8.6. Contact Forces

Can you push or lift a book lying on a table without touching it? Can you lift a bucket of water without holding it? Generally, to apply a force on an object, your body has to be in contact with the object. The contact may also be with the help of a stick or a piece of rope. When we push an object like a school bag or lift a bucket of water, where does the force come from? This force is caused by the action of muscles in our body. The force resulting due to the action of muscles is known as the muscular force.

Muscular force is the kind of force we apply through our muscles when we touch or lift something. For example, lifting a book or pushing a door open requires your muscles to contract and exert force. Without touching the object, it’s impossible to apply muscular force. Therefore, for actions such as pushing a bag or lifting a bucket, your body must physically connect with the objects to exert this force. This majorly happens through the weak involvement of your muscles, primarily responsible for these movements.

Think about how you pick up your backpack to go to school. Your arms and back muscles work together to lift the strap, demonstrating muscular force. If you tried to lift it without using your hands, it would remain on the ground, showing that contact with the object is necessary for muscular force to function.

Recall some of your experiences. A ball rolling along the ground gradually slows down and finally comes to rest. When we stop pedalling a bicycle, it gradually slows down and finally comes to a stop. A car or a scooter also comes to rest once its engine is switched off. Similarly, a boat comes to rest if we stop rowing it. Can you add some more such experiences? In all these situations no force appears to be acting on the objects, yet their speed gradually decreases and they come to rest after some time. What causes a change in their state of motion? Could some force be acting on them! The force responsible for changing the state of motion of objects in all these examples is the force of friction.

Friction is a force that occurs when two surfaces come into contact. It works against the motion of moving objects. Therefore, when a ball rolls on the ground, friction between the ball and the ground acts to slow it down until it eventually stops. This same force is responsible for bringing bicycles and trucks to a halt when we stop pedalling or switching off their engines. Friction acts in the opposite direction of motion, and it’s essential for providing grip and control when we walk or drive.

Imagine sliding a book across a table. Initially, it moves smoothly, but soon it slows down and stops. The reason? The friction between the book cover and the table surface is trying to maintain stillness. Without friction, the book would slide indefinitely, just like how ice skaters glide but will eventually use their foot to stop. This relatable example illustrates how every day, friction slows down our actions.

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