Two Questions on Newton Law Problems

[Lori]

These are conceptual questions, and i just want to really understand why the answer is the answer!

1. Homework Statement
1) A force acts to push two boxes, box 1 and box 2, across a floor (assume for this problem that friction is negligible). The two boxes are stacked such that the force directly acts on box 1 and box 1 exerts a force that pushes box 2. How does the force of box 1 on box 2 compare to the directly applied force?

2)If an object of mass m is hanging from a rope with weight w, what is the difference between the tension at the top of the rope (the part not connected to the object) and the bottom of the rope (the part connected to the object)?

Homework Equations

Action-reaction

Tension

The Attempt at a Solution

1) So, i was thinking that this problem has to do with the action-reaction law (Newton's 3rd law). Since the applied force on box 1, could i say that box 1 is applying the the same force on box 2 as the applied force?

2) I was thinking for this one that the bottom of the rope would be the weight of the object, but I'm not sure if this is right. What can be said about the difference?

 

[kuruman]

So, i was thinking that this problem has to do with the action-reaction law (Newton's 3rd law).

The directly applied force accelerates both boxes. The force of box 1 on box 2 accelerates box 2 only. If the boxes have the same acceleration, then ...

I was thinking for this one that the bottom of the rope would be the weight of the object, but I'm not sure if this is right.

It's right. What about the top of of the rope? It certainly supports the weight of the object. Is there anything else it supports?

 

[Lori]

The directly applied force accelerates both boxes. The force of box 1 on box 2 accelerates box 2 only. If the boxes have the same acceleration, then ...

It's right. What about the top of of the rope? It certainly supports the weight of the object. Is there anything else it supports?

So for problem number1, i cannot tell the difference unless i know the masses of the object?

For problem number 2, the top of the rope would just support the weight of the object ? I don't see anything else it would support

 

[kuruman]

o for problem number1, i cannot tell the difference unless i know the masses of the object?

You can tell the difference. What is the net force on the two masses as compared with the net force on one of the masses. Which one is greater?

For problem number 2, the top of the rope would just support the weight of the object ? I don't see anything else it would support

The top of the rope supports whatever is below it. What do you think that is? What makes it "the top" as opposed to "the bottom" of the rope?

 

[Lori]

You can tell the difference. What is the net force on the two masses as compared with the net force on one of the masses. Which one is greater?

The top of the rope supports whatever is below it. What do you think that is? What makes it "the top" as opposed to "the bottom" of the rope?

1) oh no. I think I understand. I'm kinda disappoint that I didn't understand this since it was on my online reading quiz. Anyways, I just realized more mass means that more force is exerted because F=ma. So the magnitude of the applied force is greater than the force of object 1 on 2.
2) Ohhh! The tension of the rope at the top is equal to the weight of the rope and the weight of the object. The tension of the rope at the bottom is equal to the weight of the object, so if we take the difference, the difference would equal to the weight of the tension!

 

[kuruman]

Yes and yes.