0

I am having a hard time understanding the tension force, especially when thinking about it at a molecular level.

Suppose I have a rope attached to a block which is sitting on frictionless surface. When I pull the rope, the molecular bonds in the rope get stretched. As a result, they try to restore (go back to "normal" position). Does this restoration pull me back or does it pull the other molecular bonds in the rope all the way to the block, until it pulls the block? or both. If the rope does pull me back, it can't be with the same force I pulled it, right? Otherwise the block would not move at all. Also, if the rope pulls the block, does the block pull the rope back too?

Is it fair to say: The person pulls the rope to the right, and the rope pulls the person to the left. The rope pulls the block to the right and the block pulls the rope to the left.

Can anyone help me understand that?

Pablo Mello
  • 35
  • It is completely fair and correct to state things as you did in you summary. – garyp Sep 20 '17 at 02:01
  • "The person pulls the rope to the right, and the rope pulls the person to the left. " correct – JMLCarter Sep 20 '17 at 02:20
  • So if the rope pulls back the person, does it pull back with how much of a force? Also, suppose I shove someone, Why don't I get shoved back with the same force? – Pablo Mello Sep 20 '17 at 02:33
  • This is exactly what Newton's Third Law is talking about. – Johnathan Gross Sep 20 '17 at 03:42
  • -1. Not useful. Your understanding is correct. You do not seem to be confused about anything. There is nothing to explain here. – sammy gerbil Sep 21 '17 at 10:41
  • 2
    Possible duplicate of With Newton's third law, why are things capable of moving? – sammy gerbil Sep 21 '17 at 11:01
  • I wonder whether this question is actually not a perfect duplicate of this old one, due to the question lost in the large middle paragraph: "Does this restoration pull me back or does it pull the other molecular bonds in the rope all the way to the block". I changing the title to reflect my opinion. Pablo, please feel free to unroll it if you disagree. – stafusa Sep 21 '17 at 12:24
  • @sammygerbil I think you did not understand my question. I understand Newton's third law. But what I was confused about was the force of magnitude. Punching a wall is not the same as punching a pillow, even if the magnitude of the force is the same. I later learned that the wall and the pillow don't push back with the same force, because it converts that energy into heat and other forms of energy. Newton's third law is confusing if one does not consider energy conservation. I guess you did not get from my question before. – Pablo Mello Sep 25 '17 at 05:39
  • @stafusa I don't agree. Please, read my comment above. – Pablo Mello Sep 25 '17 at 05:41
  • "Also, suppose I shove someone, Why don't I get shoved back with the same force?" You definitely do get push back with the same force. But don't equate force with speed. Your opponent falls from your push, but your different balance or initial motion absorbs the force one your own body. Forces are the same buy the results in motion might be different – Steeven Sep 25 '17 at 05:46
  • @PabloMello, do you realize that I'm, like you, disagreeing with sammygerbil's comment? In my comment I say I don't think your question is a duplicate: you think it is? – stafusa Sep 25 '17 at 05:46
  • @PabloMello, but here you're mistaken: "don't push back with the same force", of course it does. What happens is that the pillow, differently from the wall, is allowing you to deform it easily, "moving away" from your hand, and your punch is not exerting as much force on it as it would on a immovable wall. Think of how much force you can exert on a particle of vanishing mass (which hurtles at $v\to c$ as soon as your fist touches it) or an ideal spring of constant $k\to 0$ (which deforms arbitrarily under the slightest pressure). – stafusa Sep 25 '17 at 05:54
  • @PabloMello Whenever you push something with a force of magnitude $F$, that force always pushes back with a force of the same magnitude $F$. That is Newton's 3rd Law. If you are asking about energy conservation, which is different, you need to edit your question to make that clear. – sammy gerbil Sep 25 '17 at 13:01

2 Answers2

1

It may be easier to think of a stretched spring than a rope. A spring pulls its ends together. If you are attached to one end of the spring, and a block is attached to the other, you will be pulled towards each other. The same force is exerted by each end of the spring.

Suppose you had three identical springs identically stretched. One connects you to a wall, then second connects you to a block, and the third the block to a wall. Two identical springs pull you in opposite directions. The forces cancel. The overall effect is the same as if no forces were acting on you.

You can think of a spring as many short springs connected together. Each connection between short springs is pulled equally left and right.

If you have two identical blocks at the end of a spring on a frictionless surface, the forces on the blocks will accelerate them toward each other with identical accelerations.

If you and a block are attached to the spring, you will be pulled toward each other with equal force. Since the block is smaller than you are, the block will have a larger acceleration.

mmesser314
  • 38,487
  • 5
  • 49
  • 129
0

The force is equal and opposite according to good old Newton's 3rd law.You see if you pull the rope and rope pulls you back, there is no problem as the two forces are being applied on different objects. Since your mass is a lot greater than the rope you don't feel the pull a.k.a. the acceleration. Instead of rope, if you pulled a big rock, the rock wouldn't move, rather you would feel a pull towards the rock. In daily life you may miss that pull of rock. But suppose you are standing on a surface with very low coefficient of friction and the rock is on a surface of greater friction. Now if you pull the rock, you will move towards the rock. It is a simple experiment you can do on your own.

So, you are pulling the rope and the rope is pulling you back. A similar thing occurs during a falling object. The object pulls the Earth with the exact same force as the Earth's pull to the object. But only the object moves due to its low mass compared to Earth.

Samapan Bhadury
  • 938