Class 9 Science Solutions English Medium

Chapter 9. Force and Laws of Motion

(1) The First Law of Motion:

The first law of motion states;

"An object remains in a state of rest or of uniform motion in a straight line unless
compelled to change that state by an applied force." 

In other words: all objects resist a change in their state of motion.

• The first law of motion tells us that the motion of an object due to applying unbalanced force. When an unbalanced force is applied to any object it moves due to force. 
• The first law of motion indicates that when an unbalanced external force acts on an object, its velocity changes, that is, the object
  gets an acceleration.

Inertia: 

Defintion: There is a resistance offered by an object to change its state of motion. If it is at rest it tends to remain at rest; if it is moving it tends to keep moving. This property of an object is called its inertia.

• Inertia is a property or tendency of every object. 
• Inertia is measured by mass of an object. 
• The heavier object has the greater inertia than the light object. 

Law of inertia: 

The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia.

Types of inertia : 

(i) Inertia of Rest

(ii) Inertia of motion

(iii) Inertia of direction

Examples of inertia: 

(I) Travelling in a car: While travelling in a motorcar can be explained on the basis of the law of inertia. We tend to remain at rest with respect to the seat until the drives applies a braking force to stop the motorcar. With the application of brakes, the car slows down but our body tends to continue in the same state of motion because of its inertia. A sudden application of brakes may thus cause injury to us by impact or collision with the panels in front. Safety belts are worn to prevent such accidents. Safety belts exert a force on our body to make the forward motion slower.

(ii) Standing in bus: When we are standing in a bus and the bus begins to move suddenly. Now we tend to fall backwards. This is because the sudden start of the bus brings motion to the bus as well as to our feet in contact with the floor of the bus. But the rest of our body opposes this motion because of its inertia.

(iii) Sharp turning at high speed of a car: When a motorcar makes a sharp turn at a high speed, we tend to get thrown to one side. This can again be explained on the basis of the law of inertia. We tend to continue in our straight-line motion. When an unbalanced force is applied by the engine to change the direction of motion of the motorcar, we slip to one side of the seat due to the inertia of our body.

•  A body will remain at rest unless acted upon by an unbalanced force. 

(iv) Hitting aarom coins by a stricker: When we attempt a sharp horizontal hit at the botton of the pile using another carom coins or stricker. If the hits is strong enough. the bottom coin moves out quickly and removed and due to inertia of the other coins makes them fall vertically on the table. 

This is why, the first law of motion is also known as the law of inertia.

Mass: The quantity of material present in a object is called its mass. 

The mass of an object is a measure of its inertia. 

Differences between Inertia and Mass: