Questions about course of Energy conservation

AI Thread Summary
The discussion revolves around the calculation of work done in two scenarios: pulling a rubber band along a closed trajectory and holding a bag of groceries. Participants clarify that the initial assumption about measuring the elastic force along the trajectory does not yield the overall force, as it misinterprets the concept of work. When holding groceries stationary, the work done is zero since there is no displacement, contradicting the initial formula provided. Additionally, when moving the bag, the work done must consider the direction of forces and motion, which was not accurately represented in the responses. The conversation emphasizes the importance of understanding the principles of work and force in physics.
physicos
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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 ?
 
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physicos said:
I said it would give the overall force of F over the whole trajectory on the rubber band : IS IT CORRECT ?
What do you mean with "the overall force of F over the whole trajectory"?
No, it is not 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.
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.

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 ?
No. Think about the directions of forces and motion.
 
mfb said:
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.

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 !
 
physicos said:
1-I meant with the overall force that F*Δr where just particle of work !
There is no F*Δr in the problem.
2- I really don't know what is so special ! I've copied the excercise as it is !
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? ;)
 
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