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Don't let his size fool you. This kid plays a wicked game of tennis! His
dad coached him in the laws of physics so he can rapidly calculate the
proper velocity for the ball to just clear the 1.0-meter high net. At the
moment, he is 10 m from the net. When the ball reaches his racquet it is
20 cm above the ground.
To win this set, you'll need to calculate the velocity necessary for the ball to just clear the net. Remember, velocity is a vector! (Hint: first find the time, then the initial horizontal and vertical velocity components...) |
The ball must rise 80 cm to clear
the net. Since "...falling up is the same as falling down," t = (2dy/g)1/2
= 0.404 s. The initial vertical velocity must therefore be vy
= gt = +3.96 m/s.
The ball must travel 10 m in 0.404 s, so vx = dx/t = 24.8 m/s. Using the Pythagorean theorem, v = 25.1 m/s. Tan-1 (vy/vx) = 9.07° above the horizontal. |