Goal: Hone the concept of static friction
Source: UMPERG-ctqpe27
A mass of 5 kg sits at rest on an incline making an angle of 30° to
the horizontal.
If μs = 0.7 the friction force on the block is
- 43.3N, down the incline
- 25N, up the incline
- 10N, down the incline
- 30.3N, 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 calculated 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.