Inertia and Mass

When multiple forces are applied in the same direction on an object, the total force is the sum of all forces, making it easier to move. For example, two people pushing a heavy container together can move it more easily than one person. If forces are applied in opposite directions, the force that acts is the difference between the two. Since force has both magnitude and direction, it is considered a vector quantity.

Effects of Force,

1. A force can make a stationary object move or change the speed and direction of a moving object.
2. Force can also change the shape of an object, like when kneading dough, shaping a pot, or stretching a rubber band.

1. Inertia: When a body is in a state of motion, it tends to keep moving. If it is at rest, it tends to remain at rest. This property of an object is called its inertia. An object offers resistance to changing its state of motion. Inertia is the natural tendency of an object to resist changes in its state of motion. 

• A stationary object will remain at rest unless an external force acts on it.
• A moving object will continue to move in a straight line at the same speed unless an external force causes it to stop or change direction.

For example, when you suddenly stop a moving car, your body tends to move forward because of inertia.

Momentum (p): It is the product of the mass and velocity of a body. P = mv

2. Mass: Mass is a measure of the amount of matter in an object. It is also a measure of an object's inertia. The more mass an object has, the greater its inertia and the harder it is to change its state of motion.

• Heavier objects have more inertia, making them more difficult to start or stop moving.
• Lighter objects have less inertia and are easier to move or stop.

1. Inertia with a Coin and Card

• Place a postcard on top of a glass and put a 5 rupee coin on the postcard.
• Quickly push the postcard away.
• Observe that the coin drops straight into the glass instead of moving with the postcard.

Conclusion: The coin remains in place due to inertia, resisting the change in its state of rest, and falls into the glass due to gravity.

2. Effect of Tension in Strings

• Hang a half kg mass from a stand using string 1.
• Tie another string (string 2) to the bottom of the mass, letting it hang freely.
• Pull string 2 with a quick jerk → String 2 breaks, but the mass stays in place.
• Pull string 2 slowly → String 1 breaks, and the mass falls.

Conclusion:

• A quick pull does not move the mass due to inertia, so string 2 breaks first.
• A slow pull gradually increases tension, causing string 1 (which holds the weight) to break.
• This demonstrates how inertia and tension affect forces acting on objects.