Math Is Fun Forum

Kurtz

Member

Registered: 2007-10-01

Posts: 27

Physics: Power

I don't get how how to find the power? Do we use P=Fv?

Water flows over a section of Niagara Falls at the rate of 1.2 x 10^6 kg/s and falls 50.0m. How much power is generated by the falling water?

and also...

How long does it take a 19kW steam engine to do 6.8 x 10^7 J of work?

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luca-deltodesco

Member

Registered: 2006-05-05

Posts: 1,470

Re: Physics: Power

remember that there are two equations of power.
P = fv, P = dE/dt

p = fv can only be used when f and v are not dependant on eachover, i.e. you cant calculate v if you also dont know f at a specific point in time

in the water case, i would use p = dE/dt using gravitational potential energy, the energy generated by conversion of gravitational potential energy over 50m by 1.2 x 10^6 kg is (taking g to be 9.81) = 5.886×10^8 J, this much mass flows per second, so this much energy is gained per second, so the power is dE/dt = 5.886×10^8 W

the definition p = dE/dt can be used for second one aswell, since power is constant, dE/dt is constant, so we can write dE = P×dt so we have 6.8×10^7 = t×1.9×10^4, t = 3.6×10^3 (2sf) = 3600s = 1hour

hope im right ^^

Last edited by luca-deltodesco (2007-10-11 18:35:50)

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