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What is the magnitude of the third force?

Help with Newton's third law (should be pretty easy)?

  • Having some trouble conceptualizing this I'm pushing a book across my desk with my hand and according to Newton's third law, the book is pushing back on my hand with a force that is equal in magnitude, but opposite in direction. If the two forces are equal and opposite, why is the book accelerating in the direction of the force of my hand? I'm guessing that it's because of an external force. If so, what is the external force?

  • Answer:

    Well the two forces are indeed equal and opposite, though there is no net force between these two because they occur on different objects. Force by your hand occurs on the book and force by the book is applied on your hand, therefore ΣF does not exist:)

Comet Meebro at Yahoo! Answers Visit the source

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Forces are: 1. Gravity is pushing down on the book. 2. The book is pushing up against gravity (due to the desk). Also referred to as the normal force, F-subunit N. 3. The book has some value of µ. 4. You are pushing the book (presumably) parallel to the desk, only going against friction. So, you may be pushing at an angle against the book, so your force will be down and across. The book will be pushing back only in the across direction. If you are pushing parallel then your Force MUST be higher than µ if Fn is equal to Gravity. It is really easy to picture these types of problems with free-body diagrams. http://www.physicsclassroom.com/class/newtlaws/u2l2c1.gif

Michael McMahon

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