If s = t² + 3t gives the position of a particle in meters after t seconds, find its velocity at t = 2 s.
A
7 m/s
B
8 m/s
C
6 m/s
D
5 m/s
Analysis & Theory
Velocity v = ds/dt = 2t + 3. At t=2 ⇒ v = 2(2)+3 = 7 m/s.
The radius r of a circle increases at 0.5 cm/s. Find the rate of change of area A = πr² when r = 10 cm.
A
10π cm²/s
B
20π cm²/s
C
5π cm²/s
D
π cm²/s
Analysis & Theory
dA/dt = 2πr dr/dt = 2π(10)(0.5) = 10π cm²/s.
The radius of a sphere is increasing at 2 cm/s. Find the rate of change of its volume V = (4/3)πr³ when r = 3 cm.
A
24π cm³/s
B
36π cm³/s
C
72π cm³/s
D
12π cm³/s
Analysis & Theory
dV/dt = 4πr² dr/dt = 4π(3²)(2) = 72π cm³/s.
A ladder 5 m long is leaning against a wall. If the bottom slides away at 0.3 m/s, how fast is the top sliding down when bottom is 4 m from wall?
A
-0.24 m/s
B
-0.18 m/s
C
-0.36 m/s
D
-0.12 m/s
Analysis & Theory
x² + y² = 5² ⇒ 2x dx/dt + 2y dy/dt = 0 ⇒ dy/dt = -(x dx/dt)/y. At x=4, y=3 ⇒ dy/dt = -(4×0.3)/3 = -0.4 m/s.
The side of a square is increasing at 2 cm/s. Find the rate of change of its perimeter when side = 5 cm.
A
8 cm/s
B
10 cm/s
C
6 cm/s
D
4 cm/s
Analysis & Theory
Perimeter P = 4x ⇒ dP/dt = 4 dx/dt = 4×2 = 8 cm/s.
The side of a cube increases at 3 cm/s. Find rate of change of surface area when side = 2 cm.
A
36 cm²/s
B
48 cm²/s
C
24 cm²/s
D
12 cm²/s
Analysis & Theory
Surface area S = 6x² ⇒ dS/dt = 12x dx/dt = 12(2)(3)=72 cm²/s.
Temperature θ of a body cools according to Newton’s law: dθ/dt = -k(θ-θ₀). If θ=80°C, θ₀=30°C, k=0.1, find dθ/dt.
A
-5°C/min
B
-2°C/min
C
-3°C/min
D
-8°C/min
Analysis & Theory
dθ/dt = -0.1(80-30) = -0.1×50 = -5°C/min.
Water is poured into a cone of height 12 cm and radius 6 cm at 10 cm³/s. Find rate of rise of water level when depth = 4 cm.
A
5/π cm/s
B
5/(3π) cm/s
C
15/(π) cm/s
D
5/(12π) cm/s
Analysis & Theory
V = (1/3)πr²h, r/h = 6/12 ⇒ r = h/2 ⇒ V=(π/12)h³ ⇒ dV/dt = (π/4)h² dh/dt ⇒ dh/dt = (dV/dt)/((π/4)h²) = 10 / ((π/4)(4²)) = 5/(3π) cm/s.
If y = x², find the rate of change of y with respect to x when x = 3.
A
3
B
6
C
9
D
12
Analysis & Theory
dy/dx = 2x ⇒ at x=3 ⇒ dy/dx = 6.
The radius of a circle increases at 1 cm/s. Find rate of change of its circumference C=2πr when r=7 cm.
A
14π cm/s
B
7π cm/s
C
2π cm/s
D
π cm/s
Analysis & Theory
dC/dt = 2π dr/dt = 2π(1)=2π ⇒ At r=7, still constant ⇒ 2π cm/s × r = 14π cm/s.