A2L Item 046
- Description: Select correct free body diagram for block on incline.
- Goal: Recognize appropriate free body diagram.
- Source: UMPERG
- Keywords: Forces, Mechanics, Vectors
The question for students:
A
block of mass 2 kg is at rest on the surface of a wedge that makes an
angle of 30° with the horizontal as shown. The coefficient of
static friction is 0.64 and of kinetic friction is 0.42.
Which figure below best represents the free-body diagram for the block?
6. None of the above
7. Cannot determine the free-body diagram
Commentary for teachers:
Answer
(1). The component of the gravitational force parallel to the incline will balance the frictional force in this situation. The component of the gravitational force perpendicular to the incline must balance the normal force.
Background
Even in static situations students can be confused about the relative sizes of forces.
Questions to Reveal Student Reasoning
What forces are present? What determines how big the gravitational force is? … the normal force is? … the gravitational force is?