TPR Science WB Physics #68

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Mar 15, 2010
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An object is being acted upon by two (and only two) external forces, F1 and F2. If the object has a nonzero acceleration, which one of the following must be true?

I narrowed down to 2 choices: 1. The object cannot move at constant speed. 2. The sum F1+F2 is not zero. The answer is choice #2. However, I don't understand why choice #1 can't be true, too? If there is a net force acting on an object, then the object can't move at constant speed isn't it?

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What about an object in orbit? Like if you tie a rock on a string and started swinging it around your head.

It would have 2 forces (gravity and tension). It would have nonzero acceleration because of the centripetal acceleration required to keep the rock on the string.
It could have a constant "speed", not velocity but it didn't say constant velocity.
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