Answer to Question #115191 in Physics for yamann

Question #115191
A 73 kg object slowed down to 3.0 m/s. If the work done on this object is -2.0 x 10^3 J, then what is the initial speed
1
Expert's answer
2020-05-11T20:20:38-0400

According to the kinetic energy theorem, the work was spent to slow the body down:


KEfKEi=W,12mu212mv2=W, 12mv2=12mu2W, v=u22Wm=322(2.0103)73=8 m/s.KE_f-KE_i=W,\\ \frac{1}{2}mu^2-\frac{1}{2}mv^2=W,\\ \space\\ \frac{1}{2}mv^2=\frac{1}{2}mu^2-W,\\ \space\\ v=\sqrt{u^2-\frac{2W}{m}}=\sqrt{3^2-\frac{2(-2.0 \cdot 10^3)}{73}}=8\text{ m/s}.

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