The puck in the figure has a mass of 0.12 kg. Its original distance from the cen
ID: 2188188 • Letter: T
Question
The puck in the figure has a mass of 0.12 kg. Its original distance from the center of rotation is 55 cm, and the puck is moving with a speed of 85 cm/s in a circle. The string is pulled downward until the center of rotation has moved to r=22 cm. The table is effectively frictionless.
What is the original angular momentum of the puck?
What is the angular momentum after the puck has moved to r=0.22?
What is the original moment of inertia?
What is the moment of inertia after the puck has reached r=0.22?
How much work was done in pulling the string?
Explanation / Answer
a) L = m . v . r = 0.12 . 0.85 . 0.55 = 0.0561 kgm^2/s b) Same - no torque has been applied c) Ii = mr^2 = 0.12 . 0.55^2 kgm^2=.0363 d) Same again with different radius = Iii=.12x.22^2=.005808 e) Work done = Final KE – original KE Use angular momentum stays the same to work out the final velocity of the puck. Plug that in to 1/2 . m . v^2 to get the final energy . Work out the original energy and take them away. Thats the work done.
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