Thursday
May 23, 2013

# Homework Help: Physics

Posted by michelle quendarsv on Saturday, November 20, 2010 at 8:29pm.

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 - Damon, Saturday, November 20, 2010 at 7:27pm
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 - Damon, Saturday, November 20, 2010 at 7:29pm
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 - michelle quendarsv, Saturday, November 20, 2010 at 8:03pm
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 - Damon, Saturday, November 20, 2010 at 8:15pm
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)

Physics - michelle quendarsv, Saturday, November 20, 2010 at 8:28pm
Okay, thank you...

I have another question

If an object has a mass of 47 kg and it is moved 27 meters in 60 seconds, how much power was used?

(also, would this also include the 9.8 from g?)

Thanks.

• Physics - bobpursley, Saturday, November 20, 2010 at 8:40pm

Michelle: You need a book I know of. Go to any college bookstore, or BarnesNoble, and take a look at Schaum's Outline series, Physics. There are a number of titles in this series, I like the Physics for Scientists and Engineers, but that may be a bit mathematical for you, so College Physics may be more to your liking. But look at them. They are inexpensive, and have about 2000 solved problems in them, organized by units.

• Physics - michelle quendarsv, Saturday, November 20, 2010 at 8:55pm

Oh ok, thank you :)

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