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`“Life is like riding a bicycle.  To keep your balance you must keep moving.”–Albert Einstein`

## Thursday, June 25, 2009

### AP Physics B – Free Response Practice Question involving Friction

Free response as well as multiple choice questions involving friction are often seen in degree entrance examinations. Friction is a common force with interesting properties and it plays a very important role in our life. For instance see the post Friction Moves the Car and Friction Stops the Car at Physicsplus. The figure (Fig.1) shows a block of mass M, placed on a plane surface AB inclined at an angle θ with the horizontal AC. Answer the questions relevant to the situations given below:

(a) Initially the frictional force Ff between the inclined plane and the block is sufficient to keep the block at rest. The coefficient of static friction between the incline and the block is μ. On the dot below (Fig.3)that represents the block, draw and label a free body diagram for the block in this situation.

(b) Determine the magnitude of the frictional force on the block in the situation described in part (a). (c) A force F is now applied on the block in a direction making an angle α (Fig.2) with the inclined plane so that the block just moves up the plane.

Determine the magnitude of the minimum force (Fmin) required for pulling the block up the plane.

(d) The inclination of the plane AB with the horizontal is made equal to 30º. When a block of mass 2 kg is placed on the incline and a gentle push is given, the block moves down the plane with constant velocity of 0.2 ms–1.

Determine the magnitude of the frictional force on the block and the coefficient of friction in this situation, assuming that g = 10 ms–2.

Even though this question is meant for AP Physics B students, AP Physics C aspirants also may work it out. The question can be easily modified to suit to the AP Physics C students.

Try to answer the above question which carries 15 points. You can take about 17 minutes for answering it. I’ll be back shortly with a model answer for you.