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# Physics question help watch

1. Doing some physics work and im not sure if what im doing is right. The question is:

On a building site, a 600W electric winch and a pulley were used to lift bricks from the ground. The winch raised a load of 500N through a height of 3.0m in 25s

a) Calculate how much useful energy was transferred by the mortor

i would use the work done equation since energy transferred= work done.

b) calculate the percentage efficiency
Ok, if i am right i would use the answer from a and divide it by the input energy and multiply it by 100 to get my answer.....however how am i supposed to know the input energy?!!? Would i just convert the 600W to joules to use as my input energy?

Am i even close or right?!?!?!
So so sorry for the longgg request!

Thank You! Xxxxx
2. (Original post by A.N123)
Doing some physics work and im not sure if what im doing is right. The question is:

On a building site, a 600W electric winch and a pulley were used to lift bricks from the ground. The winch raised a load of 500N through a height of 3.0m in 25s

a) Calculate how much useful energy was transferred by the mortor

i would use the work done equation since energy transferred= work done.
Q. Are you sure the question stated 500N load and not 500Kg load?

The question asks for useful work done. The load was raised 3.0 metres. Which means it now has gravitational potential energy.

gravitational potential energy transferred = mass x gravity x height

(Original post by A.N123)
b) calculate the percentage efficiency

Ok, if i am right i would use the answer from a and divide it by the input energy and multiply it by 100 to get my answer.....however how am i supposed to know the input energy?!!? Would i just convert the 600W to joules to use as my input energy?
Correct.

We are given the power output of the motor = 600W i.e. 600W = 600 Joules / second.

Total energy input = power x time

3. (Original post by uberteknik)
Q. Are you sure the question stated 500N load and not 500Kg load?

The question asks for useful work done. The load was raised 3.0 metres. Which means it now has gravitational potential energy.

gravitational potential energy transferred = mass x gravity x height

Correct.

We are given the power output of the motor = 600W i.e. 600W = 600 Joules / second.

Total energy input = power x time

Ahhhhhh now i get a! And yes it states 500N would this therefore mean i would need to convert the 500N to kg before i figure out the answer? Thank you very much Xxxx
4. (Original post by A.N123)
Ahhhhhh now i get a! And yes it states 500N would this therefore mean i would need to convert the 500N to kg before i figure out the answer? Thank you very much Xxxx
No. Not necessary as long as you recall

g.p.e = m g h

and Newtons = m g

hence gpe = 500N x 3 meters = 1500 Joules.

5. (Original post by uberteknik)
Yes.
Thank you! Sorry to bother you again but when calculating efficiency the input energy was given in Kw. However i dont know how to convert Kw to joules and we havent even learnt it in class and the internet is quite dodgy lol (in yr10)
6. (Original post by A.N123)
Thank you! Sorry to bother you again but when calculating efficiency the input energy was given in Kw. However i dont know how to convert Kw to joules and we havent even learnt it in class and the internet is quite dodgy lol (in yr10)
1 kW = 1000 Watts = 1000 Joules per second.

To give an idea of how much power that is, 0.75kW (750 Watts) is roughly how much power an average sized horse can produce continuously.
7. (Original post by uberteknik)
1 kW = 1000 Watts = 1000 Joules per second.

To give an idea of how much power that is, 0.75kW (750 Watts) is roughly how much power an average sized horse can produce continuously.
Thank you so so soooo much!!!!!! ( and sorry for bothering you about 6000 times!)
8. (Original post by A.N123)
Thank you so so soooo much!!!!!! ( and sorry for bothering you about 6000 times!)
No worries. My pleasure.

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Updated: October 4, 2017
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