What Went Wrong with Calculating Pulley Efficiency?

AI Thread Summary
The discussion revolves around a lab experiment measuring pulley efficiency, where the calculated output work exceeded the input work, indicating a potential error. The participant used the formula E=work out/work in, calculating work out as 3.92 and work in as 1.76. Concerns were raised about the accuracy of the measurements, specifically the applied force and distances, which were taken with a spring scale and ruler. The conclusion drawn is that while the math is correct, the measurements likely contain errors, leading to incorrect results. The participant expresses frustration over the possibility of needing to redo the lab due to these inaccuracies.
livvy07
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Homework Statement



we did a lab on pulleys. The weight lifted was 19.62 N. It rose 0.20 meter with us applying a lifting force of 11 N and pulling the rose a distance of 0.16.


The Attempt at a Solution


I tried to find the efficiency of the pulley, so E=work out/work in
so work out=FD=(19.62)(0.20)=3.92
and work in=FD=(11)(0.16)=1.76
yet the work output can't be greater than the work input, so I'm clearly doing something wrong.

Any ideas? Is it a math error or bad lab results? Any help would be great :)
 
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It's not a math problem. Those measurements don't make sense. How did you measure the applied force and distances?
 
oh no. That means the whole lab is wrong, but I'm doing the math right?
We measured the applied force with a spring scale and the distances with a ruler. i think the distances are completely wrong now...
 
livvy07 said:
That means the whole lab is wrong, but I'm doing the math right?
Yep, your math is fine. :frown:
 
oh well. Thanks for the help though! Guess I'll have to start making up numbers
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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