# Homework Help: Questions about course of Energy conservation

1. Apr 5, 2014

### physicos

Thank you for helping me
1-
One end of a rubber band is tied down and you pull on the other end to trace a complicated closed trajectory.If you were to measyre the elastic force F at every point and took its scalar product with the local displacement F*r,and then summed all f these . What would you get ?

I said it would give the overall force of F over the whole trajectory on the rubber band : IS IT CORRECT ?

2- How much work do you do when you hold a bag of groceries while standing still (I said : W= m*g*h) with m mass of groceries and h height of the bag from ground.
How much work do you do when you hold the same bag along a distance d across the parking lot of the grocery store(I said W= mg*d )
IS IT CORRECT ?

2. Apr 5, 2014

### Staff: Mentor

What do you mean with "the overall force of F over the whole trajectory"?
No, it is not correct.

What's so special about the ground here? The problem statement did not say you have to pick it up. You are just holding it.

No. Think about the directions of forces and motion.

3. Apr 6, 2014

### physicos

1-I meant with the overall force that F*Δr where just particle of work !
2- I really don't know what is so special ! I've copied the excercise as it is !!

4. Apr 6, 2014

### Staff: Mentor

There is no F*Δr in the problem.
Sure, but your answer would imply there is something special about the ground...
How much work does a table need to hold something? If you give a similar answer again, where would the table get such energy from? ;)