Forms and Transformations
Name two forms of energy a lit filament bulb gives off.
Light and thermal energy (input is electrical).
What is the standard unit of energy?
Joules
Write the formula for GPE and the value of g on Earth.
GPE = mgh; g = 9.81 m/s².
Write the formula for kinetic energy.
KE = ½mv².
State the efficiency formula.
Efficiency = (useful energy output ÷ total energy input) × 100%.
What is the useful output energy of a battery-powered speaker?
Sound (input: electrical from the battery’s chemical energy).
Convert 1000 mg to g.
1 g
Find the GPE of a 2.0 kg object held 5.0 m above ground.
GPE = 2.0×9.81×5.0 = 98.1 J.
Find the KE of a 2.0 kg cart moving at 3.0 m/s.
KE = ½×2.0×3.0² = 9.0 J.
A diesel car takes in 100 kJ chemical energy and outputs 10 kJ kinetic. Efficiency?
(10/100)×100% = 10%.
At the lowest point of a pendulum’s swing, which energy is at a maximum?
Kinetic energy.
: Convert 75 mL to L
0.075 L.
A 0.5 kg drone is at a height of 4.5 m. Calculate its GPE.
0.5×9.81×4.5 = 22.1 J.
A 1000 kg roller coaster moves at 20 m/s. Find its KE.
½×1000×20² = 200,000 J.
An iPhone takes 130 J electrical and outputs 10 J light, 30 J sound, 90 J thermal. Efficiency (light+sound)?
(10+30)/130×100% = 30.8%.
What energy is stored in a wound-up pull-back car, and what does it mainly become when released?
Elastic potential → kinetic energy.
Convert 56,500 mm to km
0.0565 km
A basketball has 20 J of GPE at 2.8 m. Find its mass.
m = 20/(9.81×2.8) = 0.728 kg.
A dog has KE = 14 J and mass 17 kg. Find its speed.
v = √(2KE/m) = √(28/17) = 1.28 m/s.
A torch converts 100 J electrical into 15 J light and 85 J thermal. Efficiency?
(15/100)×100% = 15%.
List the energy transformations when boiling water using a water wheel and generator.
A: GPE (water) → KE (wheel) → electrical → thermal (hot water).
Convert 198 g to kg
0.198 kg.
A 1220 kg rock has 400 J of GPE. Calculate the height above ground.
h = 400/(1220×9.81) = 0.0335 m (≈0.034 m).
A car’s KE is 896,000 J at 120 km/h. Find its mass.
v = 33.33 m/s; m = 2KE/v² = 1,792,000/33.33² ≈ 1,613 kg.
An electric fan inputs 240 J and outputs 30 J sound, 80 J heat, 50 J kinetic (rotation), plus blades’ KE (unknown). Find blades’ KE and overall efficiency (useful = kinetic).
Blades’ KE = 80 J; Efficiency = (50+80)/240×100% = 54.2%