# Pushing a cart, using Newton's Third Law

#### Libohove90

I am trying to fully understand Newton's Third Law. I am getting there, but there are a few examples that sort of have me scratching my head for a bit.

Me and a cart are stationary, then I decide to push the cart. I exert a force on the cart and simultaneously, the cart exerts an equal and opposite force on me.

If this occurred in space, the cart and I would have went in opposite directions after the push (conservation of momentum), but on the Earth, this is not apparent. I realize the friction of the floor has a role here. Why is it that both the cart and I accelerate in the same direction as I push it?

Can someone clear this up a bit for me? I would gratefully appreciate informative replies :)

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#### willem2

I am trying to fully understand Newton's Third Law. I am getting there, but there are a few examples that sort of have me scratching my head for a bit.

Me and a cart are stationary, then I decide to push the cart. I exert a force on the cart and simultaneously, the cart exerts an equal and opposite force on me.

If this occurred in space, the cart and I would have went in opposite directions after the push (conservation of momentum), but on the Earth, this is not apparent. I realize the friction of the floor has a role here. Why is it that both the cart and I accelerate in the same direction as I push it?

Can someone clear this up a bit for me? I would gratefully appreciate informative replies :)
You're not only pushing the cart, you are also pushing the earth in the opposite direction. Both of them will push back. The earth is so massive it will hardly move at all, but the cart will move if you push it. To keep contact with the cart you must push the earth harder, so it will push back harder than the cart, so you will move in the same direction as the cart.

#### Libohove90

You're not only pushing the cart, you are also pushing the earth in the opposite direction. Both of them will push back. The earth is so massive it will hardly move at all, but the cart will move if you push it. To keep contact with the cart you must push the earth harder, so it will push back harder than the cart, so you will move in the same direction as the cart.
So the fact that the Earth is pushing me more than the cart is, is what causes me to accelerate in the same direction as the cart?

Why must I push the Earth harder to keep contact with the cart?

#### willem2

So the fact that the Earth is pushing me more than the cart is, is what causes me to accelerate in the same direction as the cart?

Why must I push the Earth harder to keep contact with the cart?
If you push the cart and the earth with the same force, you will stay in one place, because the net force on you is 0. The earth will stay in the same place because it is so massive, but the cart will accelerate. If you keep pushing it, it will roll beyond the reach of your
arms and you can no longer push it.

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