A mass whirled in a horizontal plane

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    Horizontal Mass Plane
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Discussion Overview

The discussion revolves around the mechanics of a mass tied to a string and whirled in a horizontal plane, focusing on the forces acting on the mass, particularly the tension in the string and its relationship to gravitational force. The scope includes theoretical considerations of circular motion and the implications of string orientation.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that the tension in the string provides the necessary centripetal force for circular motion, while questioning how the weight of the mass is balanced.
  • Others argue that if the string is perfectly horizontal, there would be no vertical component of tension to counteract the weight of the mass, leading to the conclusion that it cannot be whirled in a perfectly horizontal plane.
  • A participant suggests that the string cannot be perfectly horizontal, as the mass must experience an upward force to balance its weight.
  • Some participants propose that the mass can travel in a horizontal plane, but the string must make an angle with the vertical, creating a conical motion rather than a strictly horizontal one.
  • One participant introduces a hypothetical scenario involving a massless string, arguing that it would require infinite force to stretch it perfectly straight due to the lack of an upward force to balance the weight of the mass.

Areas of Agreement / Disagreement

Participants generally disagree on whether it is possible for the mass to be whirled in a perfectly horizontal plane without a balancing upward force. Multiple competing views remain regarding the orientation of the string and the implications for the forces involved.

Contextual Notes

Participants express uncertainty regarding the assumptions about the string's orientation and the forces acting on the mass, particularly the implications of a perfectly horizontal string versus one that makes an angle with the vertical.

Vijay Bhatnagar
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A mass is tied to a string and whirled in a horizontal plane. The tension in the string provides the centripetal force required for circular motion of the mass. But which force balances the weight mg of the mass?
 
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Is the string perfectly horizontal?
 
Yes, the string is perfectly horizontal. If it were not so, the vertical component of the tension would support the weight and the horizontal component provide the necessary centripetal force. However, when the string is horizontal there is no vertical component of the tension. As a result the mass will tend to fall down. Then, do we conclude that it is not possible to whirl the mass in a perfectly horizontal plane? But practically it does look possible to do so.
 
Realize that just because the mass travels in a perfectly horizontal plane, that doesn't mean that the string is horizontal. (In fact, it can't be!)
 
Get a friend to whirl an object around on a string over his head, and watch carefully. Start with the object orbiting rather slowly, then faster and faster. Does the string ever become perfectly horizontal?
 
Doc Al said:
Realize that just because the mass travels in a perfectly horizontal plane, that doesn't mean that the string is horizontal. (In fact, it can't be!)
It should not even be possible that the mass travels in a horizontal plane. It has a force mg downwards but no upward mg force.
 
lugita15 said:
It should not even be possible that the mass travels in a horizontal plane. It has a force mg downwards but no upward mg force.
It's easy to get the mass to travel in a horizontal plane. But the string can't be perfectly horizontal; if it were, there'd be no balancing upward force on the mass. (Perhaps you're saying the same thing?)
 
lugita15 said:
It should not even be possible that the mass travels in a horizontal plane. It has a force mg downwards but no upward mg force.
The upward force comes from the tension in the string.
 
Doc Al said:
It's easy to get the mass to travel in a horizontal plane. But the string can't be perfectly horizontal; if it were, there'd be no balancing upward force on the mass. (Perhaps you're saying the same thing?)
Yes, I am saying the same thing.
 
  • #10
Suppose you have a massless string and a mass m tied in the middle, then would you able to stretch the string perfectly straight by pulling it from both sides? No, that would take infinite force, because there's no verically upward force to balance the weight mg. In real life, that's why wires and cables sag down in the middle.

Similarly, the whirling string has to make an angle, say 'x', with the vertical in the downward direction. It lies on a cone, whose semi-vertical angle is 'x'. If 'T' is the tension in the string, then mg=T*cos(x). The centipetal force is T*sin(x). The mass m travels in a horizontal plane, but not the string.
 

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