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# HEEEEEELP GOT TEST TOMORROW

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if a engine has to work 100 joule and the effect is 25w how long does it take ?

physics
Nov 5, 2015

#1
+15

I'm assuming you mean the power of the engine is 25 W. "effect" is a very broad term.

Power is rate at which work can be done, shown by its formula below:

$$P=\frac{w}{t}$$

P is power, w is work, and t is time. We'd use the equation for the power and work in your question.

$$25=\frac{100}{t}$$

$$t=\frac{100}{25}$$

$$t=4s$$

The engine would take 4 seconds.

Nov 5, 2015

#1
+15

I'm assuming you mean the power of the engine is 25 W. "effect" is a very broad term.

Power is rate at which work can be done, shown by its formula below:

$$P=\frac{w}{t}$$

P is power, w is work, and t is time. We'd use the equation for the power and work in your question.

$$25=\frac{100}{t}$$

$$t=\frac{100}{25}$$

$$t=4s$$

The engine would take 4 seconds.

Will85237 Nov 5, 2015
#2
+3

Well a joule is equal to a W-s

so   J = w (time)

100 = 25 (t)

t= 4 seconds

Nov 5, 2015
#3
0

Thanks Will, and welcome back to the forum after all this time.

It is really great to see you again.

I have never understood much physics but every now an again I get inspired to try and understand.

I am a mathematicican so I like to understand the units. Not just the remember the formula.

This answer is only for my own benefit.  Will's answer is excellent.

-----------------------------------------

Now a Watt is a unit of power and  ( I googled this)

$$1W=1Kg\; m^2/s^3 = 1 \;Kg\;(ms^{-2})\;s^{-1} \qquad = \frac{1\;Kg\;m^2}{s^{-3}}$$

Now I think this means that 1W = the force needed to increase the acceleration of a mass 1Kg every second. I assume that this is in a vacuum.

Please correct me if I am wrong.

The joule (/ˈdʒuːl/), symbol J, is a derived unit of energy in the International System of Units. It is equal to the energy transferred (or work done) to an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre (1 Joule = 1 newton metre or N. · m).

ok I better google Newton now

One newton is the force needed to accelerate one kilogram of mass at the rate of one metre per second squared.

1 Newton = 1Kg m/s2

so we have

FORCE unit 1 Newton   = 1Kg m/s2

WORK unit = 1 Joule     = FORCE  * METRES =  Newton Metres  = 1Kg m/s* m  =  1Kg m2/s2

POWER  UNIT =  1 WATT  =  WORK/SEC = NEWTONS/SEC = 1Kg m2/s3

Ok..............

so

force (Newtons) mass* acceleration

work (joules) is  force exerted times by distance

power (watts)  is how much work is done every second

if a engine has to work 100 joule (work)  and the effect is 25w (power) how long (time) does it take ?

Power is how much work is done every second so that must equal 100/s

so

power = work/time

25 = 100/t

25t = 100

t = 4 seconds

I wish I could remember this stuff. ://

Nov 6, 2015
edited by Melody  Nov 6, 2015