Block on inclined plane - pushing vs pulling

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Discussion Overview

The discussion revolves around the mechanics of pushing versus pulling a block up an inclined plane, focusing on the effects of force direction on normal force and friction. Participants explore the implications of the angle of application of force and how it influences the required force to move the block.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes the vertical components of force when pushing and pulling a block up an incline, suggesting that the force required differs due to the direction of the vertical components of the applied force.
  • Another participant argues that if pushing and pulling are in the same direction, there should be no difference in the forces required.
  • A participant questions the relevance of the incline, noting that similar dynamics occur on level ground, where pulling reduces the normal force and friction while pushing increases them.
  • Further elaboration is provided on the mechanics of pulling with a rope versus pushing with a stiff wire, emphasizing that both actions can be parallel to the incline, potentially leading to identical frictional effects.
  • A later reply acknowledges a misunderstanding regarding the angle of force application, clarifying that when pulling or pushing parallel to the incline, the forces would indeed be the same.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of pushing versus pulling, with some agreeing that the angle of force application matters while others maintain that if the forces are parallel, the effects on friction and normal force are identical. The discussion remains unresolved regarding the implications of force direction on the required effort.

Contextual Notes

Participants highlight the importance of the angle of applied force and its relationship to normal force and friction, but there are unresolved assumptions about the conditions under which these dynamics apply, particularly in relation to incline versus level surfaces.

mdavis501
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When resolving all of the VERTICAL forces in a block that is being pushed up an incline with some incline amount theta, then when PUSHING with force P at an angle of alpha on the block, then the vertical component of force P is sin (alpha) * P in the downward direction (opposite of the Normal force); however, when pulling the block UP the incline with same alpha and same force P, then the vertical force is sin (alpha) * P upwards in the SAME direction as the normal force N. In both cases friction is in the direction of DOWN the incline. In both cases the Parallel and Perpendicular forces of gravity are the same. So, it would seem that the force P needed to push the block will be different than the force necessary to pull the block. Then seems counter-intuitive. Am I thinking correctly here?
 
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It only makes sense to speak of an angle relative to something.

If the pushing and pulling are truly in the same direction, then there will be no difference at all.
 
I don't understand what the point of the incline is. You have the same thing on level ground: Pulling forward & up reduces the normal force, and thus friction. Pushing forward & down increases friction.
 
A.T. said:
I don't understand what the point of the incline is. You have the same thing on level ground: Pulling forward & up reduces the normal force, and thus friction. Pushing forward & down increases friction.

You could imagine that the object is pulled by a rope attached to the center of the forward face. Since the rope will be taut under tension, it acts just like a stiff wire would under tension. The most likely direction to pull something up an inclined plane is to pull parallel with the incline. In other words, the rope/wire aims up, the handhold having the same distance from the inclined slope as it does at the attachment point. So one is lifting as one also pulls forward in this case.

Now imagine pushing the object up the slope with a stiff wire since the rope would collapse under compression. The wire is again attached to the center of a face, but the rear face this time. The most likely direction of the wire is again parallel with the incline. In this case one is pushing up (not down) as one pushes forward.

The rope (or stiff wire) can be quite short, but the push/pull efforts are still both parallel with the slope, even until they are infinately short. The two cases of friction are now identical.

Wes
...
 
Wes Tausend said:
The most likely direction to pull something up an inclined plane is to pull parallel with the incline.
OK, I misunderstood that it's about pulling/pushing at an angle to the surface. When parallel then both are the same.
 

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