Question

sliding a box is work against friction.Give reason

Answer 1

Friction is something we deal with all the time. As we all know, when you try to move something, you have to keep pushing. A car keeps rolling because it's got smooth roller bearings. A boat in the water slides through that medium smoothly enough. Between any two surfaces there is a friction coefficient. Interestingly, irrespective of how slow or fast you move it, the friction is always the same. But, when you stop, suddenly the coefficient of friction goes up by 25% to even 100%. Furthermore, friction force is proportional to the weight. Why's that?
We must look at the molecular or atomic level. Friction is caused by the tiny atomic surfaces which momentarily bond whenever contact is made, and then are broken as the objects are moved. This is the cause of the friction force. The surfaces, even of highly polished objects, are like mountains and valleys on a microscopic scale. So, when the objects are moving fast, relatively few molecular or atomic contacts are made. When they slow down and then stop, the two objects settle into each other and many sort of "Velcro" hooks are formed. That's why getting movement started again takes an extra push. And that's why, when a train, metro, or bus stops, at the last moment of braking, there is a little squeak and the vehicle rebounds just a centimeter. Watch and feel it the next time