Net force on a Swinging Pendulum

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Discussion Overview

The discussion revolves around the net force acting on a weight tied to a string that is spinning in a horizontal circle. Participants explore concepts related to Newton's third law, net forces, and acceleration in circular motion, with a focus on the dynamics of the pendulum-like system.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how there can be a net acceleration inward if the forces of the string on the weight and the weight on the string are equal and opposite, as stated by Newton's third law.
  • Another participant clarifies that forces in a third law pair act on different objects, suggesting that these should not be summed to determine net force.
  • A different participant asserts that the sum of all forces on the weight is inward, implying that this results in net inward acceleration.
  • Further elaboration indicates that while the speed of the mass tangentially does not change, its direction does, leading to radial acceleration directed towards the center.
  • It is noted that this analysis assumes no additional forces are acting on the system and that the string is moving in a circular motion under its own momentum.
  • Concerns are raised about the complexities introduced when considering a flexible string and the need for external forces to initiate circular motion.

Areas of Agreement / Disagreement

Participants express differing views on the implications of Newton's third law in this context, and the discussion remains unresolved regarding the net force and acceleration concepts.

Contextual Notes

Participants have not fully addressed the implications of additional forces or the role of the string's flexibility in the dynamics of the system, leaving some assumptions unexamined.

Conservation
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Hello everyone,

Consider a weight tied to a string spinning in a horizontal circle.

According to Newton's third law, the force of the string on the weight (inward) is opposite and equal to the force of the weight on the string (outward).

If this is the case, how can there be a net acceleration inward?

Thanks.
 
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The forces in a third law pair act on different objects, so you don't sum them to get a net force.
 
Conservation said:
how can there be a net acceleration inward?
Because the sum of all forces on the weight is inward.
 
Conservation said:
Hello everyone,

Consider a weight tied to a string spinning in a horizontal circle.

According to Newton's third law, the force of the string on the weight (inward) is opposite and equal to the force of the weight on the string (outward).

If this is the case, how can there be a net acceleration inward?

Thanks.

The speed of the mass in a tangential direction does not change but the direction does. That means the only force is inwards towards the centre. That makes the acceleration radial as well.
This only applies if there is no extra forces applied at the time - the string is just going around under its own steam. But a totally flexible string can only transmit a force along its length so getting it up to speed, you need to be moving your hand round in a circle; things get more complicated then.
 

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