Why does an object have the friction force and what is the direction of the force?

Question

BlackTom AI · Accepted Answer

To tackle this question, I will assess each option in turn and explain the underlying physics.

Option a: 'Because there is a friction between the object and the surface, direction of the force is opposite to the motion.' This reflects the standard idea that kinetic friction acts to oppose relative motion between surfaces. The friction force arises due to intermolecular interactions at the contact surface and always points opposite to the direction of sliding. This option correctly identifies both the cause (friction between surfaces) and the direction (opposite to motion).

Option b: 'Because it sits on a surface or other object, direction of the force is perpendicular to the surface.' Normal force is indeed perpendicular to the contact surface, but friction is not the normal force. The question asks about friction force, not the normal contact force. This option confuses friction with the normal reaction force, making it incorrect for the frictional description.

Option c: 'Because it is tied to a string, direction of the force is along the string but away from the object.' Tension in a string can exert forces along the string, but friction is not produced by a string, nor is its direction determined by attachment. This option describes a tensile force scenario, which is unrelated to the friction between object and surface.

Option d: 'Because it has the mass, direction of the force is downward.' Mass itself does not determine the direction of friction. Friction depends on the contact interaction and the relative motion, not on the presence of mass alone. Downward direction is associated with gravity acting on mass, not with friction direction, making this statement incorrect for friction.

In summary, option a aligns with the fundamental concept that friction arises at the interface of contacting surfaces and acts to oppose motion, while options b, c, and d conflate friction with normal forces, tension, or gravity, respectively, which do not correctly describe the friction force and its direction.

UCIC 202503 PHYS101 Quiz 2

Why does an object have the friction force and what is the direction of the force?

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