A2L Item 128

• Description: Identify the friction force on a block at rest on an incline.
• Goal: Hone the concept of static friction
• Source: UMPERG-ctqpe27
• Keywords: Dynamics, Forces, Friction, Mechanics

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The question for students:

A mass of 5 kg sits at rest on an incline making an angle of 30° to the horizontal.

If μ_s = 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

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Commentary for teachers:

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.

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