Posted by Donna on Wednesday, October 17, 2007 at 10:31am.
1. how much work do you do on a 25kg backpack when you walk a horizontal distance of 100 m?
I got 2,500 J
4. if you friend pushes a lawnmower four times as far as you do while exerting only half the force, which one of you does more work? how much more? ( I don't understand this question)
7.why does one get tired when pushing against a stationary wall when no work is done on the wall?
I got because the wall is pushing back with the same equal amount of force.
9. which requires more work streching a strong spring a certain distance or streching a weak spring the same distace? Why?

physics  drwls, Wednesday, October 17, 2007 at 10:54am
1. The answer is zero, because your motion is perpendicular to that of the weight force. Npte that you must also multiply M by g to get weight, but the answer is still zero.
4. What don't you understand? Work is force times distance. If your friend uses half the force but goes four times as far, how much greater is his work done than yours? The relative work done (His work)/(Your work) = 4 x (1/2) = ?
7. Caloriees must be burned to keep muscles in a state of tension, even when they are not in motion.
9. What do you think? Remember the definition of work. The work done stretching a spring is
(1/2)*(distance) * (maximum force)
The weaker spring requires a lower maximum force to stretch the same distance as the stronger spring.