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Old Apr 18th 2009, 01:32 PM   #1
s3a
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Newton's 3rd Law (Law of Action-Reaction) Theoretical Question

"1. Write a description of the reaction force for each of the following forces:

(c) A crane exerts a force of 6000 N[up] on a steel girder that it is lifting."
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My question is:

If the crane exerts a force of 6000N upward, does that mean that there is an unbalanced force on the steel girder for it to accelerate upwards?

I am asking because this section of my book is dedicated to Newton's 3rd law (the Law of Action-Reaction) and in my logic, if the steel girder were to react with 6000N downward then it wouldn't be moving upwards in the first place. Can someone please clear this up for me?
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Old Apr 18th 2009, 09:34 PM   #2
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You are right. There is an unbalanced force if there is an acceleration.
If the girder is only suspended, its weight acting downwards will be balanced by the tension in the cable of the crane and you have your
action - reaction pair.
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