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Name: Jeremy C.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 6/23/2004


Question:
I understand that some form of energy needs to be converted to create a force. When I push against a wall, where does the wall get the energy needed to create the reaction force of that contact+


Replies:
Jeremy,

The wall gets the energy from the electromagnetic potential energy stored in the bonds between the wall's molecules. Also, energy is not required to create a force. A transfer of energy is required to cause that force to do work.

Energy is a strange thing in that there is no definite way to define zero energy. It all depends on what you call zero velocity and what you call the origin (e.g. zero height for gravity). If you call the surface of a table zero height, an object on the table has zero gravitational potential energy. Still, gravitational force can make the object break a weak table and then cause the object to move downward.

It is difficult to say whether potential energy causes force or vice versa. Potential energy seems to be stored in the force between objects. If a force causes an object to speed up, potential energy in the force becomes kinetic energy in the object. If a force causes the object to slow down, the reverse occurs. If a force only causes the object to change direction, there is no transfer of energy. The force causes the transfer of potential energy, but not necessarily its existence. We do not really know the answer to that question.

Ken Mellendorf
Math, Science, Engineering
Illinois Central College


The repulsive energy arises from trying to force molecules to occupy the same space at the same time.

Vince Calder


Jeremy,

There is a slight misunderstanding here. Energy is required to move against a force. There are forces in nature, such as gravity, that exist without converting energy. However, in order to move against the force due to gravity (perform work) some amount of energy needs to be transferred to gravitational potential energy (energy related to position in a gravitational field).

So, while you may be expending a lot of chemical energy in pushing against a wall you are not doing any work on the wall. The wall is not doing any work in pushing back against you and is not expending any energy. The wall is being a lot more efficient than you are :- ).

Greg Bradburn



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