A mass–spring–damper is described by y″ + y′ + 4y = 0. Compare it with y″ + 2ζωny′ + ωn²y = 0 and find the damping ratio ζ.
Differential equations worksheet with answers
1.Easy
y″ + y′ + 4y = 0
ζ =
2.Easy
A quantity grows according to dy/dt = k·y with k > 0. It is 250 at t = 0 and 775 at t = 7 h. Find the rate constant k in h⁻¹.
y(0) = 250y(7) = 775
k = h⁻¹
3.Medium
A cup of coffee at 89 °C is left in a room at 21 °C. After 17 min its temperature is 36 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). How long after the start, t, does it take to reach 57 °C?
T0 = 89 °CTa = 21 °CT(17) = 36 °CT = 57 °C
t = min
4.Medium
A can of soda at 3 °C is taken out of the fridge into a room at 25 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). With k = 0.04 min⁻¹, find its temperature T after 27 min.
T0 = 3 °CTa = 25 °Ck = 0.04 min⁻¹t = 27 min
T = °C
5.Hard
Solve y″ + 3y′ + 18.25y = 0 with y(0) = 10 and y′(0) = 0 (released from rest). Find y(1.5); angles in radians.
y″ + 3y′ + 18.25y = 0y(0) = 10y′(0) = 0
y(1.5) =
6.Hard
Solve y′ + 3y = 9e−0.5t with y(0) = 1. Find y(2).
y′ + 3y = 9e−0.5ty(0) = 1
y(2) =
Answer key
- 1. ζ = 0.25
- 2. k = 0.1616 h⁻¹
- 3. t = 7.153 min
- 4. T = 17.53 °C
- 5. y(1.5) = 0.9016
- 6. y(2) = 1.318
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