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A2L Item 130

Meta-Information
Description: 
Identify the friction force on a block at rest on an incline.
Goal: 
Hone the concept of static friction
Level: 
High School
Level: 
Undergraduate
Keywords: 
Magnetic Field
Keywords: 
Mechanics
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

  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
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.