ENERGYChapter 8

Definitions - 1■ In Chapter 7 we defined the product of a force () and the time () over which it is

applied is called an impulse.

■ And we related it to the change in momentum.

■ It is also useful to consider the quantity . We call this quantity work.

or ■ So work is done by 1) the application of a force and 2) the movement of something

by that force.

Definitions - 2

■ We can see from the definition of work that the units of work are units of force (newtons) times units of distance (meters). We call the unit of work a joule:

■ That is, one joule is the work done by a force of 1 N exerted over a distance of 1 m.

■ We are also interested in how long it takes to do a certain amount of work. The rate at which work is done is called power.

■ The unit of power is the joule per second, the watt (W).

Definitions – 3■ When work is done on an object, the object acquires the ability to do

work. The ability of an object to do work is called energy. We’ll see that energy can take many forms but for now we’ll concentrate on mechanical energy, which comes in two types:

– Potential energy (PE) is the energy an object has due to its position or configuration. A common form of potential energy is gravitational potential energy. An object of mass m that is at height h above the Earth’s surface has gravitational potential energy:

– Kinetic energy (KE) is the energy an object has due to its movement and depends on its speed and mass:

or

Work and Energy

■ If we bring something to a certain speed from rest, we’ve done an amount of work to increase the object’s kinetic energy:

■ The general form of this concept is the work-energy theorem:

■ That is, if no change in energy occurs, no work is done.

Conservation of Energy

■ We’d like to find out how energy transforms from one form to another (e.g., from KE to PE).

■ One of the most important findings in physics is the law of conservation of energy:

– Energy cannot be created or destroyed. It can be transformed from one form to another, but the total amount of energy does not change.

Applications

■ Science of NHL Hockey■ Circus Physics■ Roller Coaster Energy■ Energy Transfer in a Trebuchet

Activities

■ Read section Situations in Which Mechanical Energy is Conserved, especially the roller coaster section.

■ Work and Energy Interactives– Stopping Distance– Roller Coaster Model– Chart That Motion■ Read Scientific American article