Study Answers – Additional Physics Equations #
Power (P = E/t) #
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Q: A machine does 3000 J of work in 15 seconds. Calculate its power.
P = E/t = 3000/15 = 200 W
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Q: How much energy is transferred by a 100 W bulb in 30 seconds?
E = P × t = 100 × 30 = 3000 J
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Q: A 500 W motor transfers 10000 J of energy. How long does this take?
t = E/P = 10000/500 = 20 seconds
Gravitational Potential Energy (GPE = mgh) #
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Q: Calculate the GPE when a 2 kg mass is lifted 5 meters (g = 9.81 N/kg).
GPE = mgh = 2 × 9.81 × 5 = 98.1 J
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Q: A box gains 490.5 J of GPE when lifted 10 meters. What is its mass?
m = GPE/(gh) = 490.5/(9.81 × 10) = 5 kg
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Q: A 5 kg mass has 245.25 J of GPE. To what height was it lifted?
h = GPE/(mg) = 245.25/(5 × 9.81) = 5 m
Ohm’s Law (V = IR) #
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Q: Calculate the current when 12 V is applied across a 4 Ω resistor.
I = V/R = 12/4 = 3 A
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Q: What voltage is needed to drive 2 A through a 6 Ω resistor?
V = IR = 2 × 6 = 12 V
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Q: A current of 0.5 A flows when 3 V is applied. What is the resistance?
R = V/I = 3/0.5 = 6 Ω
Electrical Power (P = VI) #
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Q: Calculate the power when 230 V drives a current of 2 A.
P = VI = 230 × 2 = 460 W
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Q: What current does a 1200 W appliance draw from a 240 V supply?
I = P/V = 1200/240 = 5 A
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Q: A current of 3 A flows through a 60 W bulb. What is the voltage?
V = P/I = 60/3 = 20 V
Heat Energy (Q = mc∆T) #
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Q: How much energy is needed to heat 2 kg of water (c = 4200 J/kg°C) by 15°C?
Q = mc∆T = 2 × 4200 × 15 = 126,000 J
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Q: A 3 kg block of iron (c = 450 J/kg°C) is heated using 27000 J. Calculate the temperature rise.
∆T = Q/(mc) = 27000/(3 × 450) = 20°C
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Q: What mass of water can be heated by 20°C using 25200 J?
m = Q/(c∆T) = 25200/(4200 × 20) = 0.3 kg
Wave Speed (v = fλ) #
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Q: Calculate the wave speed when frequency is 20 Hz and wavelength is 1.5 m.
v = fλ = 20 × 1.5 = 30 m/s
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Q: A wave travels at 340 m/s with wavelength 2 m. What is its frequency?
f = v/λ = 340/2 = 170 Hz
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Q: Sound waves travel at 330 m/s with frequency 1000 Hz. Calculate the wavelength.
λ = v/f = 330/1000 = 0.33 m
Efficiency #
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Q: A machine has input energy 1000 J and useful output energy 800 J. Calculate its efficiency.
Efficiency = (useful output/total input) × 100 = (800/1000) × 100 = 80%
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Q: A motor is 75% efficient. If the input energy is 2000 J, what is the useful output energy?
Useful output = (efficiency × input)/100 = (75 × 2000)/100 = 1500 J
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Q: A machine has useful output energy of 300 J and is 60% efficient. Calculate the input energy.
Input energy = (useful output × 100)/efficiency = (300 × 100)/60 = 500 J
