Energy Efficiency: Input & Output (Work/Energy)

[harujina]

Homework Statement

In a race, a 54 kg athlete runs from rest to a speed of 11m/s on a flat surface. The athlete's body has an efficiency of 85% during the run. How much input energy did the athlete provide?

Homework Equations

efficiency = E out/E in x 100%

The Attempt at a Solution

I know E in = E out/efficiency
and efficiency = 85%, m = 54 kg, vi = 0, vf = 11 m/s
but how can I find E out?
Eg = mgh ?
There's no height though...?

 

[rock.freak667]

The Attempt at a Solution

I know E in = E out/efficiency
and efficiency = 85%, m = 54 kg, vi = 0, vf = 11 m/s
but how can I find E out?
Eg = mgh ?
There's no height though...?

His energy output is entirely kinetic! No need for a height.

 

[harujina]

His energy output is entirely kinetic! No need for a height.

Oh, right.
Could I use Ek = 1/2mv^2 to solve this then?

 

[rock.freak667]

Oh, right.
Could I use Ek = 1/2mv^2 to solve this then?

Yes you can

 

[Matthew L]

The normal formula of efficiency is: Eout/Ein x 100% = Eff%. Why is it that when solving this question you guys got rid of the "x100%" ?

 

[Delta2]

The normal formula of efficiency is: Eout/Ein x 100% = Eff%. Why is it that when solving this question you guys got rid of the "x100%" ?

Good question regarding the basics. The answer lies in how we ll put efficiency into this equation. IF for example efficiency is 85% and we put it in the equation the efficiency as just 85 then we got to keep the 100. If we put efficiency as 0.85 then we got to remove the 100.

The symbol % in here just means division by 100. When we say efficiency is 85% we actually mean it is 85/100=0.85. When we multiply x100% we multiply by 100 and then divide by 100, so its like multiplying by 1.

 

[Matthew L]

Good question regarding the basics. The answer lies in how we ll put efficiency into this equation. IF for example efficiency is 85% and we put it in the equation the efficiency as just 85 then we got to keep the 100. If we put efficiency as 0.85 then we got to remove the 100.

The symbol % in here just means division by 100. When we say efficiency is 85% we actually mean it is 85/100=0.85. When we multiply x100% we multiply by 100 and then divide by 100, so its like multiplying by 1.

thank you!