# Related Rates - robeson.k12.nc.us

RELATED RATES Nick Bigner TJ Howard Thao Vu Definition of Related Rates Related rates problems involve finding a rate at

which a quantity changes by relating that quantity to other quantities whose rates of change are known. The rate of change is usually with respect to time. Steps to solving a related rates problem Step 1: draw a diagram

related to the problem and label accordingly Step 2: specify in mathematical for the rate of change you are looking for and record all given information

Step 3: find an equation involving the variable whose rate of change is to be found Step 4: differentiate with respect to time (t) Step 5: state the final answer

in correct form, and specifying the units that you are using World application Some real world applications of related rates includes predicting collisions, rate at which liquid drains, travel, satellites orbiting the earth, increasing the volume of an object,

mars expeditions, pretty much anything Nasa or any physicists for that matter do, and much much more! Example Tugboat Problem: A tugboat moves a ship up to the dock by pushing its stern at a rate of 3m/sec. The ship

is 200m long. Its bow remains in contact with the dock and its stern remains in contact with the pier. At what rate is the bow moving along the dock when the stern is 120m from the dock? Work Dx/dt= ?

X= ? Dy/dy= 3m/s Y= 120m Dz/dt= 0 Z= 200m Work X2 + y2 = z2

2002 1202 = x2 X= 160m Work 2x(dx/dt) + 2y(dy/dt) = 2z(dz/dt)

(160)(dx/dt) + (120)(-3) = (200)(0) (160)(dx/dt) = 360 Answer Dx/dt = 2.25m/s

Example Slag Heap Problem: Slag left over from the manufacturing of cement is being piled outside the cement plant. The resulting slag heap is a cone whose elements make an angle of 40 degrees with the horizontal. Environmentalists measure the circumference of the heap one day, finding it to be 3000ft, and increasing at 7ft/

day. About how fast is the cement plant generating slag? WORK WORK V=1/3()(r)(r2)(h)

C=2()(r)(r2) R=(c)/2()(r) H=(r)tan40

V=1/3()(r)((c)/2()(r))3 tan40 WORK dV/dt = (c2)(dc/dt)(tan40)/(8)(r2) dV/dt = (c2)(7)(tan40)/(8)(r2)

dV/dt = (30002)(7)(tan40)/(8)(r2) Answer dV/dt = 669,521.22 ft 3 /day EXAMPLE Rectangle Problem

The length of a rectangle is increasing at 3ft/min, and the width is decreasing at 2ft/min. When the length is 50ft and the width is 20ft, is the area of the rectangle increasing or decreasing? At what rate? X=50 Dx/dt=3

Y=20 Dy/dt=-2 A=1000 Da/dt=? WORK Work LxW=A

(20)(50)=1000 X(dy/dt) + Y(dx/dt) =dA/dt dA/dt = 3(20) + -2(50)

Answer decreasing at a rate of dA/dt = 40ft/min FRQ Barn Ladder Problem: A ladder to the loft in a barn is arranged so that it can be pushed up against the wall when it is not in use. The top of the ladder slides in a track on the wall, and the bottom is free to roll across the floor

on wheels. To make the ladder easier to move, a counterweight is attached to the top of the ladder by a rope over a pulley. As the ladder goes away from the wall, the counterweight goes up and vice versa. FRQ Questions A. B. C.

The ladder is 20 ft long. Write an equation expressing the velocity the counterweight moves as a function of the distance the bottom of the ladder is from the wall and the velocity the bottom of the ladder moves away from the wall. Find the velocity of the counterweight when the bottom is 4ft from the wall, and is bring pushed toward the wall at 3 ft/s. If the ladder is allowed to drop all the way to the floor with the bottom moving at 2ft/s and the top remains in contact with the wall, how fast is the counterweight moving when the top just hits the floor?

Work A 202 = x2 + y2 (20)(0) = x(dx/dt) + y (dy/dt) Dy/dt = -(x/y)(dx/dt)

V = (x)/((202)-(x2))1/2 (dx/dt) WORK B v = (4)/(384)1/2 (-3)

V = -.0623 m/s WORK C X = 20 Dx/dt = 2

V = undefined Try Me! Bacteria Spreading Problem: Bacteria are growing in a circular colony one bacterium thick. The bacteria are growing at a constant rate, thus making the area of the colony increase at a constant rate of 12 mm^2/hr. How fast is r changing when it equals 3 mm.

Answer dr/dt = 6/rr Try Me! An airplane is flying 600 mi/hr on a horizontal path that will take it directly over an observer. The airplane is 7mi high. How fast is z changing when x is 10 mi?

Answer X= 10 Dz/dt= 491.539 which mean that the distance is decreasing at about 492 miles/hour. Try Me!

Cone of Light problem: A spotlight shines on the wall, forming a cone of light in the air. The light is being moved closer to the wall, making the cones altitude decrease at 6ft/ min. At the same time, the light is being refocused, making the radius increase at 7ft/ min. At the instant when the altitude is 3 ft and the radius is 8 ft, is the volume of

the cone increasing or decreasing? How fast? Volume is decreasing at about 50.3 ft 3 / min. Answer Darth Vader Problem!

Darth Vaders spaceship is approaching the origin along the positive y-axis at 50km/sec. Meanwhile, his daughter Ellas spaceship is moving away from the origin along the positive xaxis at 80 km/sec. When Darth is at y = 1200km and Ella is at x = 500km, is the distance between them increasing or decreasing? At what rate?

Answer The distance is decreasing at about 15.4m/s Luke and Leia Problem! Luke and Leia are trapped inside a trash compactor on the Death Star. The side walls are

moving apart at 0.1 m/sec, but the end walls are moving together at 0.3 m/sec. The volume of liquid inside the compactor is 20 cubic meters, a constant. ANSWER The depth is increasing at 0.02m/s

Work Cited Foerster, Paul. Calculus Concepts and Applications. Key Curriculum Press, 1998. Print. Nicholas Bigner, TJ Howard, and Thao Vu

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