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 ?

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

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

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

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

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.

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

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.

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 ?

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.

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

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

You must not ignore the Horse/Earth forces.

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.)

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.

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Originally Posted by Capuchin

...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...

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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

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.

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Originally Posted by 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.

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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

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. :)

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.

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Originally Posted by 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.

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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

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Originally Posted by angel1106

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

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Okay, please explain how a horse would pull a cart in the vacuum of space, then.

I think you misread what I said, you don't need Earth's rotation, or gravity. You just need the Earth there. It is an integral part of the solution.

Is everyone retarded? Capuchin is corrent, you need the earth. The earth is what allows the horse to move forward. Also in grade 11, you need more than just stating newton's 3rd law to explain this question.

Ok, suppose the horse is 120kg in mass and the cart is 5 kg. The horse supplies 20N in force. So:

Acceleration cart = 20N/5kg =4m/s^2

The cart will excert the same force on the horse, hence:

Acceleration horse = -20/120 = -1/6 m/s^2

Now, we look at the horse and the cart as a system. Their net acceleration would be the sum of their individual acceleration, hence:

Net acceleration = 4-1/6 = 23/6 m/s^2 forward.

This explains why the cart moves forward not back!

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Originally Posted by Bellal

Ok, suppose the horse is 120kg in mass and the cart is 5 kg. The horse supplies 20N in force. So:

Acceleration cart = 20N/5kg =4m/s^2

The cart will excert the same force on the horse, hence:

Acceleration horse = -20/120 = -1/6 m/s^2

Now, we look at the horse and the cart as a system. Their net acceleration would be the sum of their individual acceleration, hence:

Net acceleration = 4-1/6 = 23/6 m/s^2 forward.

This explains why the cart moves forward not back!

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What would happen if the cart were heavier than the horse?? According to your argument the horse and cart would move backwards even though the horse is pulling it forwards. The EARTH is pushing on the Horse-cart. That's why it moves.

Also your calculation is far too complicated and gives the wrong answer, the net acceleration is simply

Actually, when the horse pushes the earth, the earth pushes back with equal and oppsite force, and since the horse can't move the earth(not enough mass) the earth moves the horse witch moves the cart.

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Originally Posted by YESYES922

Actually, when the horse pushes the earth, the earth pushes back with equal and oppsite force, and since the horse can't move the earth(not enough mass) the earth moves the horse witch moves the cart.

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The horse can move the Earth, and it does.

I am just in Grade 12 but I'll still try to help out. Sorry for any wrong terminology. Read Capuchin's post on the second page.

See the thing is in order to move we need something to push off from. Like swimming, if you couldn't push off from the water by kicking or flailing your arms then you wouldn't move. Or like in space where rockets cannot accelerate with external help but rather eject exhaust gas to push forward from.

Otherwise think of this, why is it so important to for people working on space stations to be strapped by something? Because if they happen to float away, they cannot generate any force by themselves to oppose their motion (unless they throw something I guess). So you need something there to push off from.

Similarly, we have the earth to push off from. As Capuchin said the horse and the cart needed to be treated as a single entity in reference to the Earth. Now remember the equation F = ma? Rearrange for acceleration gives us a = F/m. Now take the horsecart and the Earth. If both forces are equal (as a result of Newton's third law) then only their masses need to be considered.

Lets say the force exerted is 100 Newtons, the mass of the earth is 6 × 10 to the power of 24 and the mass of the horse + cart is 50 kg. Plug these values into the equation

Horsecart
a = F/m
a = 100/50
a = 2

Earth
a = F/m
a = 100/(6 x 10^24)
a = 1.666 x 10^-23!

Compare the 2 accelerations, both entities (earth and the horsecart) will move but the earth's movement is almost negligible. Sorry if I am wrong anywhere and if there is any credit then it goes to Capuchin because I would have no way formulated this answer by myself

The first thing that came to me on seeing this question is that the reason was that the net force is zero. Well, not at the beginning but later on, when the cart no more accelerates.

Firstly, the horse 'uses' friction of the ground to move forward, and since there is no net force on the cart, it accelerates in the direction of the pulling horse.

Now, friction builds up as the cart gains speed, and the frictional force becomes equal to the force exerted by the horse.

Finally, as there is not net force (weight is cancelled by Earth's reaction, forward force by friction) the cart still continues to move. This is because "any body continues in its state of rest or motion in a straight line unless acted upon by a force" by Newton I.

Anyway, if that possibility is wrong, someone tell me why, so that I understand it better. I'm too in Grade 11, well I think it's the equivalent of what I'm doing.

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Originally Posted by Unknown008

The first thing that came to me on seeing this question is that the reason was that the net force is zero. Well, not at the beginning but later on, when the cart no more accelerates.

Firstly, the horse 'uses' friction of the ground to move forward, and since there is no net force on the cart, it accelerates in the direction of the pulling horse.

Now, friction builds up as the cart gains speed, and the frictional force becomes equal to the force exerted by the horse.

Finally, as there is not net force (weight is cancelled by Earth's reaction, forward force by friction) the cart still continues to move. This is because "any body continues in its state of rest or motion in a straight line unless acted upon by a force" by Newton I.

Anyway, if that possibility is wrong, someone tell me why, so that I understand it better. I'm too in Grade 11, well I think it's the equivalent of what I'm doing.

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There is always friction, try puching something heavy along a carpet - even when the object is not moving, there is a friction that you have to overcome to get the cart moving in the first place.

The point is that it's not the horse pulling on the cart that provides the motion - because that is exactly offset by the cart pulling on the horse - however it's the horse pushing on the Earth and therefore the Earth pushing back on the horse that does provide the motion.

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Originally Posted by TinTinTinTin

I am just in Grade 12 but I'll still try to help out. Sorry for any wrong terminology. Read Capuchin's post on the second page.

See the thing is in order to move we need something to push off from.

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Capuchin and TinTinTinTin are definitely right.
:) :D

say the first smiley to the left is the horse and the smiley to the right is the cart. Now, imagine that they are connected by a rope.

In order for there to be an action and reaction force between these two, the horse must be moving to the left--in other words, the horse can't be pulling if its not moving to the left! If the horse just stood there and the cart was also there... there wouldn't be an action/reaction force right?

The horse then, is pulling on the cart with F1 and the cart is pulling on the horse with F2 (F1 and F2 are equal in magnitude).

Now, as TinTinTinTin said (and I'm sure Capuchin has metioned something along the same lines), in order for the horse to move to the left, the horse MUST be exerting a force to the right on the floor. According to Newton's third law, if the horse exerts a force to the right on the floor, the floor must be exerting a force of equal magnitude on the horse but to the LEFT. Now lets say this force that the floor exerts is F3. F3, then, is the net force (lets just ignore friction to make it easier to understand... although in reality there must be friction) of the "horsecart" (It is the net force because the action and reaction force between the cart and horse cancels out).


So, a=Fnet/m--> accel of the horsecart=F3/mass of horsecart. The horse's accel is the same as the accel of the whole horsecart and the cart's accel is the same as the accel of the whole horsecart.

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Now IF WE DO CONSIDER FRICTION (You might not want to read this part... since I'm not completely sure about it... ):
we have F1 and F2( the action and reactions forces), F3 the force the earth is exerting on the horse to the LEFT and friction of horse and cart...

so, I'm not exactly sure about this part... help me out if I'm wrong about something :S but in order for the horse and cart to move from rest, F3 must be greater than the max static friction of the horsecart (together). Once the horse + cart start moving, you would have to consider kinetic friction. If they're moving at a constant speed, F3 must equal to the kinetic friction of the horsecart. If they're accelerating, F3 must be greater than the kinetic friction of the horsecart (to give you a net force which is then divided by the mass of the whole horsecart to get your acceleration).

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SORRY IF ANYTHING IS WRONG, I'm in grade 11, and we've just finished unit 1 on mechanics >.<

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Originally Posted by Capuchin

There is always friction, try pushing something heavy along a carpet - even when the object is not moving, there is a friction that you have to overcome to get the cart moving in the first place.

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I'm not telling the opposite... What I think is that the cart is at rest because there is no net force acting upon it. Friction is present, but there is no resultant force, right? Or... wrong? It's Newton's first law... "Every body stays in its state of rest (or motion) unless a force is acted upon it."

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Originally Posted by Capuchin

The point is that it's not the horse pulling on the cart that provides the motion - because that is exactly offset by the cart pulling on the horse - however it's the horse pushing on the Earth and therefore the Earth pushing back on the horse that does provide the motion.

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But, if the horse was not tied to the cart, it would still be pushing on Earth and vice versa. However, the cart will not move. The horse uses friction and the reaction force to move forward, and hence move the cart :confused::confused::confused:

Consider all the forces acting on the horse and the cart. F(pull) is what acts on the cart. This force is directed toward the horse. The is no or little friction forces acting on the cart. This is why the net force applied to the cart is not zero, therefore the cart has to move. Horse is pulled back by the cart with the force equal to F(pull) in magnitude and opposite in direction according to the Newton's law. However, there are friction forces acting on horse's hooves. These friction forces are larger in the magnitude compared to the F(pull). What's interesting is that in this particular case these friction forces are directed the same way the horse moves. This is not unusual. Consider friction forces between a highway surface and car tires.

I was saying the net force was zero when the cart was initially at rest and at terminal velocity. When the horse pulls, this creates a force, thus an acceleration, but then, constant speed.

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Originally Posted by cool_dude

Actually charlotte doesn't make sense and capuchin makes more sense. I had the exact 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.

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Yeah you are rite the earth does not matter because both of the objects are on the same frame of reference.

I think what you need to remember here is that:

-the forces act on different objects
-N means acceleration so if the forces are equal, the cart is either in constant motion or no motion, in this case motion

well... I guess I'm late on replying but ill take the good ol college try... im a physics major and after going through hell classes I tend to make simple question super complicated... so I'm going to try to make this simple... well. You can approach this problem in a lot of ways... what tintintintin posted for the first time just simply proved that yea.. the horse moving has an effect on the earth... it didn't answer why the cart moves forward... also one thing that kind of bugged me was that you NEED the earth for this to be possible. Well, you don't... it could be a simple plane in the vacuum of space... just as long as there is friction... its possible anywhere... but anyway.. here's my answer...

First. I would draw a free body diagram and label everything that acts on it... but I'm going to talk about 4 vectors (hopefully I can explain this clearly)... one vector is the force acting on the ground when the horse is pushing off and it points to the left (assuming that the horse is pulling the cart to the right)... another vector is the force acting on the wagon by the horse caused by the horse pulling it (that points to the left) (Note: I'm not putting any values on the vectors right now, thers no need to)... the third and fourth vector are acting on the horse by the ground and by the cart that's pulling on the horse (Newton's third law) the force acting on the horse by the ground is pointing to da right and the force acting on ground cause by the horse is pointing to da left... Note that there is only one force acting on the cart on the x-axis... which is cause by the horse pulling on the cart... now the question is what does it take for the horse to move the cart?. well look at the 3rd and 4th vectors I explained... what it takes is that the force that the ground excerts on the horse is greater that the force of the cart action on the horse... now that's the simplest answer I can give... I didn't especifially go into friction or tension of the rope.. blah blah blah... just simple vectors gave the answer... of course I hope I'm right because I'm still a student and believe it or not answering this question is tough because I took this class ages ago... it always happens to me, when I start taking harder classes the easy problems just seem to be a nightmare... hopefully my explanation wasn't a nightmare... and of course, corrections are welcome since I'm someone will find something that's wrong with this explanation..

Actually the force that the horses pulls the cart on is canceled out by the equal and opposite force that the cart pulls back on the horse. You should look at what the horse does to the earth to be able to apply force to the cart.

For the horse to apply force to the cart it must push on the earth. Now the horse is accelerated forward by the earth. It is this acceleration that propels both cart and horse forward.

The horse pulling on the cart is the same but opposite of the cart pulling on the horse, but as soon as you factor in the acceleration from the earth pushing back on the horse there is unbalanced force. Which allows the horse to pull the cart.

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Originally Posted by Unknown008

But, if the horse was not tied to the cart, it would still be pushing on Earth and vice versa. However, the cart will not move. The horse uses friction and the reaction force to move forward, and hence move the cart :confused::confused::confused:

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This isn't entirely true. You don't need friction to move the cart. If such a place existed, where there was no friction, the act of you breathing in and out would cause a force large enough to cause you to move. Or if you threw an object away from you, you would move in the direction opposite of where you threw.

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Originally Posted by tmrobyn91

the horse pulling on the cart is the same but opposite of the cart pulling on the horse, but as soon as you factor in the acceleration from the earth pushing back on the horse there is unbalanced force. which allows the horse to pull the cart.

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One small error. The forces are balanced. The accelerations are not balanced.

Ok, let me restate my points:

1. Cart and horse are at rest, horse not pulling at all.
2. Horse starts to pull, unbalanced forces are generated and acceleration results.
3. Friction builds up as the speed of the cart increases.
4. When frictional force balances the pulling force of the horse, the acceleration becomes 0 and the velocity remains constant.

That's what I was saying from the beginning. I was not understanding why the Earth had to be considered in the problem, why we had to consider that the Earth pushed on the horse so that it moved if the forces were equal and opposite.