Springs & pendulums
- Mass on a spring
- T = 2π·√(m/k)
- Pendulum
- T = 2π·√(L/g)
- Angular frequency
- ω = √(k/m) = 2π/T
- Maximum speed and acceleration
- vmax = A·ω, amax = A·ω²
- Energy
- E = ½·k·A²
Waves and Optics: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
Waves, sound & optics
- Wave speed
- v = fλ, T = 1/f
- String
- v = √(T/μ), μ = m/L
- Sound level
- β = 10·log10(I/I0), I0 = 10⁻¹² W/m²
- Point source
- I = P/(4πr²)
- Standing waves
- string or open pipe fn = n·v/(2L); closed pipe fn = n·v/(4L), n odd
- Doppler effect
- f′ = f·(v ± vo)/(v ∓ vs)
- Refractive index
- n = c/v, λ = λ0/n
- Snell’s law
- n1sin θ1 = n2sin θ2
- Critical angle
- sin θc = n2/n1
- Optical fibre
- NA = sin θa = √(n1² − n2²)
- Lenses and mirrors
- 1/f = 1/do + 1/di, m = −di/do, mirror f = R/2
- Double slit
- d·sin θ = mλ, ym = mλL/d
- Grating
- d = 1/N, d·sin θ = mλ, mmax ≤ d/λ
- Single slit
- a·sin θ = mλ (dark), w = 2λL/a
Method
- Identify the kind of problem: wave speed, sound level, standing wave, Doppler, refraction, lens or mirror, or interference.
- Write the relation first: v = fλ, β = 10·log(I/I₀), fₙ = n·v/(2L) or n·v/(4L), f′ = f(v ± vo)/(v ∓ vs), n₁sin θ₁ = n₂sin θ₂, 1/f = 1/d₀ + 1/dᵢ, d·sin θ = mλ.
- Convert units: nm, mm and cm to m, and check whether angles are measured from the normal.
- Check signs and plausibility: a virtual image has dᵢ < 0, an approaching source sounds higher, a closed pipe has only odd harmonics.
2Practice problems
1.Springs & pendulumsMedium
A 0.8 kg mass on a spring with k = 210 N/m is pulled 13 cm in the positive direction and released from rest at t = 0. Find its displacement x from equilibrium at t = 0.05 s (with sign).
m = 0.8 kgk = 210 N/mA = 13 cmt = 0.05 s
x = cm
2.Springs & pendulumsHard
A 1.5 kg mass on a spring with k = 250 N/m is pulled 8 cm from equilibrium and released. Find its maximum speed vmax.
m = 1.5 kgk = 250 N/mA = 8 cm
vmax = m/s
3.Waves, sound & opticsMedium
A small source sends sound equally in all directions. The level is 80 dB at 2 m from it. Find the level β2 at 10 m, ignoring absorption and reflections.
β1 = 80 dBr1 = 2 mr2 = 10 m
β2 = dB
4.Waves, sound & opticsHard
Two thin lenses share an axis, 25 cm apart: lens 1 with f₁ = 20 cm and lens 2 with f₂ = 10 cm. An object stands 30 cm in front of lens 1. Find the position di2 of the final image measured from lens 2 (positive beyond lens 2, real-is-positive convention).
f1 = 20 cmf2 = 10 cms = 25 cmdo = 30 cm
di2 = cm
3Solutions
1.Springs & pendulums
Answer: x = 8.963 cm
Simple harmonic motion: a mass m on a spring of stiffness k oscillates with angular frequency ω = √(k/m) and period T = 2π/ω = 2π·√(m/k). A simple pendulum of length L has T = 2π·√(L/g). With amplitude A, the largest speed is A·ω, the largest acceleration A·ω² and the energy ½·k·A².
Angular frequency
ω = √(210/0.8) = 16.2 rad/s
ω·t = 16.2·0.05 = 0.8101 rad
Displacement
x = A·cos(ω·t) = 13·cos(0.8101) = 8.963 cm
Answer
x = 8.963 cm
2.Springs & pendulums
Answer: vmax = 1.033 m/s
Simple harmonic motion: a mass m on a spring of stiffness k oscillates with angular frequency ω = √(k/m) and period T = 2π/ω = 2π·√(m/k). A simple pendulum of length L has T = 2π·√(L/g). With amplitude A, the largest speed is A·ω, the largest acceleration A·ω² and the energy ½·k·A².
Angular frequency
ω = √(250/1.5) = 12.91 rad/s
Maximum speed
vmax = A·ω = 0.08·12.91 = 1.033 m/s
Answer
vmax = 1.033 m/s
3.Waves, sound & optics
Answer: β2 = 66.02 dB
Waves: a periodic wave moves one wavelength in one period, so v = fλ with T = 1/f. On a stretched string v = √(T/μ), with T the tension and μ = m/L the mass per metre. Sound in air travels at about 343 m/s at 20 °C, light in vacuum at c = 3.00 × 10⁸ m/s. Loudness is measured on a logarithmic scale: β = 10·log₁₀(I/I₀) dB with I₀ = 10⁻¹² W/m², so 10 times the intensity adds 10 dB. A point source spreads its power P over a sphere: I = P/(4πr²).
Intensity ratio
I₂/I₁ = (2/10)² = 0.04
New level
β₂ = 80 + 10·log₁₀(0.04) = 66.02 dB
Answer
β2 = 66.02 dB
Every doubling of the distance lowers the level by about 6 dB.
4.Waves, sound & optics
Answer: di2 = 7.778 cm
Geometric optics: at a boundary, n₁·sin θ₁ = n₂·sin θ₂ (Snell's law), with angles measured from the normal and n = c/v. Going into a slower medium the ray bends towards the normal. From a denser medium (n₁ > n₂) beyond the critical angle θc = arcsin(n₂/n₁) all light is reflected: total internal reflection. Thin lenses and spherical mirrors obey 1/f = 1/do + 1/di with the magnification m = −di/do. Sign convention: f > 0 for a converging lens and a concave mirror (f = R/2), f < 0 for a diverging lens and a convex mirror; di > 0 for a real image, di < 0 for a virtual one, and m < 0 for an inverted image.
Image of lens 1
1/di1 = 1/20 − 1/30 → di1 = 60 cm
Object for lens 2
do2 = 25 − 60 = −35 cm
Final image
1/di2 = 1/10 − 1/(−35) → di2 = 7.778 cm
Answer
di2 = 7.778 cm
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