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# Newton’s 3 rd Law

Newton’s 3 rd Law. Newton’s 2 nd Law : A quantitative description of how forces affect motion BUT : Where do forces come from? EXPERIMENTS show that Forces applied to an object are ALWAYS applied by another object  Newton’s 3 rd Law

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## Newton’s 3 rd Law

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1. Newton’s 3rd Law Newton’s 2nd Law: A quantitative description of how forces affect motion • BUT:Where do forces come from? EXPERIMENTSshow that Forces applied to an object are ALWAYS applied byanotherobject  Newton’s 3rd Law “Whenever one object exerts a forceF on a second object, the second object exerts an equal & opposite force -F on the first object.” • The “Law of Action-Reaction” “Every action has an equal & opposite reaction”. (Note that action-reaction forces act onDIFFERENTobjects!)

2. “If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude & opposite in direction to the force F21 exerted by object 2 on object 1.” As in the figure Another Statement of Newton’s 3rd Law

3. Example: Newton’s 3rd Law When a force is exerted on an object, that force is caused by another object. Newton’s 3rd Law: “Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.” If your hand pushes against the edge of a desk (the force vector shown in red), the desk pushes back against your hand (this force vector is shown in purple, to remind us that this force acts on a DIFFERENT object).

4. Newton’s 3rd Law: Alternative Statements 1.Forcesalwaysoccur in pairs 2.A single isolated forcecannotexist 3.The “action force” is equal in magnitude to the “reaction force” & opposite in direction a. One of the forces is the “action force”, the other is the “reaction force” b. It doesn’t matter which is considered the “action” & which the “reaction” c. The action & reaction forces must act on different objects & be of the same type

5. Action-Reaction Pairs: Act on Different Objects The key to correct the application ofNewton’s 3rd Lawis: The forces are exerted on different objects.Make sure you don’t use them as if they were acting on the same object. Example: When an ice skater pushes against the railing, the railing pushes back & this reaction force causes her to move away.

6. Rocket propulsion can be explained using Newton’s Third Law:Hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket. Note:The rocket doesn’t need anything to “push” against.

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