A quantity decays according to dy/dt = −k·y with k > 0. It is 80 at t = 0 and 16 at t = 6 h. Find the rate constant k in h⁻¹.
Differential equations worksheet with answers
1.Easy
y(0) = 80y(6) = 16
k = h⁻¹
2.Easy
A bacterial culture starts with 2000 cells and grows according to dN/dt = k·N with k = 0.16 h⁻¹. How many cells N are there after 19 h?
N0 = 2000k = 0.16 h⁻¹t = 19 h
N =
3.Medium
A can of soda at 3 °C is taken out of the fridge into a room at 27 °C. It follows Newton's law of cooling, dT/dt = −k·(T − Tₐ). With k = 0.04 min⁻¹, find its temperature T after 5 min.
T0 = 3 °CTa = 27 °Ck = 0.04 min⁻¹t = 5 min
T = °C
4.Medium
Solve y″ + 7y′ + 10y = 0 with y(0) = 4 and y′(0) = 0. Find y(1.5).
y″ + 7y′ + 10y = 0y(0) = 4y′(0) = 0
y(1.5) =
5.Hard
A system obeys y″ + y′ + 4.25y = 4.25 for t ≥ 0, starting from rest: y(0) = 0, y′(0) = 0. Find y(0.75); angles in radians.
y″ + y′ + 4.25y = 4.25y(0) = 0y′(0) = 0
y(0.75) =
6.Hard
A system obeys y″ + 3y′ + 27.25y = 136.3 for t ≥ 0, starting from rest: y(0) = 0, y′(0) = 0. Find y(1); angles in radians.
y″ + 3y′ + 27.25y = 136.3y(0) = 0y′(0) = 0
y(1) =
Answer key
- 1. k = 0.2682 h⁻¹
- 2. N = 41810
- 3. T = 7.35 °C
- 4. y(1.5) = 0.3304
- 5. y(0.75) = 0.78
- 6. y(1) = 5.004
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