Question 1Multiple choiceRelated rates
Water is pumped into an inverted conical tank (vertex at the bottom) at a constant rate of 2 m³/min. The tank is 6 m tall and its circular top has radius 2 m. How fast is the water level rising at the instant the water is 4 m deep?
- A.
m/min
- B.
m/min
- C.
m/min
- D.
m/min
Worked solution
Let be the water depth and the radius of the water surface at time . Similar triangles give , so ; this lets the volume be written in terms of alone, which is the key step in any related-rates problem.
Differentiate with respect to (chain rule, because depends on ):
Substitute the known rate m³/min and the instant m:
The most tempting wrong answer is , which comes from treating the tank as a cylinder of radius 2 m (); but the water surface shrinks as the level drops, so must be tied to before differentiating. Dropping the in the cone volume gives , and inverting the similar-triangle ratio () gives .
The water level is rising at m/min (option B).