Newton's 3rd law: Action/reaction. Need someone to illustrate~

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hellocubed

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Hey Guys, this is a very basic physics concept that has been just sailing straight over my head. I was wondering if anyone could help me out.

Newton's 3rd law: every action has an equal and opposite Reaction (acting on opposite bodies).

So we have a horse dragging a cart.
The Horse is moving forward, and hence should impart an equal and opposite force on the cart backwards.
But looking at the cart, it's going the same direction of the horse.

So... what is the opposite force?

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howlovely

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In your example, the horse is pulling the cart, but the cart is also pulling on the horse. The cart pulling on the horse completes the action-reaction pair.

Since the horse and the cart are connected, they're part of the same system. Thus, the forces that they exert on each other are internal forces and since they're equal and opposite, they are not considered in the equation Fnet = ma. (Note: even if Fnet = 0, the horse and cart could still be traveling at constant velocity.)

In this situation it's perfectly valid to view the horse and cart as a single object.

I think it's more difficult to imagine Newton's Third Law just because the horse is a living object. It's a lot easier to imagine two blocks connected with a taut string. When an external force is applied on block 1, block 1 pulls along block 2 because they're connected with string. The force of tension from block 1 on block 2 is equal to the force of tension from block 2 on block 1.

Hope this makes sense :) As I find with most concepts in physics, it's a lot easier to understand when you don't think too hard (overthink).
 
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