RELATED RATES WORD PROBLEMS AND SOLUTIONS

The derivative can be used to determine the rate of change of one variable with respect to another. A few examples are population growth rates, production rates, water flow rates, velocity, and acceleration.

Rates of change with respect to some other quantity in our daily life are given below :

1) Slope is the rate of change in vertical length with respect to horizontal length.

2) Velocity is the rate of displacement with respect to time.

3) Acceleration is the rate of change in velocity with respect to time.

4) The steepness of a hillside is the rate of change in its elevation with respect to linear distance

The average rate of change in an interval [a, b] is

whereas, the instantaneous rate of change at a point x is f′(x) for the given function

Examples

Example 1 :

A particles moves along a line according to the law s(t) = 2t3 – 9t2 +12t – 4, where t 0.

(i) At what time the particle changes direction?

(ii) Find the total distance travelled by the particle in the first 4 seconds.

(iii) Find the particle’s acceleration each time the velocity is zero.

Solution :

(i) The particle changes direction when velocity changes its sign.

Now, v(t) =  = 6t2 – 18t + 12 = 0 gives t = 1 and t = 2.

If 0    ≤ t  < 1 then both (t - 1) and (6t - 12) < 0, then

v(t) > 0

If 1 < t <  2 then both (t - 1) 0 and (6t - 12)  0, then

So, v(t) < 0.

If t > 2 then both (t - 1)  and (6t - 12) > 0, then

v(t)  > 0.

So, the particle changes direction when t = 1 and t = 2.

(ii) Given s(t)  =  2t3 – 9t2 +12t – 4 ---(1),

From (1),

s(0)  =  2(0)3  - 9(0)2 + 12(0) – 4  = -4

s(1)  =  2(1)3  - 9(1)2 + 12(1) – 4  = 1

s(2)  =  2(2)3  - 9(2)2 + 12(2) – 4  = 0

s(3)  =  2(3)3  - 9(3)2 + 12(3) – 4  = 5

s(4)  =  2(4)3  - 9(4)2 + 12(4) – 4  = 28

The total distance traveled by the particle in the first 4 seconds,

|s(1) - s(0)| + |s(2) - s(1)| + |s(3) - s(2)| + |s(4) - s(3)|

=  |1 - (-4)| + |0 - 1| + |5 - 0| + |28 - 5|

=  34

So, the total distance traveled by the particle in the first 4 seconds is 34 m

(iii)The acceleration is given by a(t)  =  12t – 18.

Velocity is zero, when t = 1 and t = 2.

Therefore, the acceleration when t = 1 sec is -6 m/sec2 and the acceleration when t = 2 sec is 6 m/sec2.

Example 2 :

If the volume of a cube of side length x is v = x3. Find the rate of change of the volume with respect to x when x  =  5 units.

Solution :

The rate of change of the volume with respect to length x is 3x2.

The rate of change of the volume with respect to x,  when x = 5 units is

=  3(5)2

= 75 units.

Example 3 :

If the mass m(x) (in kilogram) of a thin rod of length x (in meters) is given by, m(x)√(3x)   then what is the rate of change of mass with respect to the length when it is x=3 and x = 27 meters.

Solution :

The rate of change of mass with respect to the length is    dm/dx  =   √3/ √(2x)

When x  =  3, dm/dx  =  (1/2) kg/m.

When x  =  27, dm/dx  =  (1/6)  kg/m.

Example 4 :

A stone is dropped into a pond causing ripples in the form of concentric circles. The radius r of the outer ripple is increasing at a constant rate at 2 cm per second. When the radius is 5 cm find the rate of changing of the total area of the disturbed water?

Solution :

Given  = 2 cm/sec. Total area of a circle is a = π r2. The rate of change of the total area = 2 π r ((dr/dt).

The rate of changing of the total area when the radius is 5 cm

=  2π (5)(2)

=  20π

So, the rate of changing of the total area of the disturbed water when the radius is 5 cm is 20π  sq.cm/sec.

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