Question

Which of the following statements is/are true

• A. Work done by friction is always negative
• B. Static friction always does negative work
• C. If you change the reference frame, then friction can do positive, negative, or zero work depending upon what reference frame you chose
• D. Static friction can never do work

Answer 1

The correct option is C

If you change the reference frame, then friction can do positive, negative, or zero work depending upon what reference frame you chose

Friction, by its nature, always acts opposing the relative motion between two surfaces. work done by friction can be zero, when there is no relative motion. But positive work cannot be done under any circumstances, if you see it from a reference frame fixed to one of the two surfaces. But if you change the reference frame, then friction can do positive, negative, or zero work depending upon what reference frame you chose. Then the sheet example also holds good.