A block of mass M is held motionless on a frictionless inclined plane by means of a string attached to a vertical wall as shown in the drawing.
-What is the magnitude of the tension in the string?
A)zero newtons
B)Mg
C)Mg cos $\theta$
D)Mg sin $\theta$
E)Mg tan $\theta$

Question

A block of mass M is held motionless on a frictionless inclined plane by means of a string attached to a vertical wall as shown in the drawing. 
-What is the magnitude of the tension in the string?
A)zero newtons 
B)Mg 
C)Mg cos $\theta$ 
D)Mg sin $\theta$ 
E)Mg tan $\theta$

Quizplus · Accepted Answer

Since the block is motionless, the force of gravity down the plane must be balanced by the tension in the string pulling up the plane. The force of gravity down the plane is Mg sinθ, where θ is the angle of the incline. Therefore, the tension in the string must also be Mg sinθ in order to balance the downward force and keep the block stationary. Thus, the correct answer is D.

A Block of Mass M Is Held Motionless on a Frictionless