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Section 3.11 : Related Rates

  1. In the following assume that \(x\) and \(y\) are both functions of \(t\). Given \(x = 3\), \(y = 2\) and \(y' = 7\) determine \(x'\) for the following equation. \[{x^3} - {y^4} = {x^2}y - 7\]
  2. In the following assume that \(x\) and \(y\) are both functions of \(t\). Given \(x = {\textstyle{\pi \over 6}}\), \(y = - 4\) and \(x' = 12\) determine \(y'\) for the following equation. \[{x^2}\left( {{y^2} - 16} \right) - 6\cos \left( {2x} \right) = 1 + y\]
  3. In the following assume that \(x\), \(y\) and \(z\) are all functions of \(t\). Given \(x = - 1\), \(y = 8\), \(z = 2\), \(x' = - 4\) and \(y' = 7\) determine \(z'\) for the following equation. \[{x^4} + \frac{y}{z} = 2{x^2}{z^2} - 3\]
  4. n the following assume that \(x\), \(y\) and \(z\) are all functions of \(t\). Given \(x = - 2\), \(y = 3\), \(z = 4\), \(y' = 6\) and \(z' = 0\) determine \(x'\) for the following equation. \[x\,{y^2}{z^2} = {x^3} - {z^4} - 8y\]
  5. The sides of a square are increasing at a rate of 10 cm/sec. How fast is the area enclosed by the square increasing when the area is 150 cm2.
  6. The sides of an equilateral triangle are decreasing at a rate of 3 in/hr. How fast is the area enclosed by the triangle decreasing when the sides are 2 feet long?
  7. A spherical balloon is being filled in such a way that the surface area is increasing at a rate of 20 cm2/sec when the radius is 2 meters. At what rate is air being pumped in the balloon when the radius is 2 meters?
  8. A cylindrical tank of radius 2.5 feet is being drained of water at a rate of 0.25 ft3/sec. How fast is the height of the water decreasing?
  9. A hot air balloon is attached to a spool of rope that is 125 feet away from the balloon when it is on the ground. The hot air balloon rises straight up in such a way that the length of rope increases at a rate of 15 ft/sec. How fast is the hot air balloon rising 20 seconds after it lifts off?
    In the lower left corner of this image is a dot labeled “Spool of Rope”.  In the upper right corner is a dot labeled “Balloon” and it has an arrow next to it pointing directly up.  The two dots are connected with a diagonal line that has the text “This Length Increasing at 15 ft/sec” on it.  The sketch also shows that the horizontal distance between the two points is 125 feet.
  10. A rock is dropped straight off a bridge that is 50 meters above the ground and falls at a speed of 10 m/sec. Another person is 7 meters away on the same bridge. At what rate is the distance between the rock and the second person increasing just as the rock hits the ground?
    A sketch of the rock and the second person on the bridge.  The point at the top left is labeled “Rock Dropped Here” and the point 7 meters to its right is the person.  The rock is the point directly below the drop point at the bottom of the sketch with an arrow showing it falling straight down.  The line drawn from the person down to the rock is labeled “Find Rate This Distance Changing”.
  11. A person is 550 meters away from a road and there is a car that is initially 800 meters away approaching the person at a speed of 45 m/sec. At what rate is the distance between the person and the car changing (a) 5 seconds after the start, (b) when the car is directly in front of the person and (c) 10 seconds after the car has passed the person.
    In the lower left corner of this image is a dot labeled “Person”.  In the upper right corner is a dot labeled “Car” and it has an arrow next to it pointing to the left.  The two dots are connected with a diagonal line that has the text “Find Rate This Distance Changing” on it.  The sketch also shows that the horizontal distance between the two points is initially 800 meters and that the vertical distance between the two point is 550 meters.
  12. Two cars are initially 1200 miles apart. At the same time Car A starts driving at 35 mph to the east while Car B starts driving at 55 mph to the north (see sketch below for this initial setup). At what rate is the distance between the two cars changing after (a) 5 hours of travel, (b) 20 hours of travel and (c) 40 hours of travel?
    A sketch of the initial setup for the two cars.  Car A’s initial position is the point at the lower left and Car B’s initial position is the point at the lower right, with the 1200 miles between them marked.  Car A’s later position is a point to the right of its starting point with an arrow showing it driving east, and Car B’s later position is a point above its starting point with an arrow showing it driving north.  The line drawn between the two later positions is labeled “Find Rate This Distance Changing”.
  13. 13. Repeat problem 12 above except for this problem assume that Car A starts traveling 4 hours after Car B starts traveling. For parts (a), (b) and (c) assume that these are travel times for Car B.
  14. Two people are on a city block. See the sketch below for placement and distances. Person A is on the northeast corner and Person B is on the southwest corner. Person A starts walking towards the southeast corner at a rate of 3 ft/sec. Four seconds later Person B starts walking towards the southeast corner at a rate of 2 ft/sec. At what rate is the distance between them changing (a) 10 seconds after Person A starts walking and (b) after Person A has covered half the distance?
    In the lower left corner of this image is a dot labeled “Person B” and has an arrow next to is pointing to the right.  In the upper right corner is a dot labeled “Person A” and it has an arrow next to it pointing down.  The two dots are connected with a diagonal line that has the text “Find Rate This Distance Changing” on it.  The sketch also shows that the horizontal distance between the two points is initially 500 feet and that the vertical distance between the two point is 300 feet.
  15. A person is standing 75 meters away from a kite and has a spool of string attached to the kite. The kite starts to rise straight up in the air at a rate of 2 m/sec and at the same time the person starts to move towards the kites launch point at a rate of 0.75 m/sec. Is the length string increasing or decreasing after (a) 4 seconds and (b) 20 seconds.
  16. A person lights the fuse on a model rocket and starts to move away from the rocket at a rate of 3 ft/sec. Five seconds after lighting the fuse the rocket launches straight up into the air at a rate of 10 ft/sec. Is the distance between the person and the rocket increasing or decreasing (a) 6 seconds after launch and (b) 12 seconds after launch?
    A sketch of the person and the model rocket.  The person is the point at the lower left with an arrow showing them moving to the left, the initial location of the rocket launch is the point at the lower right, and the rocket is the point above that with an arrow showing it rising straight up.  The line drawn between the person and the rocket is labeled “Is This Distance Increasing or Decreasing?”.
  17. A light is on a pole and is being lowered towards the ground at a rate of 9 in/sec. A 6 foot tall person is on the ground and 8 feet away from the pole. At what rate is the persons shadow increasing then the light is 15 feet above the ground?
    A sketch of the light, the person and the shadow.  The light is at the top of a pole on the left with an arrow showing that it is being lowered, the person is a vertical line of height 6 ft standing 8 ft away from the pole, and the light ray from the light passes over the person’s head and hits the ground to the right of the person.  The shadow is the darkened portion of the ground from the person out to that point.
  18. A light is fixed on a wall 10 meters above the floor. Twelve meters away from the wall a pole is being raised straight up at a rate of 45 cm/sec. When the pole is 6 meters tall at what rate is the tip of the shadow moving (a) away from the pole and (b) away from the wall? Note the sketch below is not to scale…
    A sketch of the light, the pole and the shadow.  The light is fixed to a wall 10 m above the floor on the left, the pole stands 12 m away from the wall with an arrow showing that it is being raised straight up, and the light ray from the light passes over the top of the pole and hits the floor to the right of the pole.  The shadow is the darkened portion of the floor from the pole out to that point.
  19. A light is on the top of a 15 foot tall pole. A 5 foot tall person starts at the pole and moves away from the pole at a rate of 2.5 ft/sec. After moving for 8 seconds at what rate is the tip of the shadow moving (a) away from the person and (b) away from the pole?
    A sketch of the light, the person and the shadow.  The light is at the top of a 15 ft pole on the left, the person is a vertical line of height 5 ft to the right of the pole with an arrow showing them walking to the right, and the light ray from the top of the pole passes over the person’s head and hits the ground to the right of the person.  The shadow is the darkened portion of the ground from the person out to that point.
  20. A tank of water is in the shape of a cone (assume the “point” of the cone is pointing downwards) and is leaking water at a rate of 35 cm3/sec. The base radius of the tank is 1 meter and the height of the tank is 2.5 meters. When the depth of the water is 1.25 meters at what rate is the (a) depth changing and (b) the radius of the top of the water changing?
    A sketch of the cone shaped tank as seen from the front, with the point of the cone at the bottom.  The radius of the top of the tank is labeled 1 m and the height of the tank is labeled 2.5 m.  Water fills the bottom portion of the cone and the surface of the water is a smaller circle part way up the tank.
  21. A trough of water is 20 meters in length and its ends are in the shape of an isosceles triangle whose width is 7 meters and height is 10 meters. Assume that the two equal length sides of the triangle are the sides of the water tank and the other side of the triangle is the top of the tank and is parallel to the ground. Water is being pumped into the tank at a rate of 2 m3/min. When the water is 6 meters deep at what rate is (a) depth changing and (b) the width of the top of the water changing? Note the sketch below is not to scale…
    A sketch of the trough.  It is 20 meters long, the triangular ends are 7 meters across the top and 10 meters deep, and the water in the trough is shaded in and fills the bottom part of the trough up to some depth.
  22. A trough of water is 9 feet long and its ends are in the shape of an equilateral triangle whose sides are 1.5 feet long. Assume that the top of the tank is parallel to the ground. If water is being pumped out of the tank at a rate of 2 ft3/s at what rate is the depth of the water changing when the depth is 0.75 feet?
    A sketch of the trough.  It is 9 feet long and the triangular ends are equilateral triangles with sides 1.5 feet long, with the 1.5 foot top side parallel to the ground.  The water in the trough is shaded in and fills the bottom part of the trough up to some depth.
  23. The angle of elevation (depression) is the angle formed by a horizontal line and a line joining the observer’s eye to an object above (below) the horizontal line. Two people are on the roof of buildings separated by at 25 foot wide road. Person A is 100 feet above Person B and drops a rock off the roof of their building and it falls at a rate of 3 ft/sec. The sketches below are not to scale….

    1. At what rate is the angle of elevation changing as Person B watches the rock fall when the rock is 35 feet above Person B?
      A sketch of the two people and the falling rock.  Person A is the point at the top right, on the roof of the taller building, and is labeled as having dropped the rock.  Person B is the point at the left, 25 ft away horizontally and 100 ft below Person A.  The rock is a point on the vertical line below Person A with an arrow showing it falling and it is still above Person B.  The angle at Person B between the horizontal line and the line up to the rock is labeled \(\theta \), Angle of Elevation.
    2. At what rate is the angle of depression changing as Person B watches the rock fall when the rock is 65 feet below Person B?
      A sketch of the two people and the falling rock.  Person A is the point at the top right, on the roof of the taller building, and is labeled as having dropped the rock.  Person B is the point at the left, 25 ft away horizontally and 100 ft below Person A.  The rock is a point on the vertical line below Person A with an arrow showing it falling and it has now fallen below Person B.  The angle at Person B between the horizontal line and the line down to the rock is labeled \(\theta \), Angle of Depression.
  24. The angle of elevation is the angle formed by a horizontal line and a line joining the observer’s eye to an object above the horizontal line. A person is standing 15 meters away from a building and watching an outside elevator move down the face of the building. When the angle of elevation is 1 radians it is changing at a rate of 0.15 radians/sec. At this point in time what is the speed of the elevator?
    A sketch of the person and the elevator.  The person is the point at the lower left and the elevator is the point at the upper right with an arrow beside it showing that it is moving down the face of the building.  A horizontal line 15 m long runs from the person to the base of the building and a second line runs from the person up to the elevator.  The angle between them at the person is labeled \(\theta \), Angle of Elevation.
  25. The angle of elevation is the angle formed by a horizontal line and a line joining the observer’s eye to an object above the horizontal line. A person is 24 feet away from a building and watching an outside elevator move up the face of the building. The elevator is moving up at a rate of 4 ft/sec and the person is moving towards the building at a rate of 0.75 ft/sec. Assuming that the elevator started moving from the ground at the same time that the person started walking is the angle of elevation increasing or decreasing after 10 seconds?
    A sketch of the person and the elevator.  The person is the point at the lower left with an arrow showing them moving to the right and the elevator is the point at the upper right with an arrow beside it showing that it is moving up the face of the building.  A horizontal line 24 ft long runs from the person to the base of the building and a second line runs from the person up to the elevator.  The angle between them at the person is labeled \(\theta \), Angle of Elevation.