Energy of System: Which Statement is Always Correct?

[Fatima Hasan]

Homework Statement

Which of the following statements is always correct?

A) The work of friction force is negative.
B) The work of gravity is positive
C)The work of the spring is negative
D) The work of the normal force is negative
E) None of the above

The Attempt at a Solution

My answer is E

A) if applied force is smaller than to overcome the resistance then work of friction is 0 since no movement
B) a book on table has no movement so work of gravity = 0
C) spring can be compressed or elongated, either of which give opposite signs
D) book on table, work of normal force is 0 since no movementSo is my answer right!?
Thanks

 

[jbriggs444]

I agree with your answer and with the counter-examples you've given for the four alternatives.