# Is force relative like motion?

1. Mar 25, 2015

### manogyana25

When a book which is on table is pushed, it moves in the direction we push and the work done by us is positive and the energy gained by the body is also positive. But consider the frictional force. The work done by it is negative. So in one incident force is both positive and negative depending on the force we take into consideration. So, can we say force is relative?

2. Mar 25, 2015

### jbriggs444

The direction we push and the direction the book moves need not be the same. Consider a book on a train. We push the book rearward relative to the moving train. The book moves forward relative to the ground and the work we perform relative to the ground frame is negative.

The force of friction on that book is in the forward direction. The book is moving in the forward direction relative to the ground. Relative to the ground, the work done by friction was positive.

Energy and work are frame-relative quantities, not invariants. [The magnitude of] force, however, is invariant in classical physics. It does not matter what inertial frame you adopt, acceleration and force will not vary in magnitude from one reference frame to the next.