Questions about Energy (Work and Power)

[CPHY]

My class just started the Energy unit and I would like to make sure my answers are correct to our HW questions. I am in high school Physics. Thanks.

1. Homework Statement
A student lifts their 100N book-bag 1meter in order to put it on. How much work did the student do in lifting the book-bag?
A student wears their 100N book-bag on their back and walks 20m down the hall. How much work did the student do in carrying the book-bag?
A student lowers their 100N book-bag 1m in order to take it off. How much work did the student do in lowering the book-bag?
How much work does a bodybuilder do when bench pressing 300lb 10 times?

Homework Equations

W=f*d

The Attempt at a Solution

1. A student lifts their 100N book-bag 1meter in order to put it on. How much work did the student do in lifting the book-bag?

W=f*d
W= (100 N)*(1 m)

W= 100 J
2. A student wears their 100N book-bag on their back and walks 20m down the hall. How much work did the student do in carrying the book-bag?

W=f*d
W=(100 N)*(20 m)

W= 2000 J

3. A student lowers their 100N book-bag 1m in order to take it off. How much work did the student do in lowering the book-bag?

W=f*d
W=(-100 N)*(1 m)
W= -100 J

4. How much work does a body builder do when bench pressing 300lb 10 times?
300 lb*10=
3000 J

 

[Dr.D]

Look at #2. What direction is the force? What direction is the displacement? Is there really any work done at all?

Also, look at #4. How far does the weight get lifted? (Do we know?) Secondly, when the weight is lifted, work is done, but when weight is lowered, that work is returned or dissipated. Where does it go?

 

[haruspex]

4. How much work does a body builder do when bench pressing 300lb 10 times?
300 lb*10=
3000 J

Pay attention to units. How does pounds force suddenly become Joules? (They are not even dimensionally the same, which should give you a clue regarding a major flaw in this calculation.)

For 2, further to Dr. D's remarks, questions about muscular effort are rather fraught. The questioner expects you to take a very simple mechanical view, ignoring the subtleties of ergonomics. The true answer is that it depends on how efficiently the student walks. African tribeswomen accustomed to balancing loads on their heads are amazingly efficient, walking great distances with heavy loads. Your personal experience of carrying loads will tend to deceive you in such questions.