As a roller coaster car crosses the top of a 40-m-diameter loop-the-loop, its apparent weight is the same as its true weight. What is the car's speed at the top?
At the top of the loop, if the car has any forward speed, the centrifugal force will seem to pull the car vertically up into the air, in the opposite direction of the force of gravity, and hence it should seem to have less weight than if it was sitting on the ground.
However, at the top of the loop, the car's apparent weight is the same as it's true weight. This tells us that the centrifugal force is NOT acting. Which means that the car has zero forward speed at the top of the loop.
PS: There's no such thing as a centrifugal force. What is being experienced as a centrifugal force is actually a LACK of a centripetal force (a force that pushes the body towards the centre of motion.)