# 什麼是外力?

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!

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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&#39;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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