# What is the force that prevents an object in an accelerating inclined plane from rolling/sliding downhill? [duplicate]

A ball on a stationary frictionless wedge would naturally roll down because of the component of its weight along the plane. But I read if the wedge is accelerated fast enough it would prevent the ball from doing so. For this to be true there must be a new force that acts on the ball and whose magnitude will depend on the acceleration of the wedge (since a certain acceleration must be reached). But I'm having trouble comprehending this new force. Is it the 'reaction' force to that of causing the acceleration or is it simply inertia and the ball is just trying to be the way it was? Or is it something else entirely? It also said that with enough acceleration the ball would slide up the ramp and fall over. I can visualize this but can't seem to explain it.

• – sammy gerbil Sep 7 '17 at 8:02

You are right - when the wedge accelerates to the right, it applies a force to the ball along the normal of the plane. This force can be thought of as having a horizontal and vertical component - and if the vertical component is equal to $mg$, the ball will only accelerate horizontally (that is the direction of the net component of force on the ball).