Answer to Question #279394 in Physics for Juli

Question

You are on the second story of a building, using a rope to lower a box to your friend on the ground.

The box is moving downward at a constant velocity. Consider your system to be the box+rope. Is

the work done by you on the system positive, negative, or zero?

Answer 1

In this problem you are lowering the box to your friend from a second story window at constant velocity. So that means your change of kinetic energy is zero. Now we have the word kinetic energy theorem. W Toto. Now you have two forces acting on this. You have you and you have gravity and that's equal to change in kinetic energy. That's the word kinetic energy theorem. And like I said it tells you in the problem you're doing it at constant cross it can't speak in this case. So that means the w you she could optimize the work done by gravity. Now the question then comes is what's the sign of the work on gravity? Well think of it this way you have to maintain the kinetic energy from the beginning. You know from when you start to when you get to the to the some distance d further down you're lowering it. So gravity acting alone, what would he want to do to it if you were to release this gravity would speed it up this downward motion if you were to release it. So you've got to put the brakes on it to maintain the kinetic energy, gravity would speed it up. You got to put the brakes on. So W. G. Is greater than zero implies W. U. Is less than zero negative. It's balancing off what gravity would do. Gravity want to speed it up by itself in the downward motion. It's the direction of the force of gravity. Force of gravity. Um It want to speed it up. You got to put the brakes on, you're the one doing the brakes, you're doing negative work to balance off the what the gravity would increase the kinetic energy. And so that that really is so the word connect everything is the key first step because that connects all the works together with the kinetic energy, that's how it all connects. And then you just got to ask yourself what was what was gravity do? And then you can answer the question, which obviously, is that what you have to do is negative work. And that is the whole problem.