Conceptual questions

1. Nov 2, 2004

cougar_21

You do no net work when you walk at a constant speed . Why do you get tired?

Is the force of a tennis racket acting on a tennis ball a conservative force?

A vase falls to the floor and shatters. Is momentum conserved in the collision?What objects needto be taken into account in describing the conservation of momentum?

Please help, I'm studying for an exam and need to understand the concepts in these questions. Thankz

2. Nov 2, 2004

cougar_21

I know that getting tired is independent form work, since there is no work when on constant speed, but I don't know why you get tired does it have to do with energy

I know that the conservative force is a force for which the work is independent of the path. But nothing else.

and the third one i have no clue at all

3. Nov 2, 2004

Cyrus

You do no net work when you walk at a constant speed . Why do you get tired?
You do do work! You do work in friction of your feet, air resistance, flexing your muscles. No pain no gain.

Is the force of a tennis racket acting on a tennis ball a conservative force?
No, the tennis racket and ball get hot, sound is produced, friction takes place. But these might be exteremely small in comparison to the force of impact, and you can ignore it. In that case yes, it is conserved.

A vase falls to the floor and shatters. Is momentum conserved in the collision?What objects needto be taken into account in describing the conservation of momentum?

Yes, an inelsatic collision between the earth and the vase. The earth + vase both move when they impact in order to conserve momenutm.

4. Nov 3, 2004

cougar_21

Thank you i understand prefectly !! Hope I pass my test today

5. Nov 3, 2004

HallsofIvy

Staff Emeritus
Actually, the statement "no net work" is not quite correct. IF there were no friction, then you would not have to "do work" to maintain a constant speed on a level. Of course, there IS friction so you have to do work. Also, and this may be more relevant, you have to do work at every step because you lift your body slightly with each step. You do no "net work" in that sense because your body comes back down again- and, theoretically, you could regain the energy when you come down (that's the idea behind a pogo stick!). Unfortunately, your muscles are not springs and cannot "regain" the work done. You actually have to do still more work as you "come down" from a step in order to cushion the impact.