The batsman applies force while hitting the ball.

#### Force can change speed

The speed of a body at rest is zero. If an applied force makes the body move, the force changes the speed of the body. When a driver presses the accelerator of car, he increases the speed of car by applying force. When the driver takes his foot off from accelerator and lightly presses the break pedal, then he is reduces the speed of the car by applying force.

#### Force can change shape and size

What happens when your mom applies rolling pin on a ball of dough? The ball becomes flat and changes to chapatti. What happens when you fill a balloon with air? The shape of ballooon changes. All these changes happen because of the applied force. This shows that force can change the shape and size of an object.

### Types of Force

There are two types of forces, viz. Balanced forces and Unbalanced forces.

### Balanced Forces

In real life, more than one force is always acting on an object. If the resultant of applied forces is equal to zero, it is called balanced forces.

Example: In the tug of war contest, if both the teams apply similar magnitude of forces in oppoisite directions, rope does not move to either side. This happens becasue all the forces are balancing each other. As a result, the net sum of all the applied forces becoms zero.

Balanced forces do not cause a change in state of motion an object. Balanced forces are equal in magnitude and opposite in direction.

Balanced forces can change the shape and size of an object.

Example: When forces are applied on two opposite surfaces of a balloon, the size and shape of the balloon is changed.

### Unbalanced Forces

If the resultant of applied forces is not zero, it is called unbalanced force.

#### Effects of Unbalanced Forces:

• Move a stationary object.
• Increase the speed of a moving object.
• Decrease the speed of a moving object.
• Stop a moving object.
• Change the shape and size of an object.

## Laws of Motion:

Galileo Galilei: Galileo was the first to talk about efffect of force on motion. He said that an object moves with a constant speed when no force acted on it. In other words, when an object is in motion and no external force is acting on it, the object keeps on moving at constant speed.

But this is not possible in practical situations. It is impossible to get a condition of zero unbalanced force. Various forces always keep on acting on an object, e.g. force of friction, air drag, etc.

### Newton’s Laws of Motion

Newton worked further on the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s Laws of Motion.

Newton's First Law of Motion: An object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by an external force.

Newton's Second Law of Motion: The rate of change of momentum is directly proportional to the force applied, and is in the direction of force.

Newton's Third Law of Motion: There is an equal and opposite reaction for evrey action.

#### Newton’s First Law of Motion:

When an object is in the state of rest, it will remain in this state until an external force changes its state. Similarly, when an object is in motion, it will remain in state of motion until an external force changes its state.

Every object resists the change in its state of motion. The change can only happen after application of external force.

##### Newton’s First Law of Motion in Everyday Life:

Let us assume that a person is standing in a bus at a stop. What happens when the bus suddenly starts moving? The person tends to fall backwards. Let us see why does it happen.

When the bus is not moving, the body of the person is also in the state of rest. When the bus suddenly starts moving, the legs of the person begin to move but his torso tries to remain at rest. Due to this, the upper part of his body tends to fall backwards.

Let us now assume that a person is standing in a moving bus. What happens when the bus suddenly comes to a halt? In this case, the person tends to fall forwards. Let us see why does this happen.

When the bus is moving, the body of the person is also in the state of motion. When the driver suddenly applies brake, the legs of the person come to state of rest but his torso tries to remain in the state of motion. Due to this, the upper part of his body tends to fall forwards.

You may have observed that your mom shakes wet clothes with jerks before putting them on the laundry line. This helps in getting rid of extra moisture from the cloth. When cloth is shaken with a jerk, cloth comes into motion but water droplets trie to remain at rest. Due to this, water droplets fall on the ground.

What happens when you hit a pile of carrom-board coins with a striker. The coin at the bottom moves out without disturbing the tower of coins. Remaining coins remain in their place. The coin at the bottom comes into state of motion but other coins try to remain at rest.

## Mass and Inertia

You have read that every object tries to remain in its state of motion. In other words, it resists any change in its state of motion. This property of objects is called inertia. The property of an object to resist any change in its state of motion is called inertia.

A book on a desk will remain there till an external force makes it move. This is possible because of inertia of the book.

Inertia of an object is measured by its mass. Intertia is directly proportional to mass. This means inertia increases with increase in mass and decreases with decrease in mass.

A heavy object has more inertia than lighter one. This explains why it is more difficult to push a heavier object than a lighter one. You can easily push a box of chocolate but you may not be able to push a huge trunk.