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Work
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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