Question

A: when an object is placed on a surface the frictional force between the surface and the object is more if the mass of the object is heavy. R: static friction is directly proportional to normal reaction.

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.