Question

Whenever a body moves or tends to move over the surface of another body, a force comes into play which acts parallel to the surface of contact and opposes the relative motion. This opposing force is called friction. Static friction fS opposes the impending relative motion while the kinetic friction fK opposes the actual relative motion. Static friction is a self-adjusting force. The maximum value of static friction (fS=μsN) which comes into play when a body just starts moving over the surface of another body is called limiting friction. fs ≤ μsN and fk = μkN where N is the normal reaction between the two surfaces in contact. Both static and kinetic friction are independent of the area of surfaces in contact.

A bullet of mass 0.04 kg moving with a speed of 90 m/s enters a heavy wooden block and is stopped after a distance of 60 cm. What is the average resistive force exerted by the block on the bullet?

A horizontal force of 980 N is required to slide a sledge weighing 1200 kgf over a flat surface. Calculate the coefficient of friction.

A block of mass 2 kg is placed on the floor, the coefficient of static friction is 0.4. A force of 2.5 N is applied on the block as shown in figure. Calculate the force of friction between the block and the floor.