Chapter 3 – Differentiation Rules

1. Chapter 3 – Differentiation Rules 3.9 Related Rates Section 3.9 Related Rates

2. Related Rates • If we are pumping air into a balloon, both the volume and the radius of the balloon are increasing and their rates of increase are related to each other. • But it is much easier to measure directly the rate of increase of the volume than the rate of increase of the radius. Section 3.9 Related Rates

3. Related Rates In related rate problems, the idea is to compute the rate of change of one quantity in terms of the rate of change of another quantity (which may be more easily measured). The procedure is to find an equation that relates the two quantities and then use the Chain Rule to differentiate both sides with respect to time. Section 3.9 Related Rates

4. Book Example 1 – pg. 248 # 1 If V is the volume of a cube with edge length x and the cube expands as time passes, find dV/dt in terms of dx/dt. Section 3.9 Related Rates

5. Book Example 2 – pg. 248 # 2 • If A is the area of a circle with radius r and the circle expands as time passes, find dA/dt in terms of dr/dt. • Suppose oil spills from a ruptured tanker and spreads in a circular pattern. If the radius of the oil spill increases at a constant rate of 1 m/s, how fast is the area of the spill increasing when the radius is 30 m? Section 3.9 Related Rates

6. Book Example 3 – pg. 248 # 5 A cylindrical tank with radius 5 m is being filled with water at a rate of 3 m3/min. How fast is the height of the water increasing? Section 3.9 Related Rates

7. Example 4 If then find when x=5 and y=12. Section 3.9 Related Rates

8. Book Example 5 – Pg. 248 #11 A plane flying horizontally at an altitude of 1 mile and a speed of 500 mph passes directly over a radar station. Find the rate at which the distance from the plane to the station is increasing when it is 2 mi away from the station. • What quantities are given in the problem? • What is the unknown? • Draw a picture of the situation for any time t. • Write an equation that relates the quantities. • Finish solving the problem. Section 3.9 Related Rates

9. Book Example 6 – pg. 248 #14 At noon, ship A is 150 km west of ship B. Ship A is sailing east at 35 km/h and ship B is sailing north at 25 km/h. How fast is the distance between the ships changing at 4:00 pm? • What quantities are given in the problem? • What is the unknown? • Draw a picture of the situation for any time t. • Write an equation that relates the quantities. • Finish solving the problem. Section 3.9 Related Rates

10. Book Example 7 – pg. 249 # 23 Water is leaking out of an inverted conical tank at a rate of 10,000 cm3/min at the same time that water is being pumped into the tank at a constant rate. The tank has height 6 m and the diameter of the top is 4 m. If the water level is rising at a rate of 20 cm/min when the height of the water is 2 m, find the rate at which the water is being pumped into the tank. Section 3.9 Related Rates

11. Book Example 8 – pg. 249 # 26 A swimming pool is 20 ft wide, 40 ft long, 3 ft deep at the shallow end, and 9 ft deep at its deepest point. A cross-section is shown below. If the pool is being filled at a rate of 0.8 ft3/min, how fast is the water level rising when the depth at the deepest point is 5 ft? Section 3.9 Related Rates

12. Book Example 9 – pg. 249 # 27 Gravel is being dumped from a conveyor belt at a rate of 30 ft3/min, and its coarseness is such that it forms a pile in the shape of a cone whose base diameter and height are always equal. How fast is the height of the pile increasing when the pile is 10 ft high? Section 3.9 Related Rates

13. Book Example 10 – pg. 249 # 35 If two resistors with resistance R1 and R2 are connected in parallel (see figure), then the total resistance R, measured in ohms () is given by If R1 and R2 are increasing at rates of 0.3 /s and 0.2 /s respectively, how fast is R changing when R1=80 and R2=100 ? Section 3.9 Related Rates

14. Book Example 11 – pg. 250 # 45 A runner sprints around a circular track of radius 100 m at a constant speed of 7 m/s. The runner’s friend is standing at a distance of 200 m from the center of the track. How fast is the distance between the friends changing when the distance between them is 200 m? Section 3.9 Related Rates

15. Try these problems A spotlight on the ground shines on a wall 12 m away. If a man 2 m tall walks from the spotlight toward the building at a speed of 1.6m/s, how fast is the length of his shadow on the building decreasing when he is 4 m from the building? Section 3.9 Related Rates

16. Try these problems The altitude of a triangle is increasing at a rate of 1 cm/min while the area of the triangle is increasing at a rate of 2 cm2/min. At what rate is the base of the triangle changing when the altitude is 10 cm and the area is 100 cm2. Section 3.9 Related Rates

17. Try these problems A kite 100 ft above the ground moves horizontally at a speed of 8 ft/s. At what rate is the angle between the string and the horizontal decreasing when 200 ft of string has been let out? Section 3.9 Related Rates

18. Try these problems A television camera is positioned 4000 ft from the base of a rocket launching pad. The angle of elevation of the camera has to change at the correct rate in order to keep the rocket in sight. Also, the mechanism for focusing the camera has to take into account the increasing distance from the camera to the rising rocket. Let’s assume the rocket rises vertically and its speed is 600 ft/s when it has risen 3000 ft. • How fast is the distance from the television camera to the rocket changing at that moment? • If the television camera is always kept aimed at the rocket, how fast is the camera’s angle of elevation changing a that same moment? Section 3.9 Related Rates

19. Try these problems A Ferris wheel with a radius of 10 m is rotating at a rate of one revolution every 2 minutes. How fast is the rider rising when his seat is 16m above ground level? Section 3.9 Related Rates