# Power output physics problem

alias25
heres this question:
theres a man who does 20 press-ups (ie facing down body straight) in 50 sec what is his power output?

is it 20/50 = 0.4 press-ups per sec of power?

i know power is in watts and its P = Fv = E/t

if i take accleration to be 9.8 i could get v, a= (v-u)/t
initial vel would be 0 ithink but i still can't get force i require mass which I am not given in the Q so should i assume a mass?
or is the answer above correct? and I am thinking too much into it?

Mentor
You'll need to estimate how much work the man does during each press-up. He is lifting some percentage of his bodyweight through some distance.

alias25
isnt he lifting his whole body so 100%of his body weight so force would be 9.8 * (100% of mass) =
i could use S = 1/2 at^2 to find displacement? i tried that and i get a silly answer of 7. summin meters (thats using t as (50/20)/2 and a as 9.8 initial vel as 0)
and say i could get vel by a= (v-u)/t

then mav= mas/t = P
help how do i find some % of bodyweight and some dist.

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Mentor
alias25 said:
isnt he lifting his whole body so 100%of his body weight so force would be 9.8 * (100% of mass) =
What matters is how much force he exerts. An example: What's easier, lifting one end of a sofa off the ground or lifting the entire sofa?

Why not check this yourself? Find a bathroom scale. Stand on it to get your weight. Then do a pushup on it, and see what it reads.

i could use S = 1/2 at^2 to find displacement? i tried that and i get a silly answer of 7. summin meters (thats using t as (50/20)/2 and a as 9.8 initial vel as 0)
and say i could get vel by a= (v-u)/t
Those are formulas for freely falling bodies. I don't see anything falling here. The distance? Hint: How long are your arms?

alias25
m*9.8*12.25 = m*9.8*7.66 but the mass cancels and the numbers are silly no one got arms 7 meters long

alias25
my arms are very short lol. so are u suggesting i should assume values? like the average persons weight and arm length?etc

alias25
hang on my arm length is about a third of the length of my body or summin should i be thinking along those lines? ok maybe not a third. ( I've got a new teacher for physics, i don't want him to think I am dumb or something)

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Mentor
If you're not sure how far you press yourself up, get a ruler and check. All you need is an estimate. (Did you check what percentage of your weight is involved in a press up? Do it!)

This problem is just an exercise to see if you understand the ideas of work and power. All you will count, I presume, is the positive work done as the man pushes himself up. You can use your own weight/force/distance as an estimate, or you can express it in terms of the unknown man's weight and arm length. (But don't start talking about falling objects! )

alias25
ok thanks i think i did look too much into the question.