Friction opposes motion. How does that friction act as the force that helps us m
ID: 1624629 • Letter: F
Question
Friction opposes motion. How does that friction act as the force that helps us move in a forward direction? (i.e. why doesn't friction keep us from moving forward?) Please explain why the weight of a book on your hand and the force of your hand pushing up on the book are not a Newton's Third law action-reaction pair. What is the action reaction pair for: i) the earth pulling on the book? ii) your hand pushing up on the book? What is negative work? Describe an exercise for which a person's muscles are doing negative work. When a bug hits a windshield which is larger, the force of the bug hitting the windshield or the force of the windshield hitting the bug? Which is larger the change in momentum of the bug or the change of momentum of the car? Explain your answers.Explanation / Answer
In general, the mechanism with which static and kinetic friction act is different from mechanism with which Rolling resistanceacts. Static and kinetic frictions are a consequence of the property of the surface and rolling friction is the consequence of elasticity of the material of the bodies in contact.
A rolling body is not a perfectly elastic body. That is, when it is compressed, energy is stored and upon removal of the force, it does not give away all the stored energy back. Some of the energy is lost as heat inHysteresis. When a body is rolling, it undergoes a series of expansion and compression (due to its weight). As all energy that is stored during compression is not given out during expansion, there is a loss of energy as heat. But what energy is this that is being lost as heat? It is, whatever energy that is causing the compression and expansion of the body (or whatever is causing the rolling). In this case it is kinetic energy of the rolling body that is being converted to heat. Therefore, the body loses kinetic energy and slows down.
Therefore, there is no real frictional force (as in the static and kinetic cases) that is stopping the rolling body. Since energy is a sxalar, there is no direction.
But ofcourse there is static friction without which rolling will not occur between bodies which has a direction. You are told that if the sphere is moving forward the static friction also acts forward. But it is very important to note that the sphere do not lose energy due to static friction. .
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