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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: ChickenCh0wM1en on March 08, 2013, 09:14:35 pm

Title: propulsion + newtons 3rd law of motion
Post by: ChickenCh0wM1en on March 08, 2013, 09:14:35 pm

Hey Guys,

I have a question regarding these 2 concepts.

I understand that for Newtons 3rd law of motion that a there is a equal reaction/action force which occurs for literally every force.

And I see that propulsion is the "force which drives an object forward" making it accelerate?

Seeing that propulsion would do this, and taking Newton's 3rd law into account, wouldnt the action/reaction pair cancel out thus making the acceleration 0?

Sorry if I have made any mistakes! I'm very new to these concepts

Thank You!! :D

Title: Re: propulsion + newtons 3rd law of motion
Post by: Lasercookie on March 08, 2013, 09:47:23 pm

Third law action-reaction pairs don't cancel out because they act on different objects. e.g. say you pressing keys on your keyboard. You have the force from your fingers (object 1) acting on the keys (object 2), whilst the keys (object 1) will be applying a force on your fingers (which will be of "equal size and opposite direction")

Title: Re: propulsion + newtons 3rd law of motion
Post by: Ancora_Imparo on March 08, 2013, 09:59:32 pm

Equal and opposite forces cancel out only when they are acting on the same body. Eg: Take a book resting on a table. The forces on the book are its weight force pushing down on the table and its normal force which is upward. Both forces are equal and opposite and are both acting on the book. Thus, they cancel and the book's acceleration is 0.

Title: Re: propulsion + newtons 3rd law of motion
Post by: Conic on March 08, 2013, 10:31:47 pm

Quote from: Ancora_Imparo on March 08, 2013, 09:59:32 pm

Equal and opposite forces cancel out only when they are acting on the same body. Eg: Take a book resting on a table. The forces on the book are its weight force pushing down on the table and its normal force which is upward. Both forces are equal and opposite and are both acting on the book. Thus, they cancel and the book's acceleration is 0.

Also, when the 2 forces act on one body I'm pretty sure it isn't an action reaction pair.

Title: Re: propulsion + newtons 3rd law of motion
Post by: Ancora_Imparo on March 08, 2013, 10:55:45 pm

Quote from: Conic on March 08, 2013, 10:31:47 pm

Also, when the 2 forces act on one body I'm pretty sure it isn't an action reaction pair.

Correct. The action/reaction pairs in the 'book on a table' example are:
1) Gravitational pull downward on book by Earth/Gravitational pull upward on Earth by book
2) Table pushing book up/book pushing table down