How does the object move if forces between two objects are equal and opposite?

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In summary: The forces on the car and on the tractor must be equal and opposite (per Newton's 3rd law).In summary, in this conversation, the concept of Newton's 3rd law is discussed and how it relates to the motion of objects. The key point is that the forces between two objects are always equal and opposite, but what determines the motion of an object is the net force acting on it. In the example given, the van is able to pull the car because there is a net force in the forward direction, most likely due to friction. The forces of the ground on the van and the mud on the car can be thought of as friction. Additionally, the concept of analyzing a system as a whole or individual objects
  • #1
Overall01
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Hello,

If forces between two objects are equal and opposite, then how can a van pull a car if they were attached with, say, some rope (e.g)?

Surely the force of the car on the rope is equal and opposite to the force of the rope on the car. So is it because the van and car are of different masses or something to do with friction?

I'm not entirely sure, so any help would be much appreciated. Please base advice to secondary school (year 11) /high school student (10th grade). Thanks.
 
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  • #2
By Newton's 3rd, if object A exerts a force on object B, then object B will exert an equal and opposite force on object A. The key point is that these forces are acting on different bodies. So the motion of some object depends only on the forces that are acting on that object.
 
  • #3
CAF123 said:
By Newton's 3rd, if object A exerts a force on object B, then object B will exert an equal and opposite force on object A. The key point is that these forces are acting on different bodies. So the motion of some object depends only on the forces that are acting on that object.

Thanks for the reply.

I understand that it's because they are of different bodies (and hence masses) but could you please explain this with regards my example, since this is where I don't understand.

If a van is attached to a (stationary) car which is say, stuck in some mud, with rope and the van begins accelerating, why does the car also move? I believe it may be due to the force of the ground on the van, and the force of the mud on the car, but I'm quite confused. Does this mean friction?
 
  • #4
Overall01 said:
If forces between two objects are equal and opposite, then how can a van pull a car if they were attached with, say, some rope (e.g)?
What determines whether an object accelerates is the net force on it (per Newton's 2nd law). The fact that all forces between any two objects are always equal and opposite (per Newton's 3rd law) is irrelevant.

So, for the van to accelerate, there must be a net force in the forward direction. Most likely friction, as you suggest. Net force on van = Friction - force of car pulling back on van (via the rope).

Since they are attached via the rope, the van and car must both move forward. So that means that the net force on the car must be in the forward direction as well.
 
  • #5
http://imageshack.us/a/img825/8569/19182213.jpg
 
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  • #6
azizlwl said:
http://imageshack.us/a/img825/8569/19182213.jpg

azizlwl
isn't the horse and cart system acting effectively as the same object?
 
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  • #7
Doc Al said:
What determines whether an object accelerates is the net force on it (per Newton's 2nd law). The fact that all forces between any two objects are always equal and opposite (per Newton's 3rd law) is irrelevant.

So, for the van to accelerate, there must be a net force in the forward direction. Most likely friction, as you suggest. Net force on van = Friction - force of car pulling back on van (via the rope).

Since they are attached via the rope, the van and car must both move forward. So that means that the net force on the car must be in the forward direction as well.

azizlwl said:
http://imageshack.us/a/img825/8569/19182213.jpg
Thanks for the reply guys.

I'm still a little confused though with regards:

http://s15.postimage.org/6nn54clt7/Inthemud.jpg

What is the force of ground on tractor and force of mud on car? Is that friction? There's only one arrow, so is that the net force?

Just another thing, it doesn't mention the car moving due to the pull of rope on car; only because of the force of ground on tractor is greater than force of mud on car? So which is it?

Sorry for all the questions but I'd appreciate any help.

Thanks in advance.
 
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  • #8
Another example is an Arwood's Machine.
The tension on both sides are equal for whatever mass A and mass B.

If mass A is heavier than mass B, even though the tension on both side are equal, mass A and mass B will be accelerating.
 
  • #9
Overall01 said:
Thanks for the reply guys.

I'm still a little confused though with regards:

http://s15.postimage.org/6nn54clt7/Inthemud.jpg

What is the force of ground on tractor and force of mud on car? Is that friction?
Yes.
There's only one arrow, so is that the net force?
No, there will also be the pull from the rope (tension force). As Doc Al said, net force on car will be in same direction as the net force acting on the object moving it.
 
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  • #10
Overall01 said:
What is the force of ground on tractor and force of mud on car? Is that friction?
You can think of that as friction.
There's only one arrow, so is that the net force?
There are two arrows. One on the tractor going forward; one on the car going backward. The vector sum of those is the net force on the system of 'tractor plus car'.

Just another thing, it doesn't mention the car moving due to the pull of rope on car; only because of the force of ground on tractor is greater than force of mud on car? So which is it?
It's up to you to choose the system you want to analyze. Looking at the car by itself, the two forces are the force from the rope pulling forward and the force from the mud pulling backward. Only if the force from the rope is greater will the car accelerate forward.

But if you look at tractor + car as a single system you can ignore the force of the rope, since it would be internal to the system.

But regardless of how you analyze it, the answer must be the same.
 

1. What is Newton's Third Law of Motion?

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that when two objects interact, the force exerted by one object on the other is equal in magnitude and opposite in direction to the force exerted by the other object on the first.

2. How does Newton's Third Law of Motion relate to forces between two objects?

When two objects interact, the forces they exert on each other are equal and opposite according to Newton's Third Law. This means that if one object exerts a force on the other, the second object will also exert an equal force in the opposite direction on the first object.

3. What happens if the forces between two objects are equal and opposite?

If the forces between two objects are equal and opposite, the objects will experience equal and opposite accelerations. This means that the objects will move in opposite directions with the same magnitude of acceleration.

4. Can forces between two objects ever be perfectly equal and opposite?

In theory, forces between two objects can be perfectly equal and opposite. However, in reality, there are always external factors that can affect the forces and cause them to be slightly different. Therefore, forces between two objects are never truly equal and opposite in practice.

5. How does the motion of the objects change if the forces between them are equal and opposite?

If the forces between two objects are equal and opposite, the motion of the objects will depend on their individual masses. The object with the larger mass will experience a smaller acceleration, while the object with the smaller mass will experience a larger acceleration. This results in the objects moving at different speeds in opposite directions.

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