Here're three problems I have trouble solving, can someone please point out what formula I should be using?(adsbygoogle = window.adsbygoogle || []).push({});

1) An adult pulls a child across the ice on a sled. The combined weight of the sled and the child is 26N, and the adult pulls on a rope that joins the sled at an angle of 30deg (Pi/6 Rad) above the horizontal. Ignoring friction, how much work is done in pulling the sled 390m?

I did:

26 = F sin 30deg

F = 52N

Now, should I do W = Fd or W = Fdcos30? And why?

2) There are three charges, A, B, and C. A and B are vertically seperated by a distance of 0.060m and A has the charge -4.2 x 10^-5C and B has the charge 8.4x10^-5C. B and C are horizontally seperated by a distance of 0.140m, and C has a charge of 2.1x10^-5C.

Now, when you're calculating the total charge on B, you're finding the resultant vector..

What if this question said "Find the total charge (force) acting on charge A" or C? Is that even possible? If so, how would that be calculated..

3) An engineer can vary the rate at which water falls directly down through a generator at a hydroelectric power plant. The generator and its turbine can supply 268MW of electric power when the rate at which the water supplies power to the turbine is 335MW.

a) Efficiency? Overly simple, 268/335 = 80%

b) What is the change in potential energy of the falling water per second? No clue.. :-\

c) If water falls 16m to the turbine, what mass of water must be directed through the turbine per second to supply the power indicated above? Still.. :-\

I hope someone can help out with these questions.. I'd appreciate it as my finals are tomorrow.

Thanks.

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# Two Physics problems.. Work and Electricity!

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