Q 6.30: What is the underlying physical reason for the difference between the static and kinetic coefficients of friction of ordinary surfaces

the coefficient of static friction is larger than kinetic coefficients of friction

Explanation:

The coefficient of static friction is usually larger than the kinetic coefficients of friction because an object at rest has increasingly settled agreements with the surface it's resting on at the molecular level, so it takes more force to break these agreement.

Until this force is been overcome, kinetic coefficient of friction is not going to surface.

Note: coefficient of static friction is the friction between two bodies when the bodies aren't moving. Meanwhile, kinetic coefficient is the ratio of frictional force of a moving body to the normal reaction.

≤μN(coefficient of static friction)

where  is the static friction, μ is the coefficient of static friction and N is the normal reaction

μ = (kinetic coefficient of friction)

attached is diagram illustrating the explanation

answer: the rock sample is a sedimentary rock.

explanation:

it is given that:

mass of rock sample, m = 2.7 g

volume of rock sample,

so, density of rock sample will be

since, the density of rock sample is . hence, the rock sample is a sedimentary rock, whose densities varies between .