A2L Item 130
- Description: Identify the friction force on a block at rest on an incline.
- Goal: Hone the concept of static friction
- Source: UMPERG-ctqpe29
- Keywords: Magnetic Field, Mechanics
The question for students:
A block of mass m sits at rest on a
rough incline of angle θ. The coefficient of static friction is
μ.
The friction force on the block is
- mgcos(θ), down the incline.
- mgsin(θ), up the incline.
- μmgsin(θ), down the incline.
- μmgcos(θ), up the incline.
- none of the above.
- cannot be determined
Commentary for teachers:
Answer
(2) this is all that is needed to hold the block at rest. Some students will give #4 as the answer having specified the maximum static friction force.
It helps to classify forces as model forces obtainable from a formula, and procedure forces. Static friction is an example of a procedure force, one that cannot be determined without application of the 2nd law.