Answer to Question #184335 in Mechanics | Relativity for Nicole

Question

A block sliding with some initial velocity over a flat surface slows due to the frictional force acting between the block and the surface. This is because (select answers)

(A) intermolecular forces of attraction between the molecules of the two surfaces may become large enough to impede the motion of the block.

(B) small irregularities on both surfaces may snag and resist the sliding of the block .

(C) gravitational force changes the normal force to the surface causing the coefficient of friction between the surfaces to increase

(D) action reaction pairs between the block and the surface impede the motion .

Answer 1

Explanations & Answers

• There are several factors that cause friction from surfaces.

• Intermolecular forces are everywhere that they may involve in imposing opposing forces on objects but compared to the given situation, they are really negligible.

• It is basically known that friction depends on the surface quality (via the coefficient of friction): smoother surfaces show less friction whereas on coarser ones' friction is severe. This is a valid sentence that describes the reason for friction, hence this can be considered as an answer.

• The coefficient of friction as mentioned above depends on the nature of the surface. It is not subject to change during the motion (it does by a little amount as static to kinetic but since the situation described has already a motion, it only experiences kinetic friction thus kinetic frictional coefficient). Therefore, not a valid option.

• The final option describes how the friction applies to the block and can be considered as an answer.
• Therefore, options B and D can be considered as answers.