[sunman11010]

This is a pretty good question and I have no clue for the answer, can someone help?:confused:

According to newtons third law , when a horse pulls on a cart, the cart pulls back with an equal force on the horse. If, in fact, the cart pulls back on the horse as the horse pulls forward on the cart, how is it possible for the horse to move the cart ?

[Capuchin]

Remember that F = ma, now see if you can answer it, if not then I will help you further.

[sunman11010]

I kind of understand what you are getting at, but I'm still not entirly sure

[charlottethedinosaur]

F=ma, so the force on an object is equal to its mass multiplied by its acceleration. The cart has mass, but no acceleration.

[sunman11010]

Ooh OK thanks, I was thinking along the lines of that the horse has more mass than the cart so it will be able to move it... but thanks

[Capuchin]

Sunman, you are right, charlotte doesn't make any sense, if the cart has no acceleration how can it move??

Think about what happens between the horse and the earth, the earth is hugely massive, so although the force on the earth is the same as the force on the horse, the earth only accelerates a little away from the horse, and the horse accelerates a lot faster away from the earth.

It's important to view the horse-earth-cart system as a whole, and not in individual parts.

[sunman11010]

Now I'm confused, then what is the right answer ?

[charlottethedinosaur]

What I meant by the cart not having acceleration, was that it isn't moving away from the horse on it's own. Because the cart is an inanimate (sp?) object, so it won't move unless the horse moves it.

[sunman11010]

Capuchin I think you are making the question a lot more complicated than it is. I think I kind of understand what charlotte is trying to say. But I'm still not certain that those are 100% correct, but at least I'm starting to get an idea

So does anybody have any other ideas ?

[cool_dude]

Actually charlotte doesn't make sense and capuchin makes more sense. I had the same question in my physics class. Darn I can't remember the explanation. It had something to do with forces acting in different directions.

Edit: Capuchin I don't think the earth part plays any role in it. But you should know more since I believe your majoring in physics.

[sunman11010]

Lol, this is really bugging me. I can't think of any explanation...

[asterisk_man]

Capuchin is 100% correct. :)
Sadly for everyone else, you're 100% wrong. :(

You must not ignore the Horse/Earth forces.

[Capuchin]

Okay

The horse cannot provide any force to the cart if the earth isn't involved, otherwise the horse would just be moving his legs in free space, a humorous image, but useless to us.

Seeing as the horse and cart are unmoving on their own, we can treat them as a single entity, I will call this entity a horsecart.

Now, the horsecart pushes backwards on the earth, using the chemical energy stored in the horse's muscles, the earth pushes back with an equal force (Newton's third law). Since a = F/m we see that the acceleration of the earth caused by the horsecart is tiny, and the acceleration of the horsecart caused by the earth is of an order that is measurable.

This causes the horsecart to accelerate. This movement is completely independent of the force between the horse and the cart, since they are a single entity.

You can see that it is essential to treat the horse, cart AND EARTH as a part of the system, there will be no movement without the earth.

Charlottes argument of "the cart has no acceleration and so cannot provide a force" is incorrect. If you are in the horse's frame of reference, then the cart is accelerating away from the horse, and is so providing a force. Just because we are not accelerating and so we are in the cart's frame of reference, doesn't mean that no force exists. (If charlottes argument were valid, it would be a direct contradiction of newton's third law.)

[jcpenny]

The horse is puuling on the Cart.

The cart is pulling on the Horse.

They don't cancel each other out because the forces are acting on different objects.

[charlottethedinosaur]

...This causes the horsecart to accelerate. This movement is completely independant of the force between the horse and the cart, since they are a single entity.

You can see that it is essential to treat the horse, cart AND EARTH as a part of the system, there will be no movement without the earth.

Charlottes argument of "the cart has no acceleration and so cannot provide a force" is incorrect...

Oh, I kindof get it now.
I still don't see how the earth fits in,
But I understand that the horse and cart are moving together as, "Horsecart".
Amirite?
Or am I still not getting it?

(And I also appreciate you not calling me a moron or something, you were rather polite in telling me I was wrong. After all, My experience is in biology. Haha.)

:D

[Capuchin]

Well, the point is that the horse and cart on their own floating in free space can not accelerate, You need the Earth to be there in order for the horse to provide the force.

The trick with the question is that it is worded without the Earth and so most people don't include it in the system, but it is an integral part of the solution.

[charlottethedinosaur]

Well, the point is that the horse and cart on their own floating in free space can not accelerate, You need the Earth to be there in order for the horse to provide the force.

The trick with the question is that it is worded without the Earth and so most people don't include it in the system, but it is an integral part of the solution.

I see...
I thought you meant the Earth's rotation on it's axis was responsible for the cart moving.
But now I understand that you just need the Earth && it's gravity for a push.

:D

[Capuchin]

You don't need it's gravity, you just need another object for the horse(cart) to push against. The fact is that the Earth is so massive that it hardly accelerates for a large acceleration of horse and cart. :)

[Bunniekun]

OK Capuchin is right about the Earth + everything else view BUT what he is asking is but a simple probably Grade 11 Physics question which does NOT need the Earth in the picture all sunman actually needs to know atm, is as you said before F = ma or that even thought the cart is pulling back, the horse exterts more Total Force therefore moving the cart Charlotte's first explanation was enough but Capuchin you just added too much into a simple question. Simplely the horse horse accelerates more and increases it's Net Force while the cart stays stationary. Because this is Grade 11 Phyics not 12 or University so really keep it simple.

[angel1106]

Well, the point is that the horse and cart on their own floating in free space can not accelerate, You need the Earth to be there in order for the horse to provide the force.

The trick with the question is that it is worded without the Earth and so most people don't include it in the system, but it is an integral part of the solution.

You probably are right about the earth's rotation and such, however this is grade 11 physics, no-one in hell is going to ask an 11th grade student to put into account everything to do with the earth's gravity and such, in a simple question requiring Newton's third law. Charlottethedinosaur's answer to the original question was more close to the curriculum