Which force is causing the object to take a circular path?

When you drop a pebble from height a it reaches the ground with speed v if there is no air resistance. from what height should you drop it so it will reach the ground with three times the speed? (a) 9m (b) 3m (c) 6m (d) 18m

When you push downward on a book at rest on a table, you feel an upward force. does this force depend on friction? defend your answer.

Your town is considering building a biodiesel power plant describe at least two advantages and two disadvantages