- #1
aqdus
- 2
- 0
what will be tension on string pulled by equal but opposite forces?
What will be tension on string pulled by equal forces?
- Context: High School
- Thread starter aqdus
- Start date
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- Tags
- Forces String Tension Tension force
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Discussion Overview
The discussion revolves around the tension in a string that is being pulled by equal but opposite forces. Participants explore the implications of Newton's third law and the conditions under which tension exists in the string, considering both theoretical and conceptual aspects.
Discussion Character
- Conceptual clarification
- Debate/contested
- Mathematical reasoning
Main Points Raised
- One participant questions the tension in the string, suggesting that since there are equal and opposite forces, the net force should be zero, leading to zero tension.
- Another participant asserts that according to Newton's third law, the tension in the string will be equal to the forces applied at either end, despite the net force being zero.
- A participant emphasizes the importance of correctly identifying action-reaction pairs in free body diagrams, noting that the forces acting on the string do not constitute an action-reaction pair.
- It is proposed that the forces at each end of the string correspond to different interactions, and that tension can exist even when the net force is zero, as long as the center of mass does not accelerate.
- One participant uses the analogy of an elastic band to illustrate that while the center of mass may not move, the band will stretch, indicating the presence of tension.
Areas of Agreement / Disagreement
Participants express differing views on the relationship between net force and tension in the string. While some agree on the application of Newton's laws, others challenge the reasoning behind the existence of tension when net forces are balanced. The discussion remains unresolved regarding the implications of these concepts.
Contextual Notes
Participants reference assumptions about the mass of the string and the nature of forces involved, which may affect the conclusions drawn. The discussion does not resolve the complexities surrounding these assumptions.
- #2
- #3
aqdus
- 2
- 0
but in both cases there will be two but opposite forces on string so net force will be zero on string why the tension is not zero ?
- #4
staralfur
- 11
- 2
When you draw a free body diagram be careful not draw both forces in a action-reaction pair. In a action-reaction pair the forces don't act on the same body. If you and I both pull the string with equal but opposite forces, I am the body which you apply force to and you are the body I apply force to. The sketch I made wasn't a FBD as it contained both forces.
- #5
nasu
Homework Helper
- 4,472
- 921
The two forces acting at the two ends of the string are not an action-reaction pair. They correspond to two different interactions.
At one end (let's say where you pull by hand) there is the interaction hand-string. You pull with F1, the string pulls with -F1.
At the other end you have interaction between the string and wall and again you have F2 on the string and -F2 on the wall.
If the center of mass of the string does not accelerate, you will have F1=F2 (in magnitude) and the net force is zero. This does not prevent tensions developing in the string.
The fact that the net force is zero just tell you that the acceleration is zero. But they don't have to be equal.
Now, usually the string is assumed massless in elementary problems so even when we have acceleration the two forces are taken as equal. And the tension along the string is assumed to be constant and equal to either one of these forces.
So, to go back to the point, you can and many times do, create tension in a body by applying equal forces at different points of the body.
At one end (let's say where you pull by hand) there is the interaction hand-string. You pull with F1, the string pulls with -F1.
At the other end you have interaction between the string and wall and again you have F2 on the string and -F2 on the wall.
If the center of mass of the string does not accelerate, you will have F1=F2 (in magnitude) and the net force is zero. This does not prevent tensions developing in the string.
The fact that the net force is zero just tell you that the acceleration is zero. But they don't have to be equal.
Now, usually the string is assumed massless in elementary problems so even when we have acceleration the two forces are taken as equal. And the tension along the string is assumed to be constant and equal to either one of these forces.
So, to go back to the point, you can and many times do, create tension in a body by applying equal forces at different points of the body.
- #6
- 29,616
- 21,427
aqdus said:
Do you really expect an elastic band not to stretch if you pull it from both ends? Its centre of mass won't move because there is no nett force, but it's certainly going to stretch.
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