# Relationship Between Kinetic Friction and Mechanical Energy

• velouria131
In summary, the correct statement about friction is that statement II only is correct. The static friction force cannot change the mechanical energy of a system, while both kinetic friction and the direction of the force of friction do affect the mechanical energy of a system.
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.

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?

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.

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?

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?

velouria131 said:
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:

cepheid said:
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?

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?

## What is kinetic friction?

Kinetic friction is the force that opposes the relative motion between two surfaces in contact. It occurs when two surfaces are moving against each other.

## How does kinetic friction affect mechanical energy?

Kinetic friction converts mechanical energy into heat energy. This means that some of the energy that could have been used for motion is lost as heat due to the friction between the two surfaces.

## How is the relationship between kinetic friction and mechanical energy represented?

The relationship between kinetic friction and mechanical energy is represented by the equation W = Fd, where W is the work done, F is the force of friction, and d is the distance over which the force acts.

## What factors affect the amount of kinetic friction?

The amount of kinetic friction depends on the types of surfaces in contact, the force pressing the surfaces together, and the roughness of the surfaces.

## Can kinetic friction ever be completely eliminated?

No, it is impossible to completely eliminate kinetic friction. However, it can be reduced by using lubricants or by using smoother surfaces in contact.

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