The Story of Energy Forms and Functions
What are 5 things E helps us do?
Batteries store energy! This car uses a lot of energy Even this sleeping puppy is using stored energy. We get our energy from FOOD!
Energy O is the ability to make things move
All Energy potential kinetic with an example of each
Potential Energy Stored-up energy, or energy held in readiness potential" simply means the energy has the ability to do something useful later on
For example: When a roller coaster slows to a stop at the top of a hill, it has potential energy because of where it is (position in space). It has the potential to move because it is above the ground and has somewhere to go.
OSubstances like wood, coal, oil, and gasoline have stored energy because of their chemistry they can burn OStored energy is potential energy
Potential forms of energy Chemical: E stored in bonds of molecules Nuclear: E stored in nucleus of atoms Mechanical: E stored in objects by applying force Gravitational: E of place or position
Gravitational Potential Energy o If an object is elevated and has the force of gravity acting on it, the object has PE. o P.E. = mass x height x accel. due to gravity
Potential Energy (joules) PE = mgh Mass (kg) Height (m) Acceleration of gravity (9.8 m/sec 2 ) U g = mgh
There are a lot of advantages of knowing some physics but this one is definitely not one of them.
try a calculation What is the potential energy of a 50 kg gorilla on top of a skyscraper if he is 480 m above the street below? 50 kg PE = mgh 480 m PE = (50 kg)(9.8 m/s 2 )(480 m) PE = 235 kj
U g = (102 kg)(9.8 m/s 2 )(4 m) = 3,998 J O A cart with a mass of 102 kg is pushed up a ramp. O The top of the ramp is 4 meters high. 3,998 50 = 80 seconds to push the cart up the ramp. O How much potential energy is gained by the cart? O If an average student can do 50 J of work per second, how much time does it take to get up the ramp?
Kinetic Energy The energy of motion O If an object is moving, it has KE.
Examples of Kinetic Energy:
Kinetic forms of energy Radiant: E traveling in waves Thermal: or heat, the internal energy in substances Motion: movement of a substance from one place to another Sound: movement of E through substances in longitudinal waves Electrical: movement of electrons
O The kinetic energy of a moving object depends on two things: mass and speed. Kinetic Energy (joules) KE = 1 mv 2 2 Mass (kg) Speed (m/sec) Energy is measured in the same units as work because energy is transferred during the action of work.
Try a couple: What is the kinetic energy of a 5-g bullet traveling at 200 m/s? 5 g 200 m/s K mv 1 2 1 2 2 2 K = 100 J (0.005 kg)(200 m/s) What is the kinetic energy of a 1000-kg car traveling at 14.1 m/s? K 1 2 1 2 2 mv (1000 kg)(14.1 m/s) 2 K = 99.4 J
O Kinetic energy becomes important in calculating things like braking distance
Law of Conservation of Energy
O Potential Energy can be changed into Kinetic Energy O Also Kinetic Energy can be changed into Potential Energy
Law of Conservation of Energy O As energy takes different forms and changes things by doing work, nature keeps perfect track of the total. O No new energy is created and no existing energy is destroyed but it can change form.
for example O A falling object converts gravitational potential energy into kinetic energy O friction converts kinetic energy into vibrational (thermal) energy O makes things hot (rub your hands together) O irretrievable energy
more Energy transformation examples:
ball drop Energy Story How do your calculations and graphs demonstrate conservation of Energy? Why does the ball eventually stop bouncing?
Perpetual Motion O Why won t a pendulum swing forever? O It s hard to design a system free of energy transformations that don t include loss of heat to the environment O The pendulum slows down by several mechanisms O Friction at the contact point: requires force to oppose; force acts through distance work is done O Air resistance: must push through air with a force (through a distance) work is done O Gets some air swirling: puts kinetic energy into air O Perpetual motion means no loss of energy O solar system orbits come very close
Energy Exchange O Though the total energy of a system is constant, the form of the energy can change O A simple example is that of a simple pendulum, in which a continual exchange goes on between kinetic and potential energy pivot KE = 0; PE = mgh h PE = 0; KE = mgh KE = 0; PE = mgh height reference
Trapeze
the Russian Barre Anna bar
As Anna jumps and lands on the bar, her energy changes forms multiple times, but her total energy never changes. This is because energy can change forms, but cannot be created or destroyed in other words total energy in a system is conserved. At the top of the jump, Anna's energy is entirely in the form of gravitational potential energy, P. P depends on Anna's height, h, above the ground, the acceleration due to gravity, g, and her mass, m: P = mgh
As she begins to fall back down, her velocity increases as her height decreases. P decreases, but her energy of movement, kinetic energy, K, increases. K depends only on Anna's mass, m, and velocity, v: K = ½mv²
When Anna lands on the bar, her kinetic energy is transferred to bending the bar, and now takes the form of elastic energy, U. U depends on how deep the bar's bend is, d, and its "springiness", a constant k. U = ½kd² Even at the bottom of the bar's bend, Anna still has a tiny bit of potential energy.
solo Trapeze Regina you can trapeze
Swinging back and forth, the solo trapeze is a giant pendulum. The time it takes to swing forward, then back to where it started is called the period. This time has very little to do with the height of the swing. It depends mainly on the length of the pendulum, the longer the pendulum, the longer the period. The length of the pendulum, L1, L2, or L3 is always the distance from the pivot point near the ceiling to Regina's center of mass, m. Which of these positions, if held for the entire swing, would take the longest time to go back and forth?
A coffee mug is dropped from your hand and shatters on the floor. 1. Define the scenario (start and stop point) 2. Draw a diagram of the scenario snapshot. 3. Describe the scenario in words. 4. Use energy cubes or energy theatre to work through the scenario and map the energy transformations. 5. Finalize your drawing. 6. Show the energy conversions and conservation of energy.
O potential energy turns into kinetic energy O kinetic energy of the mug goes into: O ripping the mug apart O sending the pieces flying O sound O heating the floor and pieces through friction as the pieces slide to a stop O In the end, the room is slightly warmer
Nerf Gun O Pulling back the arming mechanism puts potential energy into the system (spring E) O Pulling the trigger releases the PE and transforms it to mechanical E (Kinetic) O Air exerts force on the nerf bullet pushing it out of the gun (some E loss to friction thermal) O Air resistance and gravity slow the bullet in flight (more E loss to atmosphere) O Once all PE is expended bullet falls to the ground. O In the end, all E loss is heat (irretrievable)
Kinetic Energy O Kinetic energy for a mass in motion is K = ½mv 2 Example: How much energy does a 0.1 kg ball have traveling at 5 m/s? K = 0.5(0.1)(5) 2 = 0.25 J of KE 47
Potential Energy O potential energy for a mass is Where g = 9.8 m/s 2 PE = mgh Problem: How much energy does a 3 kg rock have teetering on a cliff 35 m high? PE = 3(9.8)(35) = 1029 J of PE 48
If the rock from the previous question falls off the cliff, how fast is it traveling when it hits the ground? 35 m PE = KE mgh = ½ mv 2 v = 2gh = 2(35)(9.8) = 26.2 m/s
A diver of mass m drops from a board 10.0 m above the water surface. Find his speed when he hits the water. (Neglect air resistance.) mgh = 1/2mv 2 v = 2(9.8)(10) = 14 m/s
Find the diver s speed when he is 5.00 m above the water surface. At 5 m, KE = ½ PE 1/2mgh = 1/2mv 2 v = (9.8)(10) = 9.9 m/s
What is the kinetic energy of a 0.38 kg soccer ball that is traveling at a speed of 120 m/s? KE = 1/2mv 2 = 1/2(0.38)(120) = 22.8 J
KE = ½ mv 2 m = 2 x KE/v 2 = 2(105)/10 2 = 2.1 kg What is the mass of a baseball that has a kinetic energy of 105 J and is traveling at 10 m/s?