# physics

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Posted by michelle quendarsv on Saturday, November 20, 2010 at 5:26pm.

Posted by michelle quendarsv on Saturday, November 20, 2010 at 3:55pm.

if it takes 726 watts of power to move a mass 36 meters in 14 seeconds, what is the mass?

i really need like, step by step help please? asap.
thanks, michelle .
physics - bobpursley, Saturday, November 20, 2010 at 4:06pm
power= work/time= mass*g*distance/time

mass= power*time/distance*g

physics - michelle quendarsv, Saturday, November 20, 2010 at 4:24pm
so... would it be like,
726 x 14 / 36 ?
physics - bobpursley, Saturday, November 20, 2010 at 6:08pm
no, what happened to g (9.8m/s^2) in the denominator?

physics - michelle quendarsv, Saturday, November 20, 2010 at 6:40pm
but where did you get 9.8m/s2?

• physics - ,

He is assuming that you are moving the mass up against gravity. g = 9.8 m/s^2
then force up = m g = 9.8 m

If you are moving it horizontally and there is no friction and it is stopped at the start, that is another problem entirely.
It has zero kinetic energy at the start and we need to find the KE at the end to find how much work is done.
we need the acceleration
36 = (1/2) a (14)^2
a = .367 m/s^2
then
v = a t = .367 *14 = 5.14

then (1/2) m (5.14)^2 = 726*14

However I suspect that the way bobpursley told you is what your teacher wants.

• physics - ,

That's a lot of work...

umm, it says nothing about "moving up against gravity"

but if you say so... thanks, i guess.

can you give me another example with this question

-What is the mass of an object if it took 270 J of work to move it 15 meters?

• physics - ,

9.8 is g
If moving it up then
F = m g
F * distance = work = 15 m (9.8) in Joules
so
270 = 15 * m * 9.8
m = 270 / (15*9.8)