# Newton's laws of motion

## Homework Statement

A tractor is pulling a heavy weight behind it by a rope. If, according to Newton's third law, the load is pulling back as hard as the tractor is pulling forward, then why does the tractor move?

Something else I don't really get:
A fishline will break when a force of more than 600n is exerted on it. Two people pull on the line in opposite directions with a force of 400n each. Will the line break?

## The Attempt at a Solution

For the first question, I assume that if the tractor moves forward with a force of 100n, the weight will just pull back 100n, canceling out any movement.
For the second question....I don't even know how to start
Any help or suggestions would be appreciated, and thx in advance =D

Dick
Homework Helper
You had really better go back to the first one if you concluded tractors can't move. They do. I've seen them. Are the only forces involved between the tractor and the thing it's pulling? Isn't there a third body involved??

ideasrule
Homework Helper
For the first question, I assume that if the tractor moves forward with a force of 100n, the weight will just pull back 100n, canceling out any movement.
For the second question....I don't even know how to start
Any help or suggestions would be appreciated, and thx in advance =D

The tractor cannot "move forward with a force of 100n". A force is something applied by an object to another object. So what object is the tractor applying force to?

errrr
I'm not really sure to be honest
I'm assuming the tractor is applying force to the ground
So if the tractor exerts 100n onto the ground and the ground pushes the tractor 100n forward? Then the tractor exerts force on the load right? But wouldn't the load pull back with the exact same force and stop the tractor? And yes -.- i do know that tractors move

ideasrule
Homework Helper
errrr
I'm not really sure to be honest
I'm assuming the tractor is applying force to the ground
So if the tractor exerts 100n onto the ground and the ground pushes the tractor 100n forward? Then the tractor exerts force on the load right?

Yes and yes.

But wouldn't the load pull back with the exact same force and stop the tractor?

No. Why would it? Newton's laws say that if the tractor applies 100N to the ground, the ground applies 100N to the tractor. Similarly, if the tractor applies force F to the load, the load applies force F to the tractor. There's no reason why F has to be 100N, and as you discovered, it can't be if the tractor were to move.

So,
tractor applies 100n to ground
ground applies 100n to tractor