Question

A body is sliding in a plane in the direction of unit vector K = k̂, what is the direction of friction force acting on the body ?

Assuming the standard notations for the unit vectors.

1. î 
2. -î 
3. k̂
4. -k̂

Answer 1

Correct Answer - Option 4 : -k̂

Concept:

Friction is the resistance force that opposes relative motion between two objects. 

• It is not a fundamental force, like gravity or electromagnetism.
• The direction of friction is always in such a way that it opposes the relative motion

Hence, When the body is sliding in a plane in the direction of a unit vector K = k̂, the direction of friction force will be such that it will oppose the relative motion i.e in the direction -k̂.