A small block with a mass of 0.12kg is attached to a cord passing through a hole

Question

A small block with a mass of 0.12kg is attached to a cord passing through a hole in a frictionless, horizontal surface - see figure. The block is originally revolving at a distance of 0.40m from the hole with a speed of 0.70m/s. The cord is then pulled from below shortening the radius of the circle in which the block revolves to 0.10m At this new distance, the speed of the block is observed to be 2.80m/s.

What is the tension in the cord in the orginal situation with the block's speed is 0.70m/s?

What is the tension in the cord in the final situation with the block's speed 2.80m/s?

How much work is done by the person who pulled the cord?

Explanation / Answer

the tension in the cord in the orginal situation with the block's speed is 0.70m/s

=massXspeed2/radius=0.12X0.72/0.4=0.147N

the tension in the cord in the final situation with the block's speed 2.80m/s

=massXspeed2/radius=0.12X2.802/0.10=9.408N

work done by the person who pulled the cord=change in the kinetic eenergy

1/2Xmassx(V22-V12=0.5X0.12X(2.82-0.72)=0.441N

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