Relationship Between Kinetic Friction and Mechanical Energy

[velouria131]

Homework Statement

Which of the following statements about friction is correct?
I. Kinetic friction always decreases the mechanical energy of a system.
II. The static friction force cannot change the mechanical energy of a system.
III. The force of friction always points in the direction opposite to the way that a system moves.
None of the statements is correct.
Statement III only
Statements I & II only
Statement II only
Statements I & III only
Statements II & III only
Statement I only.
All three statements.

Homework Equations

W = F * D
Mechanical Energy = KE + Ug

The Attempt at a Solution

I cannot find the culprit. All appear to be correct and I cannot form any counter-examples. Kinetic friction is a non-conservative force, so it must decrease the mechanical energy of a system where friction is present. Static friction does not do work, so it must not affect the mechanical energy of a system. For the third; I cannot imagine a system in which friction points in the direction of motion. Kinetic friction is caused by this motion.

 

[cepheid]

So you think that the answer is "all of the above?"

So what's the problem? Have you been told that this answer is incorrect?

 

[velouria131]

Exactly. All of the above was taken as incorrect by the online homework program I am using. I am now stumped as to which one is the odd problem out.

 

[cepheid]

Hmm, okay, let's think this through.

If I push a box across a flat surface at a constant speed, is the mechanical energy of the box changing? Is kinetic friction present?

 

[velouria131]

There would be a change in kinetic energy but no change in potential energy. When pushing a box across a floor with friction, kinetic energy is lost to the frictional force which runs parallel to the direction of motion?

 

[cepheid]

There would be a change in kinetic energy but no change in potential energy. When pushing a box across a floor with friction, kinetic energy is lost to the frictional force which runs parallel to the direction of motion?

No, pay closer attention to the scenario I am proposing:

Hmm, okay, let's think this through.

If I push a box across a flat surface at a constant speed, is the mechanical energy of the box changing? Is kinetic friction present?

 

[cepheid]

Actually let's consider a couple of other situations:

1. Walking
2. A car moving down a road.

For situation 2, what direction does the frictional force from the road ON the tires point? What direction does the system move?

For both situations: is the friction causing the mechanical energy of the system to decrease?