A2L Item 128

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

  1. 43.3N, down the incline
  2. 25N, up the incline
  3. 10N, down the incline
  4. 30.3N, 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 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.