Work done by static friction on an object:
may be positive
must be negative
must be zero
none of these
Work done by the frictional force is negative -
When the force is large enough to overcome the friction
Work done by the frictional force is positive -
When force is applied on a body, which is placed above another body ,the work done by the frictional force on the lower body may be positive
Work done by the frictional force is zero -
When force applied on a body is insuffient to over come the friction
Consider the blocks shown in the figure to be moving together due to friction between them.
The free body diagrams of both the blocks are shown below.
Work done by static friction on A is positive and on B is negative.