Inertia

A body can not change its state of rest or uniform motion unless an unbalanced force is applied on it. The inability of a body to change on its own its state of rest or uniform motion is known as inertia. It is an inherent property of all objects. Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. It means that inertia increases with increase in mass and decreases with decreases in mass. A heavy object will have more inertia than a lighter object. For example:

1. Inertia at Rest:

If we keep a body at rest, it remains there for any length of time if no force is applied on it, is called inertia of rest. It may also be defined as the tendency of the body to oppose any change in state of rest.

Examples:

(i) Jerks while travelling: When we stand in a bus and the bus starts suddenly, we tend to fall backward. This is because our feet are in contact with the floor of the bus and the friction at the contact is high. This force does not allow the feet slip on the floor. The feet move forward with the floor. The upper body does not feel the forward force immediately and remains at rest for a while. So, the upper part of our body gets jerked backward.

Similarly, when the bus stops suddenly, the feet come to rest immediately, but the upper body continues to move in forward direction. So, we tend to fall forward.

(ii)  Card Coin Experiment: Place a coin a smooth card kept on a glass as shown. Flick the card sharply in the horizontal direction. The card flies away and coin drops into the glass.

Explanation: By applying force the card is accelerated and moves away. Since the friction between card and coin is negligible, there is no force on the coin in the horizontal direction. It remains in its original position due to inertia of rest.

(iii) Carpet beaten to remove dust particles : The carpet is beaten with a stick to remove the dust particles.

Explanation: When the carpet is beaten there is motion in the carpet but the dust particle do not change the state of rest as no force is applied on them hence do not move with the carpet. these dust particles get separated from the carpet because of inertia of rest.

(iv) Striking a Pile of Carrom Coins: In a pile of coins, by hitting the bottom coin hard with a striker. The lowest coin will move away, but the rest of the pile will remain at the original position.

Explanation: The striker applies force on the lowest coin and it moves but the rest of the pile remains at its place due to inertia of rest. As the lowest coin moves very fast, any force exerted by it on the coins above it is for a very short time, which is not able to move the upper coins in the horizontal direction.

Similarly, when the bus stops suddenly, the feet come to rest immediately, but the upper body continues to move in forward direction. So, we tend to fall forward.

2. Inertia of Motion:.

The tendency of the body to continue its state of uniform motion even until some unbalanced force is applied on it is called as inertia of motion. It may also be defined as the tendency of a body to oppose any change in its state of uniform motion. For example:

Examples:

(i) Forward jerk when moving vehicle stops suddenly:. When a running vehicle stops suddenly,the passenger is jerked forward.

Explanation:  In a running car, the whole body of passenger is in the state of motion. As the car stop suddenly, the lower part of his body being in contact with the car, comes to rest but its upper part remains in the state of motion due to inertia of motion. Thus he gets jerked forward.

(ii). In an event of long jump, an athlete runs fast before making a jump.

Explanation: Due to inertia of motion even when the athlete stops running and jumps his body is in motion and hence he is able to jump to a longer distance.

3. Inertia of Direction:

The tendency of the body to continue in the same direction until some unbalanced force is applied on it is called as inertia of direction. It may also be defined as the tendency of a body to oppose any change in its direction of motion is known as inertia of direction.

For Example:

(i) When a fast moving bus negotiate a curve on the road, then passengers fall away from the centre of the curved road.

Explanation: In a moving bus the passengers are in motion along with the bus. As the bus negotiates a curve the part of the body which is in contact with the bus will move along with the bus, but the upper part of the body will not change its direction due to inertia of direction and hence will fall away from the center of the curved road

(ii) The sparks produced during sharpening of a knife against a grinding wheel leaves tangentially to its rim.

Explanation: When grinding is being done the tiny pieces on the surface of the grinding wheel get broken and fall in the form of sparks from the wheel. Before breaking they were in motion along with the wheel, but after breaking they will not change their direction due to inertia of direction  These small stone pieces had a linear velocity in the tangential direction before breaking off from the stone.Hence they leave the grinding wheel tangentially to the rim of the wheel.

(iii) A stone tied to string is whirling in a horizontal circle. If the string breaks, then the stone flies away tangentially.

Explanation: The force required for changing the direction while whirling a stone in a horizontal circle is provided by the tension in the string. If the string breaks, the force that was acting on the stone to change the direction has become zero. The stone will not change its direction of motion because of inertia of direction and  Hence, it flies away tangentially.