Chapter 13 Work & Energy

Physics The study of motion and energy

W = F x d (Newtons x meters) Work Work: done ONLY when a force is applied to an object, and the object moves IN THE SAME DIRECTION OF THE APPLIED FORCE Work is calculated by multiplying the force by the distance over which the force is applied Work = force x distance W = F x d (Newtons x meters) The SI unit for work is Joules (J)

Work Work is ZERO if an object is not moving If you are pushing a stalled car and cannot move it, you might exert a lot of force, but no work is done If you move it, even a little, work is done

Machines Machines can help us do work, but only some of it is useful work (work the machine was designed to do) Efficiency: the amount of useful work a machine does Some work is done when the machine generates heat or noises, but it isn’t “useful,” it isn’t what the machine is intended for A pulley used on a boat may experience a decrease in useful work if heat is generated from friction or if the pulley squeaks

Power Power: 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 The SI unit for power is watt (W) One watt is the amount of power needed to do one joule of work in one second

Energy Energy: ability to do work Energy is ALL around us and exists in many forms Whenever work is done, energy is transformed or transferred from one system to another System: a portion of the universe that is chosen for studying the changes that take place within it

Energy The SI unit for energy is Joules (J) The same units as work since work is only measured when you use energy to do it All types of energy will be either potential or kinetic

Potential Energy (EP) Potential Energy (PE): stored energy, or the energy of position EVERYTHING has potential energy – it is converted into other forms of energy!

GPE = mass x free-fall acceleration x height Potential Energy (EP) Gravitational Potential Energy (GPE): potential energy due to an object’s height above ground Depends on mass and height – Heavier objects will have more GPE, and items higher up will have more GPE GPE = mass x free-fall acceleration x height GPE = mgh g = 9.8m/s2 GPE = m(9.8 m/s2)h

Kinetic Energy (EK) Kinetic Energy (KE): energy of motion – the energy an object has because it is moving Things as large as planets and as small as atoms have kinetic energy Once an object begins to move, it has the ability to do work Kinetic energy depends on the mass of the object, and the rate of acceleration

Kinetic Energy (EK) Speed matters more than mass, as a small change in speed produces a large change in kinetic energy A speeding car will do more damage because it has higher kinetic energy An apple that is falling at 10 m/s can do more work than an apple that is falling at 1 m/s Atoms and molecules are always moving, so they have kinetic energy The formula for kinetic energy involves a square root, which matters when solving for speed

Kinetic Energy (EK) KE = ½ mv2 SI unit is J The kinetic energy of a golf ball is measured to be 143.3 J. If the golf ball has a mass of about 0.047 kg, what is its speed? KE = 143.3 J m = 0.047 kg v = ?

Forms of Energy MRS CHEN 7 forms of energy that fall under KE or PE Mechanical (KE + PE) Radiant (light) (KE) Sound (KE) Chemical (PE) Heat (thermal) (KE) Electrical (PE) Nuclear (PE) MRS CHEN

Forms of Energy Mechanical Energy: energy associated with the motion and position of an object, KE + PE in an object that is used to do work. MOTION. A car moving, a ball that has been thrown, a person falling Energy that does not affect motion on a large scale is nonmechanical When you eat food the energy stored in it is non-mechanical energy

Forms of Energy Chemical Energy: Energy that is released or absorbed by the rearrangement of bonds between atoms Fuels – food, batteries, gasoline Nuclear Energy: the movement of particles in the nucleus of an atom Fission and fusion This is how the sun gets energy!

Forms of Energy Electrical Energy: energy resulting from the movement of electrons across a circuit Lightning Thermal Energy: the sum of kinetic energy in all the particles of an object Heat energy The higher the temperature, the more thermal energy The larger something it, the more thermal energy it has

Forms of Energy Light Energy: energy produced by vibrations of charged particles Also called electromagnetic energy or radiant energy Visible light, X rays, UV rays, the sun Sound Energy: vibrations travelling through the medium of the sound Sound requires a medium (something to travel through), like a wave

Law of Conservation of Energy Energy readily changes form from one type to another, however, energy cannot be created or destroyed When total energy in a system increases, the increase must be due to energy that enters the system For example, if a person on a trampoline bounces higher the second time, we can conclude their legs did work to add energy to the bounce

Energy Transformations PE can become KE and vice versa At the top of a hill, a car has high PE, but it changes to KE as it rolls down, the KE gets it up the next hill This change isn’t always complete If so, balls would always bounce to the same height and roller coasters would never stop gliding Some mechanical energy (KE + PE) will change to other forms of energy  sound or thermal energy, for example

Energy Transformations Energy transformations happen ALL AROUND YOU! Light energy from the sun is converted to chemical energy through photosynthesis, and electrical energy through solar panels A TV changes electrical energy into sound and light energy A car changes chemical energy (gas) into thermal and mechanical energy Nuclear energy generates thermal and electrical energy Potential energy is converted to kinetic energy when you stretch a rubber band and then release it

Thermodynamics Thermodynamics: study of heat and temperature and their relation to work and energy Thermodynamics explains energy conservation with the First Law of Thermodynamics: the net change in energy equals energy transferred as work or heat If heat energy is added to a system, some energy stays in the system and some leaves. The energy that leaves does work on the area around it, energy that stays creates an increase in energy of the system.

Thermodynamics If a pot of water is at room temperature and you add heat to the system: 1st, temperature and energy of water increases. 2nd, the system releases some energy and it works on the environment (maybe heating the air around the water, making the air rise).