# First Law of Motion

## •    Newton’s first law of motion is also known as law of inertia. Inertia of a body is defined as the tendency of the body to resist any change in its state of rest or motion. Mass is the measure of inertia of a body. •    Newton’s first law of motion states that every body resists a change in its state of rest or of uniform motion in a straight line unless acted upon by an external force. Mathematically,                                                                    •    Inertia of three types: Inertia of rest: It is the inability of a body to change its state of rest by itself. Inertia of motion: It is the inability of a body to change its state of motion by itself. Inertia of direction: It is the inability of a body to change its state of direction by itself. •    Newton’s first law of motion also defines force as an external cause that can change the state of the body.

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• Q1

When a motorcar makes a sharp turn at a high speed,

Marks:1

we tend to get thrown to one side

##### Explanation:

When a motorcar makes a sharp turn at a high speed, we tend to get thrown to one side. This is because the engine changes the direction of motion of motorcar by applying an unbalanced force. We slip to one side of the seat due to inertia of direction of the body.

• Q2

In Galileo’s double inclined plane experiment, the marble would continue to travel forever trying to reach the same height that it was released from

Marks:1

the slope of the plane is reduced to zero.

##### Explanation:

In Galileo’s double inclined plane experiment, if the right side plane were made horizontal then the marble would continue to travel forever trying to reach the same height that it was released from. When the plane is made horizontal, slope is reduced to zero.

• Q3

Suppose that we tie a heavy ball to the end of a string and whirl it in a circle on the floor. When the string is allowed to go off, the ball travels in a straight line. In this case, Newton’s

Marks:1

all laws are applicable

##### Explanation:

The whirling ball illustrates all three Newton’s laws of motion.
When the string is broken, the ball will travel in a straight line in the direction in which it was going when it was released. Here
Newton’s first law is applied. Centripetal force, which is always directed towards the center of rotation, changes the direction of velocity. Here Newton’s second law is applied.

The centrifugal force is the force that the whirling ball exerts in pulling outward on the string acts opposite to the centripetal force. It is equal in magnitude to the centripetal force. So, Newton’s third law is applied here.