Energy, Work & Power

The Nature of Energy energy different forms of energy the ability to cause change different forms of energy electrical – making toast chemical – stored in food radiant – from the sun thermal – warming of the atmosphere energy plays a role in every activity you do

Kinetic Energy Kinetic Energy (KE) Kinetic Energy Equation KE = ½ mv2 energy a moving object has because of its motion depends on the mass and speed of the object Kinetic Energy Equation KE = ½ mv2 KE = kinetic energy (J) m = mass (kg) v = speed (m/s)

EXAMPLE 1 KE= m= 15 kg v= 3 m/s

EXAMPLE 2 What is the kinetic energy of a 20 kg ball bouncing with a speed of 25 m/s?

Potential Energy energy does not always include motion Potential Energy (PE) stored energy an object has due to its position Example apple in a tree due to its height when it falls, potential energy transferred to kinetic

Gravitational Potential Energy Gravitational Potential Energy (GPE) energy stored by objects due to their position above Earth’s surface depends on distance above Earth’s surface and the object’s mass Gravitational Potential Energy Equation GPE = mgh GPE = gravitational potential energy (J) m = mass (kg) g = 9.8 m/s2 (acceleration due to gravity) h = height (m)

GPE = mgh 1. What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg and is 4 m above the ground?

GPE = mgh 2. An 80 kg diver jumps from a 10 m high platform. What is the gravitational potential energy of the diver halfway down?

GPE = mgh 3. A boulder on top of a mountain has mass of 66 kg and has a GPE of 1000 J. What is the height of the mountain/

Elastic Potential Energy energy stored when an object is compressed or stretched Example: stretching a rubber band has stored elastic potential energy when released, the potential energy changes to kinetic Chemical Potential Energy energy stored in chemical bonds Example gasoline food

Conservation of Energy when energy changes form, it is always conserved a light transforms electrical energy to light and thermal energy other devices that make use of electrical energy

Conversions between Kinetic and Potential ME = PE + KE ME = Mechanical Energy PE = Potential Energy KE= Kinetic Energy

Temperature and Heat temperature Kelvin (K) measure of the average kinetic energy of all the particles in an object Kelvin (K) SI unit of temperature a more commonly temperature scale used is Celsius (°C) K=°C+273 °C=K – 273 Convert 20°C to K 20°C + 273 = 293 K

Thermal Energy thermal energy sum of the kinetic and potential energy of the particles is transferred by conduction, convection, and radiation when the temperature of an object increases, the average kinetic energy of the particles in the object increases as a result, the thermal energy increases

Water °C (Celsius) °F (Fahrenheit) K (Kelvin) boiling point – 100 freezing point – 0 °F (Fahrenheit) boiling point – 212 freezing point – 32 K (Kelvin) boiling point – 373 freezing point - 273

Heat temperature and heat are not the same, but they are related heat thermal energy that flows from a warmer material to a cooler material its unit is still in Joules (J) heat always flows from warmer to cooler materials heat depends on the amount of substance you have

Transferring Thermal Energy thermal energy can be transferred three ways conduction convection radiation

Conduction Conduction transfer of thermal energy by collisions between particles in matter conduction occurs because particles in matter are in constant motion best conductors of heat  metals there are electrons that are not bound to individual atoms, but can move easily through the metal

Convection Convection fluid – liquid or a gas Convection current transfer of thermal energy in a fluid by the movement of warmer and cooler fluid from place to place fluid – liquid or a gas Convection current transfer heat from warmer to cooler parts of the fluid

Radiation Radiation transfer of energy by electromagnetic waves Energy transferred by radiation often is called radiant energy These waves can travel through space even when no matter is present

Radiant Energy and Matter materials that are light-colored reflect more radiant energy dark-colored materials absorb more radiant energy when radiant energy is absorbed by a material, the thermal energy of the material increases insulator a material in which heat flows slowly wood, plastic, fiberglass, air

Work Work for work to occur, an object must move transfer of energy that occurs when a force makes an object move measured in joules for work to occur, an object must move the motion of the object must be in the same direction as the applied force on the object

Calculating Work W = F d W = Work (J) F = Force (N) d = distance (m)

A force of 75 N is exerted on a 45 kg couch and the couch is moved 5 m A force of 75 N is exerted on a 45 kg couch and the couch is moved 5 m. How much work is done in moving the couch?

A lawn mower is pushed with a force of 80 N A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing the lawn, what is the total distance the lawn mower was pushed?

The brakes on a car do 240,000 J of work in stopping the car The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50 m while the brakes are being applied, what is the total force the brakes exert on the car?

Power Power amount of work done, or the amount of energy transferred, divided by the time required to do the work or transfer the energy measured in watts (W)

Calculating Power P = W t W = Work (J) P = Power (W) t = time (s)

To lift a baby from a crib 50 J of work are done To lift a baby from a crib 50 J of work are done. How much power is needed if the baby is lifted .5 s?

If a runner’s power is 130 W, how much work is done by the runner in 100 seconds?

The power produced by an electric motor is 500 W The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?

You exert a vertical force of 72 N to lift a box to a height of 1 You exert a vertical force of 72 N to lift a box to a height of 1.0 m in a time of 2 seconds. How much power is used to lift the box?

Using Machines machine device that makes doing work easier by increasing the force applied to an object, changing the direction of an applied force, or increasing the distance over which a force can be applied

Efficiency the same amount of work is done with a machine, but the goal is to use less force to do the work some machines change the direction of the force you apply wedge-shaped blade of an ax Using machines increases the efficiency of the work Efficiency – the percentage of work input that becomes work output in a machine