Question

Calculate the efficiency of person who is able to do 15 J of work by putting in 125 J of energy. Round the answer to the ones place. Also include the correct symbol for the unit.

Answer 1

So you recall that efficiency is \(\frac{output}{input}*100\), what did you get?

Answer 2

Aw I don't divide by 100

Answer 3

You don't divide, you multiply.

Answer 4

So would it be 12 cause you divide 15/125*100

Answer 5

12%, exactly.

Answer 6

Aw ok I get it now it's not a decimal I'm looking for it's a percent

Answer 7

Right, so you get 0.12 as a decimal and then you multiply by 100 to get your percentage.

Answer 8

Alright I see know but I did that to the next problem which is 2400/3100*100 and I got 77.4193

Answer 9

Okay, I do not know the question so I can only say that if your output was 2400 and the input was 3100, then 77.4% efficiency looks right.

Answer 10

The question is Calculate the efficiency of a machine that is able to do 2400 J of work when 3100 J of energy is added to it. Round your answer to the ones place and include the appropriate symbol with your answer.

Answer 11

Looks good to me.

Answer 12

Yea same here now what about this one


If a person is 80% efficient in lifting an object, how much work was done when the person input 150 J of energy? This answer should be rounded to the tens place. It also needs to include the appropriate unit with the answer

Answer 13

This is going to be the same equation, you will just have to solve it for the output.

Answer 14

\(efficiency = \frac{output}{input}\)

Answer 15

Plug in what you know and solve for the unknown.

Answer 16

That should have read: \(efficiency = \frac{output}{input}*100\)

Answer 17

So would I divide 80 by 150

Answer 18

You first divide by 100, so you take your 80% back to a decimal.

Answer 19

Then you are left with \(.80 = \frac{output}{input}\)

Answer 20

So how would you get your input to the other side?

Answer 21

take the .80 and multiply

Answer 22

Yes. That will give you the work (or output) of the person.

Answer 23

And that'll be the answer

Answer 24

Yep.

Answer 25

What did you get?

Answer 26

So it would be .80*125 right

Answer 27

Where did you get 125?

Answer 28

Wrong I meant150

Answer 29

Yes, .8*150

Answer 30

And that's 120

Answer 31

Exactly, good job.

Answer 32

So it's just 120 not 120%

Answer 33

Your input is not in a percentage, so it would only be 120% if you multiplied it by 100.

Answer 34

Alright is it the same for this one


A person is 67% efficient. If they are able to do 75 J of useful work, what is their total work or energy input? Round your answer to the tens place. Be sure to include the appropriate unit in the answer.

Answer 35

Make sure your units on the previous question is Joules.
For this question you need to take note that they are giving you the "output".

Answer 36

So take your same equation, \(efficiency = \frac{output}{input}*100\), fill in the parts you know and solve for the part you don't.

Answer 37

So what's the symbol for joules

Answer 38

J

Answer 39

Right I knew that

Answer 40

I did this wrong didn't I 67*100*75

Answer 41

Efficiency = 67%
input = ?
output = 75
\(67\% = \frac{75}{input}*100\)

Answer 42

What do you do first?

Answer 43

You would take 75*100

Answer 44

I would take the 100 over so that I only had to work with \(.67 = \frac{75}{input}\)

Answer 45

Alright and so I would then .67*75 right

Answer 46

Think about that, how would you remove the 75 from the other side? What would you be left with? You want to get the "input" alone.

Answer 47

I would divide?

Answer 48

Or no

Answer 49

Sure, just look at what you would have though: \(\frac{.67}{75} = \frac{1}{input}\)

Answer 50

What if you multiplied by the "input"?

Answer 51

\(input * .67 = 75\)

Answer 52

So what's the input?

Answer 53

Assuming you took my suggestion, what would be the next step to solve for the input?

Answer 54

I can't multiply the .67 by 75 and I can't divide either so I have no clue

Answer 55

Why can you not divide out the .67 from both sides?

Answer 56

It gives me 0.0089

Answer 57

Really? Because that's not what I get.

Answer 58

What you get

Answer 59

\(input = \frac{75}{.67}\)

Answer 60

But I don't know what the input is so I took .67/76 and got that answer above

Answer 61

You are dividing .67 by 76, though I presume you meant 75, which is not what the problem is.

Answer 62

It looks like you are doing this: \[\frac{.67}{75}=\frac{1}{input}\] which means that the number you got is the "inverse" of the input. So if you take your answer and invert it: \(\frac{1}{your ~answer}\) you will get the correct number.

Answer 63

so 0.008933333333 is my answer

Answer 64

Which needs to be inverted.

Answer 65

I did didn't I?

Answer 66

Not if you have 0.0089333

Answer 67

Slow down a bit and look at this. You had \[.67=\frac{75}{input}\] and you want to isolate input. Now the first logical step is to get it out of the denominator: \[input*~.67=\frac{75}{input}*input\rightarrow input*.67 = 75\] but you still want to isolate "input" so the second logical step is to multiply \(\frac{1}{.67}\)(or divde .67) to both sides\[\frac{1}{.67}*input*.67=75*\frac{1}{.67}\rightarrow input = \frac{75}{.67}\]

Answer 68

So now you know that your input is equal to \(\frac{75}{.67}\)

Answer 69

Right

Answer 70

And what is 75 divided by .67?

Answer 71

111.940

Answer 72

YES! Very good. :D

Answer 73

So what's next

Answer 74

You're done.

Answer 75

Aw for real that's it

Answer 76

Yep, that's it.

Answer 77

thanks again

Answer 78

You're welcome.