if a engine has to work 100 joule and the effect is 25w how long does it take ?
+561 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.
+561 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.
+118608 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.
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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.
I also googled Joule.
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/s2 * 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. ://
