Newton's Third Law: Explaining Why You Can't Lift Yourself

[astrololo]

Homework Statement

I need to explain why I can't lift myself by pulling my shoes' laces.

Homework Equations

Newton's third law

The Attempt at a Solution

My explanation is that by pulling on your laces, there is always going to be a force that goes against your force, so this is why you can't pull yourself up. (So we're using NEwton's third as an explanation) Is this correct ?[/B]

 

[Herman Trivilino]

Homework Statement

I need to explain why I can't lift myself by pulling my shoes' laces.

The Attempt at a Solution

My explanation is that by pulling on your laces, there is always going to be a force that goes against your force, so this is why you can't pull yourself up. (So we're using NEwton's third as an explanation) Is this correct ?[/B]

No this is not correct. Let me rephrase your answer in a way that might help you see this.

You pull up on your laces, your laces pull down on you. One of these two forces (they're an action/reaction pair) is on your shoe laces. The other is on you. Since the forces act on different objects, they can't cancel each other out. For two forces to be able to cancel each other they would need to act on the same object.

 

[andrewkirk]

I think they may be looking for a little more detail than that. Your hands exert an upwards force on the shoelaces. What is the opposite force to that? What exerts it, and upon what? How does that opposite force end up eventually countering the lift? There's an old song about Ezekiel and bones that may help with this.

 

[astrololo]

No this is not correct. Let me rephrase your answer in a way that might help you see this.

You pull up on your laces, your laces pull down on you. One of these two forces (they're an action/reaction pair) is on your shoe laces. The other is on you. Since the forces act on different objects, they can't cancel each other out. For two forces to be able to cancel each other they would need to act on the same object.

Sorry, I didn't phrase correctly what I meant. But is what you wrote the explanation for why I cna't pull myself ?

 

[Herman Trivilino]

Sorry, I didn't phrase correctly what I meant. But is what you wrote the explanation for why I cna't pull myself ?

No, of course it's not. I merely explained why your answer was wrong. You said that two equal-but-opposite forces produce a net force of zero. An action/reaction pair of of forces can never produce a net force of zero because they act on different objects.