Question

which of the following statements is true for a block with height h and area a?
the coefficient of friction depends on the surface area, a, of the block that is in contact with the surface as measured by an ordinary ruler, but does not depend on h.
the coefficient of friction depends on the material properties of the block and the surface, and does not depend on a or h.
if the block is experiencing static friction must be accelerating at a constant, non - zero rate.
the force of friction on the block never depends on the normal force exerted on an object from the surface.

Explanation:

• The coefficient of friction is a property that depends on the nature of the surfaces in contact (material properties). It is independent of the surface area \(A\) and height \(h\) of the block.
• Static friction acts when there is no relative motion between the surfaces. If a block is experiencing static friction, it is either at rest or moving with a constant velocity (zero acceleration), not accelerating at a non - zero rate.
• The force of friction (both static and kinetic) is given by \(F_f=\mu F_N\), where \(F_N\) is the normal force. So, the force of friction does depend on the normal force.

Answer:

The coefficient of friction depends on the material properties of the block and the surface, and does not depend on \(A\) or \(h\).