Goal: Hone the concept of static friction
Source: UMPERG-ctqpe29
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:
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.