[问答题]
A child wishes to determine the speed a slingshot imparts to a rock. Howcan this be done using only a meter stick, a rock, and the slingshot?
参考答案:
To find the initial speed, use the slingshot to shoot the rock directly horizontally (no initial vertical speed) from a height of 1 meter. The vertical displacement of the rock can be related to the time of flight by Eq. 2-11b. Take downward to be positive.
$y=y_{0}+v_{y 0} t+\frac{1}{2} a t^{2} \rightarrow 1 \mathrm{~m}=\frac{1}{2} g t^{2} \rightarrow t=\sqrt{2(1 \mathrm{~m}) /\left(9.8 \mathrm{~m} / \mathrm{s}^{2}\right)}=0.45 \mathrm{~s}$
Measure the horizontal range R of the rock with the meter stick. Then, if we measure the horizontal range R , we know that $ R=v_{x} t=v_{x}(0.45 \mathrm{~s}) $, and so $v_{x}=R / 0.45 \mathrm{~s}$ . The only measurements are the height of fall and the range, both of which can be measured by a meter stick.
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