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什麼是外力?

什麼是外力? 外力包含地心吸力和normal reaction嗎...

Update:

Here is another question.

A body will maintain uniform motion if there is no external force acting on it. Is that right?

Please explain in detail. THX!

1 Answer

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  • 1 decade ago
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    for the question what external force is, it just depends on how you define the system.

    For example, there is a block B placed on the floor and another block A placed on top of block B and there is friction between all contact surfaces.

    if we take both blocks as a system,

    then the external force is just the frictional force acting on block B by the floor

    but if we onli take block B as a system,

    then the external forces are frictional force acting on block B by the floor AND also the frictional force acting on block B by block A.

    according to Newton's first law, if there is no external force acting on a object, there are 2 cases.

    case 1:

    if the object is stationary, then it will stay stationary

    case 2:

    if the object is moving, then it will keep on moving with uniform velocity

    so it is RIGHT to say that a body will maintain uniform motion if there is no external force acting on it.

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