Download 7 Forms of Energy

7 Forms of Energy
Energy appears in many forms, such as heat, light, sound, electricity, and radioactivity. It even takes the form of mass, as celebrated
in Einstein’s famous E = mc2 equation.
1. Chemical – given off during chemical changes due to breaking bonds. (May occur very quickly or slowly.)
Examples: TNT, Campfire, digesting food
2. Electrical – produced by moving charges.
Example: Lightning
3. Sound – Vibrating particles
Example: Drum beat
4. Radiant or Solar (light) – energy that travels using electromagnetic waves.
Examples: Microwaves, radio waves, x-rays
5. Mechanical – Energy in moving objects.
Examples: Car moving down a road, falling object
6.
Nuclear – released when the forces that hold the nucleus of an atom together are overcome.
Examples:
Fusion = Sun
Fission = Nuclear Reactors
7. Heat – caused by energy being transferred by colliding particles.
Examples: Occur on the atomic level
Law of Conservation of Energy (LOCOE) – Energy cannot be created nor destroyed, but may be changed from one form to another.
Work
It takes energy to push something and make it move. How much energy depends on the force exerted and how long it is exerted. By
“how long,” we mean distance. The quantity “force × distance” is equal to the change in energy.
Work - The product of the force and the distance through which the force moves:
Work = force × distance
The unit of work combines the unit of force (N) with the unit of distance (m), the newton-meter (N · m). We call a newton-meter the
joule (J)
The amount of work done on an object depends on (1) how much force is applied and (2) how far the force causes the object to move.
Work is done in lifting the barbell.
Lifting it twice as high requires twice
as much work
.
He may expend energy when he pushes on the
wall, but if it doesn’t move, no work is done on
the wall.
Effort Force is the force applied to do work
Resistance Force is the force that opposes motion

To move an object, the effort force must be greater than the resistance force.
Work = force x distance
W
W=fxd
W=N∙m
f
f = Newtons
d
d = meters
Problems:
1. How much work is needed to lift an object that weighs 500 N to a height of 4 m?
2.
How much work is needed to lift it twice as high?
3.
How much work is needed to lift a 1000 N to a height of 8 m?
Power
Lifting a load quickly is more difficult than lifting the same load slowly. If equal loads are lifted to the same height, the forces and
distances are equal, so the work is the same. What’s different is the power.
Power is the rate at which energy is changed from one form to another. Also, power is the rate at which work is done.
Power equals the amount of work done divided by the time interval during which the work occurs.
The unit of power is the joule per second, called the watt.
One watt (W) of power is used when one joule of work is done in one second.
One kilowatt (kW) equals 1000 watts. One megawatt (MW) equals one million watts.
Power – the amount of work done in a period of time. (watts)
Power = work ÷ time
1watt( w) 
1 joule
1sec ond
1 Kw = 1000 w
Problems:
1. How much work is done when a person moves a box with a force 20N over a distance of 5m?
2.
How much work is done when 2 people lift a bed 0.2m up with a total force of 300N. If they each do
exactly half of the work, how much work did one person do?
3.
If the work in the previous problem took 3s to do, how much power did each person use?
4.
A person weighing 600N gets on an elevator. The elevator lifts the person 6m in 10s. How much
power was used by the elevator?
Mechanical Energy
Work is done in lifting the heavy ram of a pile driver. When raised, the ram then has the ability to do work on
a piling beneath it when it falls. The ability to do work has been acquired by an object. This ability to do work
is energy. Like work, energy is measured in joules.
Potential energy is stored energy that arises because of an object’s position.
Kinetic energy is energy of motion possessed by moving objects.
Potential and kinetic energy are both considered to be kinds of mechanical energy. So mechanical energy may be in the form of either
potential energy, kinetic energy, or both.
Potential Energy
Energy that is stored is called potential energy (PE). In the stored state, energy has the potential to do work.
The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in lifting it–the
force required to move it upward multiplied by the vertical distance moved (W = F × d).
Gravitational potential energy = weight × height
An object’s weight is its mass m multiplied by the acceleration of gravity g. We write weight as mg. So the work done in lifting mg
through a height h is equal to its gain in gravitational potential energy (PE):
PE = mgh
Note that the height h is the distance above some base level, such as the ground or the floor of a building. The potential energy is
relative to that level and depends only on weight and height h.
You can see in Figure 6.8 that the potential energy of the boulder at the top of the structure depends on height only, and not on the
path taken to get it there.
The PE of the 10-N ball is the same (30 J) in all three cases. That’s because the work done in elevating it 3 m is the same whether it is
(a) lifted with 10 N of force, (b) pushed with 6 N of force up the 5-m incline, or (c) lifted with 10 N up each 1-m stair. No work is
done in moving it horizontally (neglecting friction).
Kinetic Energy
When you push on an object, you can make it move. Then a moving object becomes capable of doing work. It has energy of motion,
or kinetic energy(KE). The kinetic energy of an object depends on its mass and speed. Specifically, kinetic energy is equal to one half
the product of the mass and the square of the speed:
Kinetic energy = 1/2 mass × speed2
KE = 1/2 mv2
Based on the equation heavy objects have more kinetic energy than light ones moving at the same speed. Kinetic energy also depends
on speed. In fact, kinetic energy depends on speed more than it depends on mass. Kinetic energy depends on speed multiplied by
speed.
Work-Energy Theorem
To increase the kinetic energy of an object, work must be done on it. Or, if an object is moving, work is required to bring it to rest.
If there is no change in an object’s energy, then we know no work was done on it.
Work equals change in kinetic energy. Work = ΔKE
Push against a box on a floor. If it doesn’t slide, then you are not doing work on the box. This theorem applies to changes in potential
energy also. When an object is held stationary, no further work is being done because there is no change in position.
When the car goes twice as fast, it has
four times the kinetic energy (and will
need four times the stopping distance
when braking).
Problems:
1. When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it’s moving 3 times as fast?
2.
Can an object have energy?
3.
Can an object have work?
Conservation of Energy
The Law of Conservation of energy states:
Energy cannot be created or destroyed; it may be transformed from one form into another or transferred from one object to
another, but the total amount of energy never changes.
Energy transitions in a pendulum. PE is relative to the lowest point of the pendulum when it is vertical.
The potential energy of the
elevated ram is converted to
kinetic energy when released.
A circus diver at the top of a pole has a potential energy of 10,000
J. As he dives, his potential energy converts to kinetic energy.
Note that at successive positions one-fourth, one-half, threefourths, and all the way down, the total energy is constant.
As stated in the LOCOE, energy cannot be created nor destroyed.
All Kinetic Energy must have been there at one time as Potential
Energy in one form or another. This means that the total kinetic
energy equals the total potential energy.
KE = PE
A ball with a mass of 5kg falls 6m. What is the final velocity?
Machines Notes
What do machines do for us?
Make work easier by:
1) reducing the amount of effort force needed (multiplies effort force)
2) Multiplies the distance through which the effort force moves.
Does a machine allow us to do less work?
NO! You use less force but over a greater distance.
Work done on one end equals the work done on a
load at the other end
Force is multiplied. Note that a small input force ×
large distance = large output force × small distance.
Simple Machines – change the size and/or direction of the force being used to do the work.
Types of simple machines:
1. Levers – a rigid bar that is free to turn about a fixed point called the fulcrum.
2. Inclined Planes – a sloping surface
3. Pulleys - a pulley is a grooved wheel that turns around an axle (Fulcrum), and a rope or a chain is used in
the grove to lift heavy objects.
a. A Pulley changes the direction of the force – instead of lifting up, you can pull down using your body
weigh against the Resistance (load, what you are lifting).
Input force × input distance = output force ×
output distance. Note the load is supported
by 7 strands of rope. Each strand supports
1/7 the load. The tension in the rope pulled
by the man is likewise 1/7 the load.
This pulley acts like a lever. It changes only
the direction of the input force.
Efficiency
Any machine that multiplies force does so at the expense of distance. Likewise, any machine that multiplies distance does so at the
expense of force. No machine or device can put out more energy than is put into it. No machine can create energy; it can only transfer
it or transform it from one form to another.
Work input is greater than work output due to friction (HEAT).
Given the same energy input, some machines can do more work than others. The machines that can do more work are said to be more
efficient. Efficiency is the percent of the work put into a machine that is converted into useful work output.
Work Output (WO) = Work Done = Resistance Force x Resistance Distance
Work Input (WI) = Energy Used = Effort Force x Effort Distance
4m
1m
The box weighs 3200N and it takes an effort force of 1200N to move the box up the ramp.
WI = 1200N x 4m = 4800J
WO = 3200N x 1m = 3200J
Efficiency = WO  WI x 100%
3200J  4800J x 100% = 67% efficiency
Even a lever converts a small fraction of input energy into heat when it rotates about its fulcrum. We may do 100 joules of work but
get out 98 joules. The lever is then 98 percent efficient, and we waste 2 joules of work input on heat. In a pulley system, a larger
fraction of input energy goes into heat. If we do 100 joules of work, the forces of friction acting through the distances through which
the pulleys turn and rub about their axles may dissipate 60 joules of energy as heat. So the work output is only 40 joules, and the
pulley system has an efficiency of 40 percent. The lower the efficiency of a machine, the greater the amount of energy wasted as heat.