Q16DQ - University Physics with Modern Physics

Question

When a book slides over a tabletop, the force of friction does negative work on it. Can friction ever do positive work? Explain. (Hint: Think of a box in the back of an accelerating truck.) 

Step-by-Step Solution

Answer

Yes, friction can also do positive work.

Step 1: About work-energy theorem

The total work done is given by:

${\mathbf{W}}_{\mathbf{tot}}\mathbf{=}{\mathbf{K}}_{\mathbf{2}}\mathbf{‐}{\mathbf{K}}_{\mathbf{1}}\mathbf{=}\mathbf{∆}\mathbf{K}$ 

When forces act on a particle while it undergoes a displacement, the particle’s kinetic energy changes by an amount equal to the total work done on the particle by all the forces.

Step 2: Explanation of the statement.

When a book slides over a tabletop, the friction force opposes the motion of the book. Hence the work done by the frictional force is negative.

Yes, friction can also do positive work if the displacement of the object is in the direction of friction force. 

For example, for a box in the back of an accelerating truck, the static friction on the box causes the box to accelerate along with the truck is the horizontal force on that box.