A2L Item 130

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

The friction force on the block is

  1. mgcos(θ), down the incline.
  2. mgsin(θ), up the incline.
  3. μmgsin(θ), down the incline.
  4. μmgcos(θ), up the incline.
  5. none of the above.
  6. cannot be determined



(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.