# Hacked off with college over Velocity Ratio and Efficiency question.

1. Jul 4, 2011

### RichMortimer

1. The problem statement, all variables and given/known data

If the work put in is 3000j, calculate the useful energy output.

[PLAIN]http://www.mkwebman.com/question [Broken]

2. Relevant equations

VR = Distance covered by effort / Distance covered by load
Efficiency = MA / VR
Efficiency = useful output energy / input energy

3. The attempt at a solution

I firstly worked out the distance moved vertically by both the effort and load. These are 0.71 for the effort and 4.24 for the load based on an angle of 45 degrees.

VR = 0.71/4.24 VR = 0.17

MA = 1962/11772 MA = 0.17

As the question doesn't mention friction I've used efficiency = MA/VR.

efficiency = 0.17/0.17
efficiency = 1 1 x 100 = 100%

Finally, using efficiency = useful output energy / input energy

useful output = 1 x 3000

Useful = 3000j

I've been told I'm wrong as it cannot be 100% efficient. However, as the question doesn't state a friction variable and which ever ratio I use returns 0.17 I can't see how this is marked incorrect!!

Rich.

Last edited by a moderator: May 5, 2017
2. Jul 4, 2011

### lewando

There is a problem viewing your attached image.

3. Jul 4, 2011

### RichMortimer

it is at www.mkwebman.com/question[/URL]

Thank you!!

Last edited by a moderator: Apr 26, 2017