Posted by Eric on Friday, September 30, 2011 at 11:15am.
You and your friend are practicing pass plays with a football. You throw the football at a 35.0° angle above the horizontal at 19.0 m/s. Your friend starts right next to you, and he moves down the field directly away from you at 5.50 m/s. How long after he starts running should you throw the football? Assume he catches the ball at the same height at which you throw it.

Physics  Henry, Saturday, October 1, 2011 at 4:43pm
Vo = (19m/s,35 deg.).
Xo = hor.= 19cos35 = 15.56m/s.
Yo = ver.= 19sin35 = 10.90m/s.
h = (Vf^2  Yo^2) / 2g,
h = (0  (10.9)^2) / 19.6 = 6.06m.
t(up) = (Vf  Yo) / g,
t(up) = (0  10.9) / 9.8 = 1.11s.
t(dn) = t(up) = 1.11s.
T = t(up) + t(dn) = 1.1 + 1.11 = 2.22s
= Time in flight.
d = Xo*T = 15.56 * 2.22 = 34.5m = hor
distance traveled.
t = d/V = 34.5 / 5.5 = 6.27s
td + T = 6.27s,
td = 6.27  T = 6.27  2.22 = 4.07s.
after the receiver starts running, the
ball should be thrown.
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