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Work energy power

22219215_10210319911261383_1031198474_o.jpg
Question 5 - I Have tried this, but failed, I used MGH = 50 x 10 X 0.35 and found Heat energy is 120 KJ to its 120000 J, since power = output/input X 100%
I used that but my answer did not match. The ans is 15% but how? please show me the steps, Thank you

Question 6 - I have no clue.
Original post by Gartley222
22219215_10210319911261383_1031198474_o.jpg
Question 5 - I Have tried this, but failed, I used MGH = 50 x 10 X 0.35 and found Heat energy is 120 KJ to its 120000 J, since power = output/input X 100%
I used that but my answer did not match. The ans is 15% but how? please show me the steps, Thank you

Question 6 - I have no clue.


You need to include the number of times the woman repeats the exercise... 4 minutes at 30 reps per minute as per the question.

Fridges are insulated boxes that use a motor to drive a heat pump. the heat pump moves heat from the inside of the box to the outside of the box... but it doesn't remove the heat from the room in any way. The motor is also less than 100% efficient and will produce some heat while it is running.
mgh so 50 x 10 x 0.35 = 175j so you got that right.
She does it 30 times per minute and question says 4 minutes. So 175j x 30 x 4 = 21000j. This is the useful energy
The total energy is the useful energy + energy wasted which is 120000j so add that to 21000j = 141000
So then useful output divide total input so 21000 divide 141000 x100 to get 14.89 percent = 15percent
Reply 3
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Gartley222
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Original post by Chocolate...s
mgh so 50 x 10 x 0.35 = 175j so you got that right.
She does it 30 times per minute and question says 4 minutes. So 175j x 30 x 4 = 21000j. This is the useful energy
The total energy is the useful energy + energy wasted which is 120000j so add that to 21000j = 141000
So then useful output divide total input so 21000 divide 141000 x100 to get 14.89 percent = 15percent

Thank you very much! Now I am clear.
Any thoughts about the question 6?
Original post by Gartley222
Thank you very much! Now I am clear.
Any thoughts about the question 6?


We havnt went into too much detail about the law of conservation energy yet in our gcse level only to knoe that energy cant be destroyed or created. Maybe take into account that when a fridge door is open, compressor still works at its optimum level and so room temperature is increased
Reply 5
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Gartley222
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Original post by Chocolate...s
We havnt went into too much detail about the law of conservation energy yet in our gcse level only to knoe that energy cant be destroyed or created. Maybe take into account that when a fridge door is open, compressor still works at its optimum level and so room temperature is increased

I have a question to you! how did you understand from the question that which one is output and which one is input? Any clues?
Reply 6
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Gartley222
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Original post by Joinedup
You need to include the number of times the woman repeats the exercise... 4 minutes at 30 reps per minute as per the question.

Fridges are insulated boxes that use a motor to drive a heat pump. the heat pump moves heat from the inside of the box to the outside of the box... but it doesn't remove the heat from the room in any way. The motor is also less than 100% efficient and will produce some heat while it is running.

Thank you :biggrin:
Original post by Gartley222
I have a question to you! how did you understand from the question that which one is output and which one is input? Any clues?


i think you are talking about question 5 right as question 6 doesnt say anything about input or output.
The question says that heat is generated from her legs muscles so we assume that this is the energy wasted. Input energy is the amount of energy supplied to a device so useful output energy + energy wasted = your total input
Reply 8
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Gartley222
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Original post by Chocolate...s
i think you are talking about question 5 right as question 6 doesnt say anything about input or output.
The question says that heat is generated from her legs muscles so we assume that this is the energy wasted. Input energy is the amount of energy supplied to a device so useful output energy + energy wasted = your total input

Yes! got it! Thanks

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