Question

A 2.0-kg white rock is spinning at the end of a light (massless) string in a circular path with a radius of 1.25 m on a horizontal frictionless surface (figure below). The rock has a tangential speed of the rock was 5.0 m/s. As you can imagine, the string slowly winds around a vertical rod and several seconds later the length of the string has shortened to 0.50 m. What is the tangental speed of the rock just when the string reaches this length?

3.9 m/s

7.5 m/s

9.3 m/s

12.5 m/s

22.5 m/s

Answer 1

To find the final speed, we should use the fact that the angular momentum is conserved.

Therefore,

$v_f = \frac{v_ir_i}{r_f} = 12.5 m/s$