Chapter 3: Applications of Differentiation
Section 3.10: Antiderivatives
Example 3.10.4
From the edge of a cliff 500 ft above ground level, a rock is thrown upward with a speed of 60 ft/s.
Obtain the position function yt.
When does the rock reach its maximum height?
What is the maximum height attained?
When does the rock drop to ground level?
Solution
Mathematical Solution
Near the surface of the earth, objects are are subjected to a downward gravitational acceleration denoted by g, with value 32 ft/s2 (9.8 m/s2). It is this gravitational acceleration that accounts for the force pulling objects earthward.
Establish a coordinate system with y positive upward, measured from ground level. Then, the problem is described by the following.
y″t=−32, y′0=60, y0=500
As in Example 3.10.2, the position function yt is found by antidifferentiating twice, paying attention to the initial conditions y′0=60 and y0=500. Hence, y′t=−32 t+c, with c=60 to satisfy the condition y′0=60, so yt=−32 t+60.
The position function is yt=−32 t22+60 t+C, with C=500 to satisfy the condition y0=500., so yt=−16 t2+60 t+500.
The rock reaches its maximum height when y′t=−32 t+60=0, or at t = 60/32 = 15/8.
The maximum height above ground is y15/8=−16 15/82+60 15/8+500 = 2225/4≐556.25 ft, or some 56.25 ft above the top of the cliff.
The rock reaches ground level when yt=0, so the quadratic equation −16 t2+60 t+500=0 must be solved for t=53 ±89/8. Of course, just the positive time is meaningful here, and that time is approximately 7.77 seconds after the rock was launched.
Maple Solution
As in Example 3.10.2, use the AntiderivativePlot command to obtain the required antiderivatives. After obtaining the position as a function of time, render it a function by selecting the Assign Function option in the Context Panel.
Click the restart icon in the toolbar or execute the restart command at the right.
restart
Tools≻Load Package: Student Calculus 1
Loading Student:-Calculus1
Apply the AntiderivativePlot command to the acceleration y″t; include the initial velocity condition.
Apply the AntiderivativePlot command to the velocity y′t; include the initial position condition.
AntiderivativePlot−32,t=0..10, output=antiderivative,value=0,60
−32⁢t+60
yt=AntiderivativePlot,t=0..10, output=antiderivative,value=0,500
y⁡t=−16⁢t2+60⁢t+500
→assign as function
y
The answers to the remaining three questions can be extracted from the position function yt.
Find the time at which the maximum height is attained
Set the velocity equal to zero Press the Enter key.
Context Panel: Solve≻Solve
y′t=0
−32⁢t+60=0
→solve
t=158
Find the maximum height attained
Write y15/8 Context Panel: Evaluate and Display Inline
Context Panel: Approximate≻5 (digits)
y15/8 = 22254→at 5 digits556.25
Find the time at which the rock reaches ground level
Write yt=0 and press the Enter key.
Control-drag the positive solution Context Panel: Approximate≻5 (digits)
yt=0
−16⁢t2+60⁢t+500=0
t=158−58⁢89,t=158+58⁢89
158+58⁢89→at 5 digits7.7712
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