Hi, I'm struggling with the following 3 questions and would appreciate some help - thanks. I know what the answers are but don't know why they're right.
1. Two electrical heating coils, L and M, are made from wires of the same material. The wires are of equal length but different diameters. L runs at twice the voltage of M but both coils dissipate the same power. What is the correct conductance ratio for the two coils L/M. (answer is 1/4)
2. A ball rolls up a ramp which is at angle 20 degrees to the horizontal. The speed of the ball at the bottom of the ramp is 2.2m/s. L is the distance the ball moves along the ramp before coming to rest. What is the distance L? Ignore the effects of friction and rotation. (answer is 0.72m)
3. A firework rocket with a mass of 0.4kg is launched vertically upwards with an inital acceleration of 6.2m/s/s. What is the force on the rocket from the burning fuel. (the answer is 6.4N, I see that comes from (9.8+6.2) x 0.4, but I don't understand why we're taking gravity into account, and if we are, why we aren't using -9.8)
Thanks!