Physics (10th Edition)

Published by Wiley
ISBN 10: 1118486897
ISBN 13: 978-1-11848-689-4

Chapter 6 - Work and Energy - Problems - Page 169: 57

Answer

The minimum initial speed the puck should have is $2.4m/s$.

Work Step by Step

According to the work-energy theorem, $$E_f-E_0=W_{nc}$$ The puck never leaves the ground, so in this case, $PE=0$. Therefore, $E=KE$ $$\frac{1}{2}mv_f^2-\frac{1}{2}mv_0^2=W_{f}$$ $$\frac{1}{2}m(v_f^2-v_0^2)=W_{f}$$ In the first case, the hockey player gives the puck $v_0=1.7m/s$ and after traveling only half a distance, $v_f=0$, so $$W_{f, 1}=-1.445m $$ $$-fs_1=-1.445m$$ $$fs_1=1.445m (1)$$ (the negative sign shows the force negates the motion) In the second case, the hockey player gives the puck $v_0$ enough to travel to a teammate and come to a halt $v_f=0$, so $$W_{f, 2}=-\frac{1}{2}mv_0^2 $$ $$-fs_2=-\frac{1}{2}mv_0^2 $$ $$fs_2=\frac{1}{2}mv_0^2 (2)$$ Divide (2) over (1), $$\frac{s_2}{s_1}=\frac{1/2v_0^2}{1.445}=\frac{v_0^2}{2.89}$$ We know $s_2=2s_1$, so $v_0=2.4m/s$
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