27. balloons and motorcycles a hot - air balloon is 150 ft above the ground when a motorcycle (traveling in…

27. balloons and motorcycles a hot - air balloon is 150 ft above the ground when a motorcycle (traveling in a straight line on a horizontal road) passes directly beneath it going 40 mi/hr (58.67 ft/s). if the balloon rises vertically at a rate of 10 ft/s, what is the rate of change of the distance between the motorcycle and the balloon 10 seconds later?
Answer
Explanation:
Step1: Set up variables
Let (y) be the height of the balloon, (x) be the horizontal distance of the motorcycle from the point directly beneath the balloon initially, and (z) be the distance between the balloon and the motorcycle. By the Pythagorean theorem, (z^{2}=x^{2}+y^{2}).
Step2: Find (x) and (y) at (t = 10s)
The motorcycle's speed (v_{x}=58.67\ ft/s), so (x = 58.67\times10=586.7\ ft). The balloon's initial height (y_{0}=150\ ft) and its rising speed (v_{y}=10\ ft/s), so (y=150 + 10\times10=250\ ft).
Step3: Differentiate (z^{2}=x^{2}+y^{2}) with respect to (t)
Differentiating both sides with respect to (t) gives (2z\frac{dz}{dt}=2x\frac{dx}{dt}+2y\frac{dy}{dt}), or (\frac{dz}{dt}=\frac{x\frac{dx}{dt}+y\frac{dy}{dt}}{z}).
Step4: Find (z) at (t = 10s)
Using (z^{2}=x^{2}+y^{2}), (z=\sqrt{586.7^{2}+250^{2}}=\sqrt{344216.89 + 62500}=\sqrt{406716.89}\approx638\ ft).
Step5: Substitute values into (\frac{dz}{dt}) formula
Substitute (x = 586.7\ ft), (\frac{dx}{dt}=58.67\ ft/s), (y = 250\ ft), (\frac{dy}{dt}=10\ ft/s), and (z\approx638\ ft) into (\frac{dz}{dt}=\frac{x\frac{dx}{dt}+y\frac{dy}{dt}}{z}). (\frac{dz}{dt}=\frac{586.7\times58.67+250\times10}{638}=\frac{34421.689+2500}{638}=\frac{36921.689}{638}\approx57.9\ ft/s)
Answer:
The rate of change of the distance between the motorcycle and the balloon (10) seconds later is approximately (57.9\ ft/s)