Energy: Forms and Changes

Nature of Energy Energy is all around you You can hear energy as sound You can see energy as light You can feel energy as wind

Energy Energy is defined as the ability to do work The property of an object that allows it to produce change in the environment or itself All energy units are Joules=Nm All energy is scalar but change in energy can be a negative number

States of Energy

Kinetic Energy Kinetic Energy= Energy of motion K.E. = mass x (velocity2 ) 2 Or we can write: KE = .5mv2 or KE = 1/2mv2 What has a greater affect of kinetic energy, mass or velocity? Why?

KE example An object with a 45kg mass has 10.0m/s velocity. What is the KE of the object?

Potential Energy Potential Energy is stored energy. Or stored because of the work done on it: Stretching a rubber band. Winding a watch. Pulling back on a bow’s arrow. Lifting a brick high in the air.

Several types of Potential Energy Stored chemically in fuel(gasoline), the nucleus of atom(nuclear energy), elastic qualities and in foods. Calories are a measure of food energy ****Gravitational Potential Energy=potential energy that is dependent on height of the object and its mass. The bigger they are the harder they fall” is not just a saying. It’s true. Objects with more mass have greater G.P.E. The formula to find G.P.E. is G.P.E. = (Weight X Height) = (m x g x h)

PE example A 6.1kg bowling ball is lifted to a height of 2.1m above the ground. Calculate the PE of the ball. A bell with a 7.5N weight is placed on a rope 16.2m high. Calculate the PE.

Kinetic and Potential Energy Conversions Law of Conservation of Energy states that energy can be neither created or destroyed. “What you start with you end with” It can however be converted from one form to another If energy seems to disappear scientists look for what it converted to….heat, light, sound, etc…

Mechanical Energy Mechanical Energy is the total of all energy in a system. KE + PE = Mechanical Energy

Kinetic Potential Conversions At the point of maximum potential energy, the car has minimum kinetic energy.

Law of Conservation of Energy Formula for Conservation of Energy KEi + PEi = KEf + PEf Example Problem: A 3.0 kg mass is moving toward a hill at 4.7 m/s. How high will it travel up the hill before it comes to a stop? m = 3.0 kg v = 4.7 m/s KE = .5mv2

m = 3.0 kg v = 4.7 m/s KE = .5mv2 KEi + PEi = KEf + PEf .5mv2i + mghi = .5mv2f + mghf .5(3.0)4.72 + 0 = 0 + 3.0(9.81)h 33.135 = 29.43 h h = 1.12589... h = 1.1 m

Elastic Potential Energy potential energy spring = 1/2(spring constant)(distance from equilibrium)2 U = 1/2kx2 U = potential energy of a spring at a certain position k = the spring constant, specific to the spring, with units N/m. x = distance the spring is stretched or compressed away from equilibrium

A spring, which has a spring constant k = 7  A spring, which has a spring constant k = 7.50 N/m, has been stretched 0.40 m from its equilibrium position. What is the potential energy now stored in the spring? k = 7.50 N/m x = 0.40 m U = 1/2kx2 U = 1/2(7.50 N/m)(0.40 m)2 U = 0.60 N∙m U = 0.60 J The elastic potential energy stored by the spring when it is has been stretched 0.40 m is 0.60 J.