Energy The fuel for motion

Learning Goals I can differentiate between kinetic and potential energy. I can calculate kinetic energy and gravitational potential energy. I can determine the change in kinetic energy in a collision. I can explain different forms of energy conversions.

What is Energy? The ability of something to change its self or the environment The ability to do work

Types of Energy Kinetic Energy Moving energy More mass, more kinetic energy More velocity, more kinetic energy ▫Kinetic energy = 0.5(mass) x velocity 2 ▫KE =0.5mv 2 Units for energy – Joules

Example A 1500kg car is traveling at a velocity of 25m/s. How much kinetic energy does the car possess? ▫Known:  m = 1500kg  v = 25m/s  KE = 0.5mv 2 KE = 0.5mv 2 KE = 0.5(1500)25 2 KE = J

Work-Energy Theorem the net work done on an object is equal to its change in kinetic energy (W = KE f – KE i ) Example: A boy pushes a 5.00 kg cart in a circle, starting at m/s and accelerating to 3.00 m/s. How much work was done on the cart? ▫W = KE f – KE i  KE f = 0.5(m)(V f 2 ) = 0.5(5.0)( ) = 22.5J  KE i = 0.5(m)(V i 2 ) = 0.5(5.0)(0.5 2 ) = J ▫W = KE f – KE i = 22.5 – = J

Potential Energy Stored energy due something’s position Types of Potential ▫Elastic Potential ▫Stored by something that can stretch or compress ▫Ie: a rubber band or spring

Chemical Potential Stored in chemical bonds Released during chem. Reactions ▫Found in the food you eat, released as it is digested ▫Natural Gas- When burned the Chem Potential is released as heat

Gravitational Potential Stored in anything that can fall ▫The farther from the ground, the more potential ▫The more mass, more potential GPE = mass (kg) x 9.8m/s 2 x height (m) ▫Changes once an object falls

Example A 2kg lamp is hung 3m above the floor. What is the gravitational potential energy of the lamp? ▫Known:  m = 2kg  h = 3m  g = 9.8m/s 2  PE = mgh ▫Unknown  PE = (2)(3)(9.8)  PE = 58.8 J

Warm Up How much potential energy does a 2.0kg coconut possess while in a tree branch 3.5m above the ground?

Law of Conservation of Energy Energy cannot be created or destroyed only transferred ▫The total amount of energy in the universe is constant  Total Energy stays the same  PE i + KE i = PE f + KE f  Or  0.5mv MV 2 = 0.5m(v 2 ) M(V 2 ) 2

Conversions between Kinetic and Potential Mechanical energy ▫The total amount of energy in a system ▫Mechanical energy = PE + KE ▫When shooting a rubber band, elastic potential is converted into kinetic energy

Energy Conversions Falling objects ▫When an object falls the energy is converted from Gravitational potential energy into kinetic. ▫As height decreases, GPE decreases, but velocity (kinetic energy) increases.

Energy Conversions Pendulums (swings) As an object swings, ▫at the highest point PE is highest, KE is lowest.PE = max ▫At the lowest point, PE is lowest, KE is the highest ▫Velocity increases heading toward lowest point, decreases toward highest point

Other Energy Changes Collisions ▫During Collisions energy is transferred btn bodies ▫Types of Collision  Elastic – if all potential is changed back into kinetic  Ie pool balls  Use conservation of energy equation  Inelastic – if some of the kinetic is changed into other forms of energy  If two clay balls hit, they will change shape and stick  Find the change in kinetic energy ▫Most collisions aren’t entirely one or the other, generally a combination of the two

Energy Changes Elastic Inelastic

Other Energy Changes Friction ▫Causes an object to slow down. ▫Changes kinetic energy into thermal energy/heat

Warm Up How much kinetic energy does a 250kg bowling ball have just as it hits the ground if it is dropped from a height of 10m?

Other Energy Changes Converting Mass into Energy ▫Nuclear fussion  Taking two atoms and making a new one plus neutrons  Releases radiant energy (radiation) ▫Nuclear Fission  Splitting atoms into two new ones  Releases radiation  Method behind nuclear power and nuclear weapons

Einstein and Energy Einstein’s famous equation determines how much energy is available in mass ▫Mass-energy equivalence ▫Relates to his theory of special relativity E = mc 2 ▫E – energy ▫M – mass ▫C – speed of light  3.0 x 10 8 m/s

Energy Conversions Energy Conversions in the body ▫Food (chemical potential) is used and changed into heat, kinetic energy (movement), and used in chemical processes ▫Calorie is the unit used by nutritionists for energy  1C = 4184 J

Warm Up A baseball player throws a 0.5kg ball into the air with a velocity of 12.5m/s. What is the maximum height the ball with reach? (hint… law of conservation of energy..)