What does a pulley system need in order to reduce the force needed to lift an object?
A. exactly one pulley that has a fixed position
B. No pulley system can reduce the force needed to lift an object.
C. at least one movable pulley
D.
a load that is not too heavy

photocells convert light energy to electricity

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me