Understanding Force Statements: Solving a Confusing Homework Problem

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Homework Help Overview

The problem involves a person pushing an object of mass m along the ground, considering the effects of kinetic and static friction. The scenario describes the object accelerating while being pushed, then sliding to a halt, and subsequently not moving when pushed again, raising questions about the forces involved and the conditions for motion.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the differences between static and kinetic friction, questioning the validity of the provided statements. There are suggestions to analyze the forces acting on the object through free-body diagrams and to clarify the choices presented in the problem.

Discussion Status

The discussion is ongoing, with participants expressing confusion about the problem's statements and the physical implications of the forces involved. Some guidance has been offered regarding comparing forces, but no consensus has been reached on the correct interpretation of the problem.

Contextual Notes

There is mention of the problem being perceived as a single scenario rather than two separate cases, which may affect the interpretation of the forces at play. Additionally, one choice has been noted as potentially invalid based on physical units.

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Homework Statement



A person is pushing an object of mass m along the ground with force F. The coefficient of kinetic friction between the object and the ground is (mu)k. The object is accelerating, but then the person stops pushing and the object slides to a halt. The person then starts push8ing again with a force F, but the object doesn't budge. The maximum coefficient of static friction between the object and the ground is ms. Which of the following statements is true?

Choices:
F > (mu)k/(mu)s
(mu)kmgsmg
(mu)smg=F
The scenario described is physically impossible.

Homework Equations


I read through this again and again and couldn't figure it out at all. I think the last one is possible but I am not sure.


The Attempt at a Solution


This is a thinking problem... no equation needed. I am finding the equation.

Thoughts? What do you think it is?
 
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The problem hinges on the difference between static and kinetic friction.

I don't understand several of the choices. Have you copied them exactly as given?
 
OK, you might want to draw the free-body diagram for the object in the first case. You know that the object is moving with an acceleration, so there is a net horizontal force acting on the object. What does this tell you about the magnitudes of the forces acting on the object?

In the second case, the object is not moving. What does this tell you about the magnitudes of the forces acting on the object?

PS: you might want to type in the choices again. they are rather messy.
 
F>(mu)k/(mu)s
(mu)kmg<=(mu)smg
(mu)smg=F
The scenario described is physically impossible.

This is exactly how they look. I am so confused on this one. Just so you know... this is one scenario not two.
 
You can eliminate one of the choices based simply on physical units. (i.e. one of the choices doesn't make sense in terms of physical units)
 
vballkatie22 said:
F>(mu)k/(mu)s
(mu)kmg<=(mu)smg
(mu)smg=F
The scenario described is physically impossible.
OK, that's much clearer.

Hint: Compare F with kinetic friction (μk mg) when the object is moving. Then compare F with the maximum value of static friction (μs mg) when it's not moving.

When I say compare, I mean: Decide which is bigger.
 
How am I supposed to do that? This question is so confusing.
 

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