Re: Kinectic and static friction?

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Posted by Barrie Cooper on November 02, 1999 at 13:03:10:

In Reply to: Kinectic and static friction? posted by Stephanie on October 24, 1999 at 15:38:50:

Hi Stephanie!
Sorry that the reply's a little late. This is basically what I know about friction. It might not be 100% accurate but it got me through A level Physics and the 1st year at Uni.

Friction is a force which acts on a body to oppose motion. We consider two cases; when the object is moving - kinetic/dynamic friction, and when the object is stationary - static friction.

In the first case, friction is constant;
F = @mg
where m is the mass of the object, g is the acceleration due to gravity and @ is the coefficient of dynamic friction (I haven't got a mu on my keyboard!). It will act as long as the object is moving, in an attempt to bring it to a halt.

Static friction on the other hand aims to prevent an object from starting to move. In this case,
F <= &mg
where & is the coefficient of static friction.

Imagine trying to push a car. One person pushing may not be exerting enough force to start it moving. Static friction is keeping it in place. However, 2 people may exert a large enough force to get it rolling. In this case their force would have to be greater than &mg because otherwise static friction would keep it in place.

The "<" less-than sign is very important. Static friction only acts to oppose a force threatening to move an object; it cannot be greater than it because else the object would move in the opposite direction!!!

Finally, friction is essentially a force caused by tiny chemical bonds between the surface of the object and the floor. To start the object moving we have to break these bonds and so a little more energy is required to start it moving than to keep it moving.

I hope by now that the answer to your question is clear; either an object is moving (in relation to the floor) or it isn't. Therefore, either static friction or dynamic friction is exerted by the floor, not both.

I hope this helps. If you have any further questions please don't hesitate to contact me.

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Comments:
: Hi Stephanie! : Sorry that the reply's a little late. This is basically what I know about friction. It might not be 100% accurate but it got me through A level Physics and the 1st year at Uni. : Friction is a force which acts on a body to oppose motion. We consider two cases; when the object is moving - kinetic/dynamic friction, and when the object is stationary - static friction. : In the first case, friction is constant; : F = @mg : where m is the mass of the object, g is the acceleration due to gravity and @ is the coefficient of dynamic friction (I haven't got a mu on my keyboard!). It will act as long as the object is moving, in an attempt to bring it to a halt. : Static friction on the other hand aims to prevent an object from starting to move. In this case, : F <= &mg : where & is the coefficient of static friction. : Imagine trying to push a car. One person pushing may not be exerting enough force to start it moving. Static friction is keeping it in place. However, 2 people may exert a large enough force to get it rolling. In this case their force would have to be greater than &mg because otherwise static friction would keep it in place. : The "<" less-than sign is very important. Static friction only acts to oppose a force threatening to move an object; it cannot be greater than it because else the object would move in the opposite direction!!! : Finally, friction is essentially a force caused by tiny chemical bonds between the surface of the object and the floor. To start the object moving we have to break these bonds and so a little more energy is required to start it moving than to keep it moving. : I hope by now that the answer to your question is clear; either an object is moving (in relation to the floor) or it isn't. Therefore, either static friction or dynamic friction is exerted by the floor, not both. : I hope this helps. If you have any further questions please don't hesitate to contact me.

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