Third, energy can change forms. When water falls over a dam, its energy of position

becomes the energy of motion (kinetic energy). The kinetic energy from the moving

water can cause a turbine to spin in a dam, generating electric energy. If that electricity

is used to power a blender to make a milkshake, the energy is transformed again, this

time into the rotational kinetic energy of the blender’s spinning blades.

In Concept 1, an archer does work by applying a force to pull a bowstring. This work

increases the elastic potential energy of the bow. When the string is released, it

accelerates the arrow, transferring and transforming the bow’s elastic potential energy

into the kinetic energy of the arrow.

One can trace the history of the energy in the bow and arrow example much farther

back. Maybe the chemical energy in the archer that was used by his muscles to stretch

the bow came from the chemical energy of a hamburger, and the cow acquired that

energy by digesting plants, which got energy via photosynthesis by tapping

electromagnetic energy, which came from nuclear reactions in the Sun. We could go

on, but you get the idea.

Energy is a scalar. Objects can have more or less energy, and some forms of energy

can be positive or negative, but energy does not have a direction, only a value. The joule is the unit for energy, just as it is for work. The fact

that work and energy share the same unit is another indication that a fundamental relationship exists between them.

Energy

Transfers between objects

Exists in many forms

6.4 - Kinetic energy

Kinetic energy: The energy of motion.

Physicists describe the energy of objects in motion using the concept of kinetic energy (

KE). Kinetic energy equals one-half an object’s mass times the square of its speed.

To the right is an arrow in motion. The archer has released the bowstring, causing the

arrow to fly forward. A fundamental property of the arrow changes when it goes from

motionless to moving: It gains kinetic energy.

The kinetic energy of an object increases with mass and the square of speed. A

74,000 kg locomotive barreling along at 40 m/s has four times as much kinetic energy

as when it is going 20 m/s, and about five million times the kinetic energy of a 6-kg

bowling ball rolling at 2 m/s.

With kinetic energy, only the magnitude of the velocity (the speed) matters, not

direction. The locomotive, whether heading east or west, north or south, has the same

kinetic energy.

Objects never have negative kinetic energy, only zero or positive kinetic energy. Why?

Kinetic energy is a function of the speed squared and the square of a value is never

negative.

Because it is a type of energy, the unit for kinetic energy is the joule, which is one kg·

(m/s)^2. This is the product of the units for mass and the square of the units for velocity.

Kinetic energy

Energy of motion

Proportional to mass, square of speed

KE = ½ mv^2

KE = kinetic energy

m = mass

v = speed

Unit: joule (J)

(^124) Copyright 2007 Kinetic Books Co. Chapter 06