Work and Energy Review

Links and useful resources

Lesson-specific resource links

LOL charts
Work, energy, and system analysis
#hw (science) Read NASA finds hints of ancient life on Mars. 2025-02-10
#hw (science) In Class: Write a one-sentence summary of the life on Mars article based on the kernel, "It found evidence," that answers who, when, how 2025-02-10

While exploring Cheyava Falls on Mars, the Perseverance rover found possible signs of ancient life.

cheyava the perseverance exploring while found of possible life signs rover mars on falls ancient

Lightning Round Questions

#quiz What is the connection between Work and Energy?
#quiz What is the formula for calculating work?
#quiz An object collides with an identical, but stationary object in space. They stick together as they move after the collision. How fast is the combined object going compared to the speed of the first object?
#quiz A bowl of water on a scale reads 1kg. You dip a steel marble into it by dangling it from a string, but do not allow it to touch the bottom. The marble has a volume of 5 $c m^{3}$ . What does the scale read now? What would it read if the marble was ten times denser?
#quiz If you drop a brick from the first story window, it hits the ground with a certain speed. How much higher would you have to drop from in order to double the speed at the moment of impact?
#quiz Fill in the blank: Work changes ________
gr10 [lightning:: 3]
gr7 [lightning:: 2]

Concept summary and connections

Work, Energy, and Systems
LOL Charts

A LOL Chart is a funny name for a way to keep track of how energy moves and changes form within a system. The name comes from the shape of the diagram: There are two bar-charts with a circular area in the middle, so it looks like the letters "LOL," lol.

Energy Bar Charts

At any given time, a system will have energy within it in various forms:

Kinetic - energy of motion
Potential - energy of motion that could happen because of a force
Thermal - energy of heat
Chemical - energy of incomplete chemical reactions
Electrical - energy of charge movement

An energy bar chart is just a bar chart that shows the energy content of a system for each kind of energy you're interested in. The simplest one for our purposes just shows kinetic and potential energy:

LOL Charts

A LOL chart is a graphical way to describe how the energy of a system changes during some scenario. It has two bar charts along with a system diagram between them. The bar chart on the left shows the system's energy before the scenario, and the one of the right shows it afterward. The system diagram shows whether energy enters or leaves the system during the scenario.

This chart shows the energy change in the scenario, "A ball flies upward until it reaches the top of its trajectory." The system is the ball-earth system. Since the ball and earth are directly working on each other, there is no external force, and the system doesn't have an energy change (there's no arrow in the center circle).

In the next chart, the scenario is, "A chemical rocket launches into the sky." The system is just the rocket and fuel. In this case, only part of the chemical energy of the fuel is turned into kinetic and potential energy for the rocket. The rest is lost as heat, so energy leaves the system.

$E_{c h}$ = Chemical energy, stored in the fuel
$E_{k}$ = Kinetic energy, increased when the rocket gains speed
$E_{p}$ = Potential energy, increased when the rocket gains altitude
Thermal energy is lost when the fuel is burned, because not all of it could be turned into thrust.

Work changes energy

Remember: the unit of work is $\frac{k g \cdot m^{2}}{s^{2}}$ , just the same as energy! If you do work on a thing, you are changing its energy (and yours, in the form of chemical energy from food and accumulated body heat from exertion). There are many ways this can happen:

You rapidly knead bread, applying force on it over short distances repeatedly as you press the dough. The dough heats up (increasing thermal energy, by spending your own chemical energy)
You lift a book from the floor to a shelf (increasing gravitational potential energy by spending chemical energy)
You carefully lower a heavy object to the floor (decreasing gravitational potential energy... negative work!, sill by spending your chemical energy).
You pedal your bike furiously to accelerate at the start of a race (increasing kinetic energy by spending your chemical energy)

The amount of work done is determined by the equation $w o r k = f o r c e \cdot d i s t a n c e$ . Note that a force is measured in Newtons, which are $\frac{k g \cdot m}{s^{2}}$ . Multiplying that by a distance yields the energy unit of the joule, or $\frac{k g \cdot m^{2}}{s^{2}}$ .

There are many, many uses for energy in physics computations. It can simplify all sorts of things that would otherwise be hard to analyze:

Burning fuel to accelerate
Speed while rolling down an unevenly-shaped hill
How high a projectile will go
How fast a falling object is moving

In fact, it's so useful that I recommend asking yourself if you can solve a problem using energy before you even consider other methods. It can often make solutions simpler for kinematics problems especially.

Media resources

Guided practice

LOL Chart: A heavy rock slides down a hill at a constant speed. The system is the rock and hill.
LOL Chart: An asteroid crashes into jupiter. The system includes just Jupiter.
LOL Chart: A motorcycle accelerates from the starting line. System: motorcycle-track.
How much work do you do when you do just the upward movement part of a pushup?

Homework

#hw (physics) In one sentence, summarize the content of the article we read. 2025-02-10
#hw (physics) Do the tasks assigned to you at physicsclassroom.com 2025-02-10