Answer:
She must launch the balloon at a 59.9° angle above the horizontal plane.
Explanation:
Refer to the accompanying diagram for a visual explanation of the scenario.
The position and velocity vectors of the balloon at time "t" can be computed using these formulas:
r = (x0 + v0 · t · cos θ, y0 + v0 · t · sin θ + 1/2 · g · t²)
v = (v0 · cos θ, v0 · sin θ + g · t)
Where:
r = position vector at time "t".
x0 = initial horizontal position.
v0 = initial velocity.
t = time.
θ = launch angle.
y0 = initial vertical position.
g = acceleration due to gravity (-9.81 m/s² if upward is treated as positive).
v = velocity vector at time "t".
We can establish the origin of the reference frame at the launch point, making x0 and y0 = 0.
At the maximum altitude (276 m), the balloon's velocity vector is horizontal (refer to v1 in the diagram). Thus, the y-component of the velocity vector is zero. We can express this using the velocity vector's y-component equation:
At maximum altitude:
vy = v0 · sin θ + g · t
0 = v0 · sin θ + g · t
Also, at maximum height, the y-component of the position vector is 276 m (see r1y in the diagram). Therefore:
At peak height:
y = y0 + v0 · t · sin θ + 1/2 · g · t²
276 m = y0 + v0 · t · sin θ + 1/2 · g · t²
We have two equations involving two unknowns (θ and t):
276 m = y0 + v0 · t · sin θ + 1/2 · g · t²
0 = v0 · sin θ + g · t
To solve for the variables, we can rearrange the velocity equation for sin θ and substitute it back into the position equation to find t and eventually θ:
0 = v0 · sin θ + g · t
0 = 85.0 m/s · sin θ - 9.81 m/s² · t
9.81 m/s² · t / 85.0 m/s = sin θ
Now, substituting sin θ in the position equation:
276 m = y0 + v0 · t · sin θ + 1/2 · g · t² (y0 = 0)
276 m = 85.0 m/s · t · (9.81 m/s² · t /85.0 m/s) - 1/2 · 9.81 m/s² · t²
276 m = 9.81 m/s² · t² - 1/2 · 9.81 m/s² · t²
276 m = 1/2 · 9.81 m/s² · t²
276 m / (1/2 · 9.81 m/s²) = t²
t = 7.50 s
With t known, we can find the angle θ using the previously derived formula:
9.81 m/s² · t / 85.0 m/s = sin θ
9.81 m/s² · 7.50 s / 85.0 m/s = sin θ
θ = 59.9°
She should launch the balloon at a 59.9° angle above the horizontal.