physics questions?

Question

i need help. can anyone help me????

10. A 3.20 102 kg piano is being lifted at a steady speed from ground level straight up to an apartment 10.0 m above the ground. The crane that is doing the lifting produces a steady power of 4.00 102 W. How much time does it take to lift the piano?

12. A 50 g ball is thrown from a window with an initial velocity of 6.0 m/s at an angle of 30° above the horizontal.
(a) Using energy methods, determine the kinetic energy of the ball at the top of its flight.
(b) Determine its speed when it is 3.0 m below the window.

Answer 1

10. Power is a rate of energy flow. Work done =potntial energy gained. We have
P=Pe/t
so
t=Pe/P
t=mg h /P
t=3.2E+2 x 9.81 x 10.0/4.00 E+2=78.5 sec

12. You only need to know
Pe=mgh and Ke=(1/2) mV^2

A) At the very top you will have max potential energy Pe and Vh the only contributor to kinetic energy Ke.
a) Vh=Vcos(30)
Ke'=(1/2) mVh^2

b) At 3 m below the window
Ke= Pe

Pe= mg H where H= h+3m
h=(1/2) g t^2
t=Vv/g where Vv=Vsin(30)
h=(1/2) g (Vsin(30)/g)^2

then since Pe + Ke'=Ke Ke' is horizontal potentia l energy we have computed earlier.

(1/2) m V^2 = Pe + Ke'
so V= Sqrt( 2(Pe + Ke' )/m)