Understanding Force Statements: Solving a Confusing Homework Problem

[vballkatie22]

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?

 

[Doc Al]

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?

 

[horatio89]

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.

 

[vballkatie22]

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.

 

[turin]

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)

 

[Doc Al]

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.

 

[vballkatie22]

How am I supposed to do that? This question is so confusing.