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Name: Laura R.
Status: student
Age: 12
Location: N/A
Country: N/A
Date: 2000-2001


Question:
A question from a test I just took:

Who is doing more work: a girl in a wheelchair, pouring liquid into a glass on a table, or a boy at the table, writing on a piece of paper?

I said it was the girl.

Reasoning: Work equals force times distance. The water is moving under the force of gravity over a distance, at least half a foot. The boy is pushing a pen, which doesn't weigh as much as the water does, across a smaller distance.

My science teacher disagreed. He says the girl is not exerting much force--not as much as the boy is. But I say, she is causing more force to be exerted--the force of the gravity. It is as if I pushed a rock off the top of a tall building. I push a little, gravity does the rest of the work.

Please help me understand this.


Replies:
Laura,

I would have to say that is not a good test question at all, if anything your teacher should at least pass you on that question you obviously stated the correct reasoning-Work=force x distance. If that is the complete question than it really can not be answered, a girl in a wheelchair?? Is she moving the wheelchair or just sitting there? Is the boy writing a word or a novel? In other words the question should have more detail!!

Michael Baldwin


The girl is causing more work to be done, but she isn't doing any of it. If she holds the pitcher still while liquid runs out of it, she is doing zero work.

Tim Mooney


Given the information you have given me, I think I would have to agree with you. In order to pour the water into a glass from, I assume, another glass, she would have to lift it some degree and tilt the glass over. In this case, she is raising the glass and thus creating potential energy due to the position of the glass. This potential energy increase is due to the work she put into it, i.e. a force times a distance to lift the glass. Thus, the total energy in the system (the girl and the glass and the water) has increased due to her putting work into the system by raising the glass to pour the water. The boy writing does not cause the total energy in the system to change because he does not change the potential or kinetic energy in the system. In order to have work done in a system, there has to be a change of energy in the system.

Chris Murphy


Also, the weight of the pen has little to do with the work the boy is performing (I'm assuming that the paper is horizontal so that he is not having to lift the pen as he is writing, just push it around). He just has to overcome the force of the friction between the paper and the pen. This force (to overcome the friction) times the distance he moves the pen gives the work he is performing.

Let us look at your example of pushing a rock off a building. The person doing the work exerts enough force to move it over the edge but once he is not in contact with it he can't exert force on it so his work is done. A person who pushes a rock off a ten story building does the same amount of work as a person who pushes the same rock off of a one story building?

On the other hand, the person who carries a rock up 10 flights of stairs certainly does more work than a person who carries it up 1 flight! 10 times as much since they carried it 10 times as far against the same force. With the rock on top of the building this work against gravity is stored as potential energy. This potential energy of lifting the rock is released when the rock falls.

Greg Bradburn


I would say that you are both correct or both incorrect. The problem is not really specified sufficiently. What is the weight of the pitcher? What is the weight of the pencil. What is the displacement for each of the subjects performing the work? Without this information, one can only guess.

Harold Myron


The force of gravity is not exerted by the girl. Gravity from the Earth exerts the force that causes the liquid to fall, so the energy enters the liquid from the gravitational force. It began as potential energy stored within the force between the liquid and the Earth. It became kinetic energy within the liquid. Very little energy left the girl's body.

The girl did not cause the force to be exerted. The gravitational force was pulling on the liquid even before the girl poured it out. Before pouring, the girl exerted a force in the opposite direction to prevent the jar and liquid from falling. By pouring, she stopped pushing up on the liquid. This allowed the liquid to fall.

Energy did pass from the boy's body to the pencil, through the force between his fingers and the pencil. The boy did work on the pencil with this force.

Kenneth Mellendorf



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