# Newton's 3rd Law

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#1

Scroll down to Example 1: Fridge Push

A person drives a cart, Cart 1, to the right while pushing another cart, Cart 2, that has a massive refrigerator on it. The total mass of Cart 2, cart plus fridge, is three times the total mass of Cart 1, cart plus person. If the person is driving with enough force that the two carts accelerate to the right, what can be said for sure about the magnitudes of the forces on the carts?
My question is:
Why is the answer not D?

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Force = Mass*Acceleration, so let's give Cart 1 the pretend mass 5Kg and Cart 2 the pretend mass 10Kg. The masses are different, so to find out the force, there has to be an acceleration. If the acceleration for Cart 1 is 10km/h then the total force will be 50N. If the acceleration for Cart 2 is 5km/h then the total force will be 25N. However, if the force for Cart 1 stayed at 50N, and Cart 2 hit the gas so it accelerated to 20km/h then the total force will be 100N. This means that Cart 2 will overpower Cart 1, whereas before, Cart 1 overpowered Cart 2.
If so, why isn't the answer D - it depends on the acceleration. To simplify, let's call the mass x. Then the force will be xA. Whatever the acceleration is will affect the force.

0
3 years ago
#2

option D:

The force on Cart 2 exerted by Cart 1 could be larger or smaller depending on the size of the acceleration.

and what I think it's trying to say is:

The force on Cart 2 exerted by Cart 1 could be larger or smaller than the force exerted on cart one by cart two depending on the size of the acceleration.
(which it should be clear is false)

The forces aren't constant regardless of acceleration... but they are always equal to each other regardless of acceleration

P.S.

it's force = mass x acceleration... not force = mass x area
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#3
Thanks, and I have some questions:

(Original post by Joinedup)
The force on Cart 2 exerted by Cart 1 could be larger or smaller depending on the size of the acceleration.

and what I think it's trying to say is:

The force on Cart 2 exerted by Cart 1 could be larger or smaller than the force exerted on cart one by cart two depending on the size of the acceleration.
(which it should be clear is false)
Sorry, but I still don't get it. What's the difference?
And how do you know it's false?

(Original post by Joinedup)
The forces aren't constant regardless of acceleration... but they are always equal to each other regardless of acceleration
What does constant mean in this sense?
Why would they always be equal though? I thought my explanation showed that it depended on the acceleration.

(Original post by Joinedup)
P.S.

it's force = mass x acceleration... not force = mass x area
Yeah, sorry, I put that at the beginning accidentally, but I used acceleration for the rest of it. Edited

Thanks
0
3 years ago
#4
(Original post by HowToBeABlobfish)
Thanks, and I have some questions:

Sorry, but I still don't get it. What's the difference?
And how do you know it's false?

What does constant mean in this sense?
Why would they always be equal though? I thought my explanation showed that it depended on the acceleration.

Yeah, sorry, I put that at the beginning accidentally, but I used acceleration for the rest of it. Edited

Thanks
some people have difficulty with the idea of two things being equal in size to each other - but that the size doesn't have to be fixed.

if cart one pushes on cart two with a force of 5N then cart two pushes on cart one with a force of 5N

but cart one pushes on cart two with a force of 7N then cart two pushes on cart one with a force of 7N

it's actually quite difficult enough to understand without kahn academy badly worded questions tbh - haven't you got any notes from a real teacher or a text book?
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