
William M. answered 09/25/24
Mechanical Engineer - I use physics daily
I am assuming this is in regard to the sport of shot put, and the height of the thrower is such that the shot will be 2 meters off of the ground at release.
I am also assuming that the angle of 37o refers to the angle up from the ground to the initial shot trajectory. Therefore the shot has an initial upward velocity of (10 m/s)*(sin(37)), and it has an initial horizontal velocity of (10 m/s)*(cos(37)).
Neglecting air resistance, we can use the equation of vertical position (Y) for an object with some initial position (Yo), velocity (Vyo) and acceleration due to gravity (9.806 m/s2 downward).
Since the shot is starting from 2 m off of the ground, traveling upwards, then back down, it must pass the 2 m high point before returning to the ground. The time that it takes to go up then back down to the 2 m high point can be found from:
Y = Yo + (Vyo)*(t) -(1/2)*(9.806 m/s2)*(t)2
Substituting known quantities gives:
2 m = 2 m + [(10 m/s)*sin(37)]*(t) - (1/2)*(9.806 m/s2)*(t)2
[(10 m/s)*sin(37)]*(t) = (1/2)*(9.806 m/s2)*(t)2
[(10 m/s)*sin(37)] = (1/2)*(9.806 m/s2)*(t)
t = [(10 m/s)*sin(37)] / [(1/2)*(9.806 m/s2)]
t = 1.227 s
So it takes 1.227 seconds for the shot to rise up in its trajectory and fall back to the starting height of 2 m off of the ground.
The remainder of the time that the shot spends in the air is the time it takes to fall from 2 m high. Using the equation Vy = Vyo - (g)(t), with Vyo = [(10 m/s)*sin(37)] , and g=9.806 m/s2, we can find the vertical velocity of the shot as it passes the 2 m high mark. Vy = -6.01 m/s (which is approximately the opposite of the initial vertical velocity upwards). Using an initial vertical velocity of -6.01 m./s and an acceleration of -9.806 m/s2 with an initial height of 2 m, we can form the equation:
Y = Yo + (Vyo)*(t) -(1/2)*(9.806 m/s2)*(t)2
0 = 2 m +(-6.01 m/s)*(t) - (4.903 m/s2)*(t)2
rearranging into quadratic form:
(t)2 + (1.227)*(t) - (0.4079) = 0
[solve by quadratic equation solution]
Here the negative value of (t) can not be used, so t = 0.272 s.
So it takes an additional 0.272 s until the shot hits the ground. Adding the 1.227 s it took for the shot to go from 2 m, into the air, and back down to 2 m, gives a total of 1.5 s.
In 1.5 seconds, the shot goes a horizontal distance of [(10 m/s)*cos(37)] *(1.5 s) = 12 m.
Mark S.
Hi William! I like the detail of your solution here, but I would like to point out that adding the extra step of finding the maximum height of the shot is unnecessary. It however, is good for practice as it implements another kinematic equation!09/25/24