Energy & Work
A 4 kg rock sits on a 15 m cliff. Find its gravitational potential energy relative to the ground (g ≈ 9.8 m/s²).
What is about 588 J?
A wave has wavelength 2.0 m and frequency 30 Hz. What is its speed?
What is 60 m/s?
A runner starts from rest and accelerates at 1.5 m/s² for 5.0 s. What is the runner’s final speed?
What is 7.5 m/s?
In an insulated container, 0.25 kg of water at 80°C is mixed with 0.15 kg of water at 10°C. What is the final temperature? (Assume no phase change.)
What is about 53.75°C?
A device uses 40 J of electrical energy and produces 10 J of visible light. What is its efficiency (based on useful output)?
What is 25%?
A 25 N force pushes a cart 4.0 m along a horizontal surface. How much work is done?
What is 100 J?
A wave has a frequency of 5.0 Hz. What is its period?
What is 0.20 s?
Using the same runner (a = 1.5 m/s² for 5.0 s from rest), how far does the runner travel?
What is 18.75 m?
Describe the direction of heat flow when a hot object is placed in cooler water, and what happens when thermal equilibrium is reached.
What is: heat flows from higher temperature to lower temperature, and net heat flow stops when both reach the same temperature?
An elevator motor uses 60,000 J of energy to lift a 500 kg load straight up by 8.0 m. If the useful output is the increase in gravitational potential energy, what is the motor’s efficiency? (Use g ≈ 9.8 m/s².)
What is about 65% (approximately 65.3%)?
A 2.0 kg cart speeds up from 3.0 m/s to 7.0 m/s. By how much does its kinetic energy increase?
What is 40 J?
In the photoelectric effect, keeping the light frequency above the threshold but increasing the intensity causes more electrons to be ejected, but does not increase their maximum kinetic energy.
What is because higher intensity means more photons (more electrons), while the max energy depends on photon frequency?
A ball is thrown straight up at 12 m/s. Neglecting air resistance and using g ≈ 9.8 m/s², what is its velocity after 2.0 s?
What is about 7.6 m/s downward (v ≈ -7.6 m/s)?
Microwave ovens use this region of the electromagnetic spectrum, while medical imaging with X-rays uses this higher-energy region.
What are microwaves and X-rays?
On a velocity–time graph, the area under the curve between two times represents this quantity.
What is displacement (or change in position)?
A 0.75 kg rock is dropped from a 5.0 m cliff. Ignoring air resistance, what is its speed just before hitting the ground? (Use g ≈ 9.8 m/s².)
What is about 9.9 m/s?
Ultraviolet light can eject electrons from a metal, while red light cannot—even if the red light is brighter. This observation supports the idea that light energy comes in discrete packets that depend on frequency.
What is the photon (particle) model of light and the frequency-dependent photoelectric effect?
A 2.0 kg object has a net force of 10 N toward the north. What is its acceleration?
What is 5.0 m/s² north?
An electromagnet can be made stronger (without changing the battery) by doing one of these: increasing the number of wire loops, inserting an iron core, or reducing the air gap.
What is add an iron core (or add more turns of wire)?
Three 6 Ω resistors are connected in series in a circuit. This is the circuit’s equivalent resistance.
What is 18 Ω?
A 1200 kg car is moving at 25 m/s. The brakes apply an average force of 12,000 N opposite the motion. Ignoring friction and air resistance, how far does the car travel before stopping?
What is about 31 m (approximately 31.25 m)?
Name one experiment that is strong evidence for the wave nature of light and one that is strong evidence for the particle nature of light.
What are double-slit interference (wave) and the photoelectric effect (particle)?
A car’s velocity is -12 m/s (west is negative). Its acceleration is +3 m/s². Explain what happens to the car’s speed and direction over the next few seconds.
What is: it slows down while moving west, comes to rest after about 4 s, then starts moving east?
Explain two key differences between sound waves and electromagnetic waves.
What is: sound is a mechanical wave that needs a medium and travels much slower, while electromagnetic waves can travel through a vacuum at about 3.0×10^8 m/s?
On a power vs. time graph, the area under the curve represents this. For a device that delivers a constant 15 W, describe the graph.
What is energy (work)? It increases linearly with a slope of 15 J/s.