No resultant force means no external force?

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Discussion Overview

The discussion revolves around the relationship between resultant forces and external forces, particularly in the context of a conveyor belt system where sand falls onto the belt. Participants explore whether the presence of external forces necessarily implies a resultant force and how this applies to the dynamics of the sand and the belt.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether no resultant force implies no external force, suggesting that external forces can exist even when the net force is zero, as illustrated by a book on a desk.
  • There is a discussion about the forces acting on the sand as it falls onto the conveyor belt, with some participants asserting that the friction force is necessary for the sand to match the speed of the belt.
  • One participant notes that while the belt maintains a constant speed, the sand must be accelerated, implying that there is a net force acting on the sand during this acceleration.
  • Another participant emphasizes that the belt exerts a force on the sand, which is necessary for the sand to reach the speed of the belt, while the sand exerts an equal and opposite force on the belt.
  • There is a contention regarding whether the belt slows down momentarily as sand falls onto it, with one participant arguing that the problem states the belt's speed is constant, and thus it should be assumed as such for the analysis.
  • One participant introduces the idea of needing to consider relativistic and quantum mechanical effects, although this is presented in a somewhat dismissive manner regarding their significance in the current scenario.

Areas of Agreement / Disagreement

Participants express differing views on the implications of resultant and external forces, particularly in the context of the conveyor belt system. There is no consensus on whether the belt slows down when sand falls onto it, as some participants maintain that the speed is constant while others suggest a temporary slowdown occurs.

Contextual Notes

Participants acknowledge the complexity of the forces involved and the assumptions required for the analysis, such as the constant speed of the belt and the need to account for various forces acting on the system. There is also mention of the potential need to consider additional physical effects, though this remains unresolved.

Who May Find This Useful

Readers interested in dynamics, force interactions, and the principles of mechanics may find this discussion relevant, particularly those exploring concepts of resultant and external forces in systems involving motion.

makeAwish
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Hmm. I'm quite confused with this:
Does no resultant force means no external force?
i.e. if there is an external force, there is a resultant force?


Like say, sand falls onto a moving conveyor belt at the rate of 5.00 kg/s. The conveyor belt is supported by frictionless rollers and moves at a constant speed of 0.750m/s under the action of a constant horizontal external force Fext supplied by the motor that drives the belt.

so the friction force is the force required for the sand to move at same speed as the belt right? then can i say there is no resultant force acting on the sand?


anyway i duno if this is considered as h/w qns.. sorry.
 
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janettaywx said:
Hmm. I'm quite confused with this:
Does no resultant force means no external force?
i.e. if there is an external force, there is a resultant force?
Not at all. Consider a book sitting on your desk. The net force is zero, yet both gravity and the desk exert forces (external forces) on the book.


Like say, sand falls onto a moving conveyor belt at the rate of 5.00 kg/s. The conveyor belt is supported by frictionless rollers and moves at a constant speed of 0.750m/s under the action of a constant horizontal external force Fext supplied by the motor that drives the belt.

so the friction force is the force required for the sand to move at same speed as the belt right? then can i say there is no resultant force acting on the sand?
Note that the sand must be accelerated from zero speed to the speed of the belt. That requires a force.
 
Doc Al said:
Not at all. Consider a book sitting on your desk. The net force is zero, yet both gravity and the desk exert forces (external forces) on the book.



Note that the sand must be accelerated from zero speed to the speed of the belt. That requires a force.

yah, so that force is the friction force?
 
janettaywx said:
yah, so that force is the friction force?
The conveyor belt exerts a horizontal force on the sand, if that's what you mean by "friction force".
 
hmm. okay!

then since they are moving at same speed can i say there is no resultant force on the system?
 
What system?
 
the system of sand and belt?
 
janettaywx said:
the system of sand and belt?
It's unclear what you mean by "sand and belt" as your system. All the sand?

Focus on a section of the belt alone. Since its speed remains fixed, the net force on it must be zero.

The falling sand is being accelerated by the belt, so there must be a net force on that sand during its acceleration.
 
Oh. okay sorry, i was actually referring to the sand on belt and the belt itself. for this, is no net force?
 
  • #10
then since they are moving at same speed can i say there is no resultant force on the system?
Strictly speaking, the belt DOES slow down a bit as the sand particles fall on it. As the belt exerts a force to speed up the sand, the sand exerts an equal and opposite force on the belt slowing it down.
 
  • #11
Since the belt maintains a constant speed, the net force on it is zero. Since the sand exerts a force on the belt, the mechanism must exert an equal and opposite force on the belt to maintain its constant speed.
 
  • #12
Ya. But i think, the belt should slow down for a very small interval of time before getting 'reinforcements' from the motor to maintain the speed.
 
  • #13
sganesh88 said:
Ya. But i think, the belt should slow down for a very small interval of time before getting 'reinforcements' from the motor to maintain the speed.
First lesson:

NEVER, EVER, break the PREMISE given in the exercise!

You are GIVEN that the velocity is constant, so that is what you are to assume for the remainder of the exercise.


Besides, STRICTLY speaking, you'd have to take into account relativistic and quantum mechanical effects, at least after you have included the gravitational effects from the planets impinging upon the scenario.

How do you know that these would be insignificant compared to the error source you happened to mention?
 

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