What should I know?

1. A force can sometimes cause a change in an object's position, giving it some displacement.
2. It is useful to think of both the force that causes a displacement and the displacement over which that force is applied as a single physical concept, called work.
3. Work is the product of the force and the displacement over which it is applied.
4. The units for work should be Newtons x meters, but we summarize this as Joules, named for James Joule.
5. Work, with its formula of W = F · Δx, is similar to but distinctly different from the concept of impulse discussed in Unit 03, with its formula of J = F · Δt.
6. Just as the impulse applied to an object is stored in its momentum, the work applied to an object is stored in its energy.
7. Energy stored in an object in some way can be used to do work at another time.
8. Energy is technically defined as the ability to do work, but, in reality, it is a broad concept that is tricky to define. It is also arguably the most important concept in all of science. Since physics studies energy directly, and all other branches of science study the application of energy concepts to specific situations, physics is sometimes considered the foundation science.
9. Since work changes energy and energy is used to do work, they have the same units: Joules.
10. The amount of work done is the amount that energy is changed. The amount of energy used is the amount of work done. This is called the work-energy relationship (or, sometimes, the work-energy theorem.) It can be summarized in the simple formula W = ΔE.
11. The time-rate at which work is done or energy is transferred is called power. The formula for power is P = W/Δt or P = ΔE/Δt.
12. Power is measured in Joules/second, but abbreviated as Watts, after James Watt.
13. We will divide the concept of energy into three major types: kinetic energy, potential energy and thermal energy. These are not the only kinds of energy, just the major types we will discuss in this class.
14. Kinetic energy is energy of motion. The more velocity an object has, the more kinetic energy it has. The more mass an object has, the more it resists changes to its velocity and the more kinetic energy it has.
15. For nearly all objects, kinetic energy can be calculated using the formula K = (1/2)mv² .
16. By colliding with a second object, an object can do work on that second object, transferring some or all of its kinetic energy to that second object.
17. Potential energy is energy of position. Work becomes potential energy when it move an object from one position to another against a force that resists that displacement. This happens often in a gravitational field, but also in electric and magnetic fields, and when an object is attached to something elastic, like a spring.
18. In general terms, the amount of potential energy gained against a resistive force is equal to the work done to move that object: U = F · Δx.
19. Since we will often use gravitational potential energy with the Earth as an example, it is helpful to use the more specific form for gravitational potential energy: U_g = mgh.
20. Thermal energy (Q) is the energy of the random motions of the particles that make up an object. In a sense, it is a kind of kinetic energy, but is random in its nature, consisting of translational, vibrational and rotational motions. (Note: this is a part of what often called internal energy.)
21. Since these random motions are very often not easy to convert back into work, we will often consider it as energy lost from a system, but it is not destroyed -- simply not useful energy.
22. When analyzing the workings of a machine or a process, its efficiency (𝜂) can be quantified by comparing the amount of work and energy gotten out of it and the amount of work and energy put into it. The formula for efficiency is 𝜂 = E_out / E_in .
23. No device or process can get more work or energy out than is put into it.
24. The reason people use devices or machines to complete a task is not that it increases the amount of work or energy, but that it increases the amount of force that you can apply.
25. The force advantage that you gain from using a device or a machine is called the mechanical advantage. Its formula is MA = F_out / F_in . The mechanical advantage is often, but not always, a number greater than 1.
26. The useful forms of energy for this class -- those that are easy to convert back to work -- are kinetic and potential energy. Together, these make up the general category called mechanical energy.
27. In a closed system of objects, the total amount of energy and work are a constant. This principle is one of the most important in all of science. It is called the law of conservation of energy.
28. In an ideal, closed system, the efficiency of all processes is 1 (or 100%.)