A*R*C*S 7.3.1. Pitching Speed.
A baseball player wants to determine his pitching speed. You have him stand on a ledge and throw the ball horizontally from an elevation \(5 \mathrm{~m}\) above the ground. The ball hits the ground \(30 \mathrm{~m}\) away. What is his pitching speed?
Solution.
1. Analyze and Represent
- \begin{equation*} y_i = 5 \mathrm{~m} \end{equation*}\begin{equation*} y_f = 0 \end{equation*}\begin{equation*} x_i = 0 \end{equation*}\begin{equation*} x_f = 30 \mathrm{~m} \end{equation*}\begin{equation*} a_y = -g = -10 \mathrm{~m/s^2} \end{equation*}\begin{equation*} v_{ix} = ? \end{equation*}
- Particle model: ignore rotation of the ball. Near Earth with gravity as the only force, leading to an acceleration of \(-g\hat{y}\text{.}\)
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A simple physical picture will probably do here.
Figure 7.3.1. A baseball’s path.
2. Calculate
- \begin{equation*} x_f = x_{i} + v_{ix}t \end{equation*}\begin{equation*} y_f = y_{i} + v_{iy}t + \frac{1}{2}a_y t^2 \end{equation*}
- \begin{equation*} 0 = y_{i} - \frac{1}{2}g t^2 \end{equation*}\begin{equation*} t = \sqrt{\frac{2y_i}{g}} \end{equation*}\begin{equation*} x_f = v_{ix}t = v_{ix}\sqrt{\frac{2y_i}{g}} \end{equation*}\begin{equation*} v_{ix} = x_f\sqrt{\frac{g}{2y_i}} \end{equation*}
- \begin{equation*} v_{ix} = 30 \mathrm{~m/s} \end{equation*}
3. Sensemake
- \begin{equation*} \mathrm{m/s} = \mathrm{m}\sqrt{\frac{\mathrm{m/s^2}}{\mathrm{m}}} = \mathrm{m/s} \end{equation*}
- \(30 \mathrm{~m/s}\) is about \(65 \mathrm{~mph}\text{.}\) Professional baseball pitchers regularly reach around \(100 \mathrm{~mph}\text{,}\) so while my friend is doing okay, he might need a little more practice before going pro.
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\(v_{ix}\) depends on three quantities, two of which are of interest: \(y_i\) and \(x_f\text{.}\) For \(x_f\text{,}\) if you are able to throw the ball a greater horizontal distance without it falling farther, you must be throwing it faster. Sure enough, \(x_f\) is in the numerator of our expression, so this agrees with our physical reasoning.On the other hand, a larger value of \(y_i\) means that the baseball will fall a greater distance without actually traveling any farther horizontally. This can only be true if the ball is thrown with a smaller initial speed. The equation has \(y_i\) in the denominator, so once again this agrees!