You get home from class and throw your backpack on the table. The mass of the ba

Question

You get home from class and throw your backpack on the table. The mass of the backpack is 3 kg. The backpack lands on the table with a horizontal velocity of 1.5 m/s and then slides across the table, coming to a stop in a time of 1.2 seconds. Assume all the motion of the backpack is purely in the horizontal direction, that a constant frictional force acts on the backpack, and that the acceleration of the backpack is constant as it slides to a stop. What is the magnitude of the frictional force acting on the backpack as it slides?   

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Explanation / Answer

Initial velocity of the backpack (u) = 1.5 m/s
Final velocity of the backpack (v) = 0 m/s (finally it stops)
time taken by backpack to stop (t) = 1.2 s
Now using kinematic equation
v = u+ at
0 = 1.5 +a*1.2
a = -1.25 m/s2
-ve sign indicate that the acceleration is in opposite direction of velocity.
hence the friction force will be
f = ma
where m is mass of backpack
f = 3*1.25 = 3.75 N
Hence the friction force will be 3.75 N

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