-1

I have heard many people tell me that the tensional force is bi-directional. Consider the following case where a (mass-less) rope is used to transmit tension.

The rope is being pulled (by hand) with a force of 5 newtons. Thus the mass (along with the rope) will have an acceleration of 5 ms^(-2). (Neglect friction)

enter image description here

1) Considering a point P on the rope, have I represented the tensional force on the rope correctly?

2) By Newton's Third Law, if the rope is pulling on the block, the block must exert an equal and opposite force on the rope. So, shouldn't the body not have any motion? A similar question was asked here: With Newton's third law, why are things capable of moving?. According to the answer provided, it is the force of the muscles that is responsible for the resulting acceleration.

So then what force is transmitted across the rope? It has to be the force of the muscles and not any other force since the tension in the rope is 5 newtons. But if it is so, the force exerted by the block on the rope (reaction) should also be 5 newtons. This means that the object will have no motion! Am I misunderstanding something here?

Qmechanic
  • 220,844
Gokulakrishnan Shankar
  • 456

1 Answers1

2

There are three parts to the situation you are considering:
Yourself Rope
Block

You exert a force of 5 N on the rope to the right and by Newton’s third law the rope exerts a force of 5 newton on you to the left.

The rope exerts a force of 5 N on the block to the right and by Newton’s third law the block exerts a force of 5 N on the rope to the left.

The end result is that

  • the block has on it a net force of 5 N to the right and it will accelerate to the right.
  • there is a net force of 5 N to the left on you. If you were not anchored to the ground this force would cause you to accelerate to the left.
  • the rope has a net force of zero on it which is a consequence of the assumption that the rope is massless. You can think of the rope as transferring forces between you and the block and there is no reason why the rope cannot move.
Farcher
  • 104,498