Work and Energy Chapter 13 section 1,3, 4

What is work? Work is done ONLY when force is applied to an object and the object moves IN THE SAME DIRECTION OF THE APPLIED FORCE Work is calculated by: Work= force x distance W= F x d

Work is ZERO when an object is not moving If you are trying to push a stalled car You might exert a lot of force but are unable to actually move it No work was done If the car actually moves, even a little.. work was done

Work is measured in Joules Remember: W = F x d F is measured in N (newtons) D is measured in m (meters) J (joules) is equal to N x m So: 1 N x m = 1 J = 1 kg x m2/s2 These units are interchangeable

Power Power is the rate at which work is done, or how much work is done in a given amount of time. Power = work/time P= W/t Power is measured in WATT (W)

One watt is the amount of power needed to do one joule of work in one second BE CAREFUL The W for work is not italicized The W for watt is italicized

One of the easiest things to understand and hardest to define….. Energy is really only observable when it is being used so the definition is what you can do with it. Energy- the ability to do work The units of energy are joules These are the same units as work since work is only measured when you use energy to do work.

Two Types of Energy… Potential is stored energy or energy of position We will use gravitational potential energy (GPE) energy due to height above the ground. Kinetic is action energy, it is when something is moving …. Total Energy of a system is KE + PE

Gravitational Potential Energy GPE depends on both and height If you drop two apples of different mass but from the same height The HEAVIER apple has more gravitational potential energy than the lighter one does. GRAVITATIONAL POTENTIAL ENERGY EQUATION GPE = mass x free-fall acceleration x height PE= mgh

Going on…. “mg” is the weight of the object in newtons This is mass x gravity. Height is relative- usually measured from the ground, but sometimes a different height might be more important For instance, if an apple is going to fall onto a bird’s nest at a lower branch in a tree 12 feet off the ground

Kinetic Energy Once an object begins to move it has the ability to do work The energy that an object has because it is moving is KINETIC ENERGY (KE) The kinetic energy depends on the mass of the object, and the rate of acceleration Kinetic Energy Equation: Kinetic energy = ½ x mass x speed squared KE = 1/2mv2

So what is the big deal? An apple that is falling at 10 m/s can do more work than an apple that is falling at 1 m/s.. Kinetic energy depends on speed more than it does on mass A small change in speed produces a large change in kinetic energy (and possibly more damage) Atoms and molecules have kinetic energy

Forms of Energy One way or another, all forms of energy involve motion. All forms of energy have a kinetic and potential type!!!

Mechanical Energy Mechanical Energy- the energy of an object actually moving The amount of work that something can do because of its potential and kinetic energy This is the sum of kinetic energy and the potential energy in a system ME = KE + PE Like a car moving, a ball that has been thrown, a person falling etc.

Chemical reactions involve potential energy Chemical energy is a kind of potential energy The energy is in the bonds that hold things together The breaking of chemical bonds involves changes in energy Reactions that release energy decrease the potential energy Reactions that consume energy increase the potential energy

Chemical energy Chemical energy- energy released or absorbed by the rearrangement of bonds between atoms Exothermic reaction- releases energy and makes the surrounding area hotter fire Endothermic reaction- absorbs energy making it colder (instant ice pack)

Where does all the energy on this planet come from? Virtually all energy on this planet comes from the sun With the exception of nuclear energy and geothermal The sun is releasing energy from nuclear fusion reactions fusing hydrogen into helium

How does all our energy come from the sun? Obviously heat energy on the surface of the Earth is from the sun But the sun’s energy is also used for: plant life … and animal life water cycle wind

Plant life Plants use light energy to make (sugars) from carbon dioxide. Light energy has been converted to chemical energy Burning these sugars releases the energy the plants used to create them. Animals burn the sugars during digestion, YES animals eat plants. Burning of fossil fuels (like gasoline) also release this energy

Life light energy sugar carbon dioxide chemical energy

Think of the NUCLEAR REACTIONS… The Sun gets energy from nuclear reactions (fusion) Nuclear power plants use nuclear fission to release nuclear energy

Energy can be stored in “electric fields The appliances in your home run off of electricity This is because if electrons flow from areas of high potential energy concentration to areas of low PE concentration the ELECTRONS GAIN ENERGY (remember the boiling water example) Certain things have high electric potential while others have low

Law of conservation of energy Energy can be neither created nor destroyed, it can only change forms. So if we can neglect friction, the total energy of a system will always be the same.

So for a ball thrown in the air max PE 0 KE At all cases TE is the same TE = KE + PE KE = PE max KE 0 PE max KE 0 PE

Energy Transformations Roller coasters: Energy changes form: The conveyor belt is the most important part of the ride it is the initial energy input It is stored as gravitational potential energy at the top of the first hill It turns first into kinetic energy, then potential etc Some of the energy is lost as heat and vibrations that make a sound http://www.youtube.com/watch?v=7Tredv0Yq1M

Potential energy can become kinetic energy Example: A car sits on the top of a hill. As it rolls down the hill the energy slowly changes from potential energy to kinetic energy When the car reaches the lowest point, there is no potential energy

Kinetic energy can become potential energy If the car the kinetic energy takes the car up the next hill; the car slows down and most the energy turns back into potential energy This is an energy transformation…

Mechanical Energy can change to other forms of energy……. Kinetic to potential (and vice versa) isn’t always complete A bouncing ball: Doesn’t bounce back up to the original height Some of the energy changes to elastic potential energy Some of the energy is released as a sound from the plastic compressing The air around the ball gets warmer

Thermodynamics describes energy conservation Energy can be transferred as work or heat For any system: The net change in energy equals the energy transferred as work and as heat

Systems may be open, closed or isolated Open system: a system in which energy and matter are exchanged with the surroundings (most things) Closed system: if energy, BUT NOT MATTER, is exchanged Isolated system: no matter nor energy is exchanged (true ones can’t exist in nature)

Efficiency of Machines Only a portion of the work done by any machines is USEFUL work- meaning work that the machine is designed or intended to do There is a difference between the total work and the useful work done by a machine Some work is done in the addition of heat, or noises created by the machine but it isn’t “useful” because it is not what the machine is intended for

Efficiency equation: Efficiency = useful work output/work input Efficiency: a quantity usually expressed as a percentage, that measure the ratio of useful work output to work input

Perpetual motion machines are impossible This is a machine that would keep moving without the input of energy.. A perpetual motion machine could would in the absence of friction and air resistance -which doesn’t exist in this world Machines require an input of energy because some energy “leaks” from the system.

But wait…. Aren’t there more types of energy?

Light energy (also electromagnetic energy) Light Energy- Movement of “packets of energy” called photons causing a vibration in charged particles Anything you can see also X-rays, microwaves (the invisible wave cooking the food not the oven) UV rays and infrared waves. Electromagnetic waves are made of electric and magnetic fields

Thermal Energy (heat energy) all particles in an object are moving. Thermal Energy- the sum of the kinetic energy of all the particles in an object. hotter objects have more thermal energy. larger objects also have more thermal energy (it takes more heat to make something big hot).

Electrical energy (electromagnetic energy) Electrical Energy- energy resulting from a movement of electrons across a circuit. Ex- TV, radio, computer, video games (anything you plug in or put batteries in)

Sound Energy Sound requires a medium (something to travel through) Sound energy- A vibration moving through the medium of the sound. like a wave (in the ocean or at a football game)

Nuclear energy Nuclear energy- movement of particles of a nucleus of an atom either by fission, splitting a large atom (uranium or plutonium) or by fusion, joining smaller atoms (hydrogen to make helium)