Two Questions on Newton Law Problems

• Lori
In summary, the first problem deals with the application of the action-reaction law and the comparison of forces between two boxes stacked on top of each other. The second problem involves determining the tension at different points on a rope supporting a hanging object and understanding the concept of net force.
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)?

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?

Lori said:
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 ...
Lori said:
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
kuruman said:
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

Last edited by a moderator:
Lori said:
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?
Lori said:
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
kuruman said:
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!

Yes and yes.

1. What are Newton's three laws of motion?

Newton's first law states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless acted upon by an external force. Newton's second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Newton's third law states that for every action, there is an equal and opposite reaction.

2. How do I solve problems involving Newton's laws?

To solve problems involving Newton's laws, you must first identify the forces acting on the object in the problem. Then, use Newton's second law to calculate the net force and acceleration of the object. Finally, use this information to solve for any unknown quantities, such as velocity or distance.

3. What is the difference between mass and weight?

Mass is a measure of the amount of matter in an object, while weight is a measure of the force of gravity acting on an object. Mass is measured in kilograms (kg), while weight is measured in newtons (N).

4. How do Newton's laws apply to everyday life?

Newton's laws of motion can be seen in everyday life in activities such as driving a car, throwing a ball, or riding a bike. These laws help explain the motion and forces involved in these activities and are essential for understanding the physical world around us.

5. Can Newton's laws be broken?

No, Newton's laws are fundamental laws of physics and have been tested and proven to hold true in countless experiments and real-world scenarios. However, in extreme conditions such as in outer space or near the speed of light, these laws may behave differently, but they are not technically being broken.

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