Download The Nature of Energy Notes

The Nature of Energy Notes
Unit III Matter & Energy
When wind moves a leaf, or even a house, it causes a change. In this case, the change is in the
position of the object. Work is done when a force moves an object through a distance. The ability to
do work or cause change is called energy. So the wind has energy.
When an object or organism does work on an object, some of its energy is transferred to that object.
You can think of work, then, as the transfer of energy. When energy is transferred, the object
upon which the work is done gains energy. Energy is measured in joules-the same units as joules.
Kinetic Energy
There are two general kinds of energy. The two kinds of energy are kinetic energy and potential
energy. Whether energy is kinetic or potential depends on whether the energy is being transferred or
stored. The examples you have read about so far have involved things that were moving. A moving
object can collide with another object and move it some distance. In that way, the moving object
does work. For example, a bowling ball knocks over a bowling pin. Because the moving object can
do work, it must have energy. The energy of motion is called kinetic energy. The word kinetic
comes from the Greek word kinetos, which means “moving.”
Mass and Velocity The kinetic energy of an object depends on both its mass and its velocity. Think
about rolling a golf ball and a bowling ball so that they travel at the same velocity. Which ball would
you have to roll more forcefully? You would have to exert a greater force on the bowling ball because
it has more mass than the golf ball. Since energy is transferred during work, the more work you do,
the more energy you give to the ball, So a bowling ball has more kinetic energy than a golf ball
traveling at the same velocity. Kinetic energy increases as mass increases. What would you have to
do to make the bowling ball move faster? You would have to throw it harder, or use a greater force.
In other words, you have to do more work on the bowling ball to give it a greater velocity. Kinetic
energy increases when velocity increases.
Calculating Kinetic Energy Kinetic energy depends on both mass and velocity. The mathematical
relationship between kinetic energy, mass, and velocity is written as follows.
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Kinetic Energy = (Mass x Velocity ) / 2
Do changes in velocity and mass have the same effect on kinetic energy? No—changing the velocity
of an object will have a greater effect on its kinetic energy than changing its mass. This is because
velocity is squared in the energy equation. For instance, doubling the mass of an object will double
its kinetic energy. But doubling its velocity will quadruple its kinetic energy.
Potential Energy
Sometimes when you transfer energy to an object, you change its position or shape. For example,
you lift a book up to your desk or you compress a spring to wind a toy. Unlike kinetic energy, which is
the energy of motion, potential energy is stored. It might be used later on when the book falls to the
floor or the spring unwinds. Energy that is stored and held in readiness is called potential energy.
This type of energy has the potential to do work.
An archer gives potential energy to a bow by pulling it back. The stored energy can send an arrow
whistling to its target. The potential energy associated with objects that can be stretched or
compressed is called elastic potential energy. You give a different type of potential energy to an
object when you lift it. Potential energy that depends on height is gravitational potential energy.
The gravitational potential energy an object has is equal to the work done to lift it. Work = Force x
Distance. The force is the force you use to lift the object, or its weight. The distance is the distance
the object moves, or its height. This gives you the following formula.
Gravitational potential energy = weight x height
When weight is measured in Newton’s and height is measured in meters, the unit of energy is the
Newton-meter. This unit is also know as the joule (J). Recall from Chapter 4 that a joule is the
amount of work you do when you exert a force of 1 Newton to move an object a distance of 1 meter.
Work and energy share the same unit because energy and work are so closely related. Once you
know weight and height, you can calculate gravitational potential energy. Suppose that a hiker climbs
4 meters up a hill and that he weighs 680 Newton’s. The hiker has gained 27,200 joules (680
Newton’s 40 meters) of gravitational potential energy at the top of the climb.
The greater the weight of an object or the greater the height it is lifted, the greater its gravitational
potential energy. The hiker would gain more gravitational potential energy by climbing to a greater
height or by increasing weight, maybe by wearing a backpack. What if you known the mass of an
object instead of its weight? Then you multiply the mass of the object by the acceleration of gravity
(9.8 m/s2) to find its weight in Newton’s. In this way you can write a second equation for gravitational
potential energy.
Gravitational Potential energy = Mass x Gravitational Acceleration x Height