I need some help on a couple of physics problems. I appreciate the help!?

Question

1) What is the work done by you on a book that you lift straight up off a table?
- positive
- negative
- zero
- can not determine
2) You are in a game of toss the water balloon. When you catch the balloon that is thrown to you, the work you do is
- positive
- negative
- zero
- not enough information
3) You are holding your cell phone to your ear. You are doing
- positive work on the phone.
- negative work on the phone.
- zero work on the phone
- not enough information
4) You are throwing a ball against a wall so that it bounces off the wall with the same speed that it hits the wall with, the work done by the wall on the ball is
- positive
- negative
- zero
- not enough information
5) You are parachuting out of an airplane. Once your parachute opens, as you fall the work done by gravity on you is
- positive
- negative
- zero
- not enough information

Answer 1

1) What is the work done by you on a book that you lift straight up off a table?
- positive since you are lifting the book against the gravity through a distance d. Work done is wt. of the book multiplied by the distance and yes, the work done by your muscles in lifting themselves.

2) You are in a game of toss the water balloon. When you catch the balloon that is thrown to you, the work you do is

What is negative work? The balloon is moving towards you and you stop it, by absorbing momentum. Your hands and the balloon may move backwards.

3) You are holding your cell phone to your ear. You are doing

Your hand muscles are doing work to keep the phone at that position. Where is the work? Work is defined as force x distance. Here, no distance is involved. So, there is no work on the phone but that does not mean you can hold it for ever in that position.


4) You are throwing a ball against a wall so that it bounces off the wall with the same speed that it hits the wall with, the work done by the wall on the ball is
- zero

If the ball rebounds with the same speed, the work done by the wall is zero. In practice, this is not possible.

5) You are parachuting out of an airplane. Once your parachute opens, as you fall the work done by gravity on you is
- positive

Gravity continues to work on you to pull you towards the earth with the same acceleration but the air resistance of the parachute is working against the gravity. The net force is less and the work is less.

Positive and negative are arbitrary terms and are defined by the direction of movement.

Most of the questions are poorly constructed.

Answer 2

1. By convention, positive.
2. Negative. The kinetic energy of the balloon could be converted to useful work.
3. No work. Nothing is moving.
4. Zero.
5. Positive. Suppose that you were holding a string, connected to a reel at the top of a tall tower. You will be unwinding the string from the reel. The reel could be connected to a generator to make electricity.

Answer 3

1) Book lifted from table: Gravity is pulling down, you are pulling UP. the book moves UP. So the work done by you is positive: work = force x displacement 2) Catching the balloon: You are pushing AWAY, but the balloon is moving TOWARDS you. So the work done by you is negative, because the force and the displacement are in the opposite direction. 3) Cellphone: You are exerting a force against gravity, but there is no motion, no displacement. Therefore: work = force x displacement = 0 4) Throwing a ball against the wall: Tricky: although there is force and motion, the wall does not move during the impact. The ball moves into the wall as it compresses, but then it decompresses: so the net displacement is 0. total work = 0 (This is validated by the fact that it bounces back with the same speed as before.) 5) Parachuting Gravity is pulling you DOWN, and you are moving DOWN. So work is positive.

Answer 4

1 positive
2 negative
3 zero work
4 zero
5 negative

When you lift something, you are giving it potential energy, so work is added. When things are lowered, the reverse is true, so negative work is done.