A block of mass M slides down a frictionless plane inclined at an angle θ with the horizontal. The normal reaction force exerted by the plane on the block is
A) Mg.
B) Mg sin θ.
C) Mg cos θ.
D) Mg tan θ.
E) zero, since the plane is frictionless.

Question

Quizplus · Accepted Answer

The normal force is perpendicular to the surface of the inclined plane and opposite to the weight of the object (which is Mg in this case, pointing straight down). Therefore, the vertical component of the weight is balanced by the normal force so that the block does not fall into the plane. Using trigonometry, the vertical component of the weight is Mg cos θ. So, the normal force (which is equal in magnitude and opposite in direction to the vertical component of the weight) is Mg cos θ.

A Block of Mass M Slides Down a Frictionless Plane