# Efficiency! What makes it less than 100%?

1. Apr 27, 2011

### iRamie

1. The problem statement, all variables and given/known data
List a couple factors that make efficiency less than 100%.

2. Relevant equations
Efficiency = Eout / Ein
Eout = mgh
Ein = Work = FD

3. The attempt at a solution

IDK :\

2. Apr 27, 2011

### thegreenlaser

A better way to look at efficiency is (useful output energy)/(input energy). A tank of gasoline has a certain amount of stored energy, however your car's motor produces a lot more than forward motion. All of that energy in the gasoline goes somewhere, but only a certain part goes to moving your car forward. For example, your engine gets very hot as it runs: Where does the energy for this heat come from? If all you want is forward motion, how is the production of this heat going to affect the amount of useful energy you get out of a tank of gasoline? (in this case useful energy is used for forward motion of the car)

Essentially, I understand the question to be: what sort of things 'waste' energy in a system? What sort of things decrease the amount of useful energy you get?

3. Apr 27, 2011

### iRamie

No like i mean, in a situation where there is an inclined plane (RAMP). And you are pushing an object up there.
In that situation what makes it less than 100%

4. Apr 27, 2011

### AlexChandler

With this definition of efficiency it will be less than 1 if the object starts at some speed V0 and ends at some speed V1 with V0<V1.

This is because

$$W = \Delta K.E. + \Delta G.P.E.$$

if the change in kinetic energy is positive, then

$$W > mgh$$

and

$$\frac{mgh}{W} < 1$$

But i'm not sure that this definition for efficiency really makes sense.

5. Apr 27, 2011

### iRamie

Im in grade 11 btw so its introductory mechanics lol

We have a lab in which there is a ramp with one end on the floor and the other end is on top of a few textbooks. Therefore, it has a slope, and an object is lifted using the ramp and the force is measured using a newton scale.
So in this case, what makes it less than 100%.
Assume the object is moving at constant velocity.

6. Apr 27, 2011

### AlexChandler

Or you can take the advice of thegreenlaser, and say that friction will cause efficiency to be less than 100%. Friction is a much simpler and better answer in my opinion.

7. Apr 27, 2011

### iRamie

Okay so can i say Friction will cause more applied force as a result it causes less efficiency.
Is there anything else? Because it asks for 2 Major Factors.

8. Apr 27, 2011

### AlexChandler

Perhaps air resistance. What do you think?

9. Apr 27, 2011

### thegreenlaser

In the case of an inclined ramp, you're pushing/pulling, but not all that energy goes into moving the object. I think they just want you to list off possibilities for where that energy goes, though I'm pretty sure they want you to think about it, and not just copy down what someone tells you, which is why I'm not just giving you a list of things.

So, think about it. Why is it that it's harder to pull an object along carpet than it is to pull it on ice? Where does that energy go? That sort of energy loss is exactly why you can't get 100% efficiency. As far as what form this 'waste energy' takes, think about what happens when you rub your hands together really fast: what sort of energy is created? That sort of energy is one example of "waste" energy that keeps a machine from being 100% efficient.