What will be tension on string pulled by equal forces?

AI Thread Summary
Tension in a string pulled by equal but opposite forces is not zero, despite the net force being zero, due to the interactions at both ends of the string. According to Newton's third law, the forces acting on the string from both ends are not action-reaction pairs because they act on different bodies. When equal forces are applied, the string experiences tension as it stretches, even though its center of mass does not accelerate. In elementary physics, strings are often considered massless, leading to the assumption that tension is constant and equal to the applied forces. Thus, applying equal forces at different points can indeed create tension in the string.
aqdus
Messages
2
Reaction score
0
what will be tension on string pulled by equal but opposite forces?
 
Physics news on Phys.org
Newton's thirld law. If you pull a string fixed at the other end it will have the same tension as the if the string is pulled by equal but opposite forces.
 

Attachments

  • Snapshot.jpg
    Snapshot.jpg
    2.8 KB · Views: 715
  • Like
Likes aqdus
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 ?
 
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.
 
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.
 
aqdus said:
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 ?

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.
 
The rope is tied into the person (the load of 200 pounds) and the rope goes up from the person to a fixed pulley and back down to his hands. He hauls the rope to suspend himself in the air. What is the mechanical advantage of the system? The person will indeed only have to lift half of his body weight (roughly 100 pounds) because he now lessened the load by that same amount. This APPEARS to be a 2:1 because he can hold himself with half the force, but my question is: is that mechanical...
Hello everyone, Consider the problem in which a car is told to travel at 30 km/h for L kilometers and then at 60 km/h for another L kilometers. Next, you are asked to determine the average speed. My question is: although we know that the average speed in this case is the harmonic mean of the two speeds, is it also possible to state that the average speed over this 2L-kilometer stretch can be obtained as a weighted average of the two speeds? Best regards, DaTario
Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?
Back
Top