Hacked off with college over Velocity Ratio and Efficiency question.

[RichMortimer]

Homework Statement

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

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

Load = 1962N

Homework Equations

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

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!


Please please help if you can.

Thanks in advance,

Rich.

 

[lewando]

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[RichMortimer]

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

Thank you!