Introduction to Energy!

For: you From: Mrs. Geiselhart

To find this PowerPoint go to Computer EVHS Shared U:Drive Depts Science Geiselhart PowerPoint on Energy

Kinetic Energy (K.E.)

Kinetic Energy is the energy due do an object’s motion.

The equation for kinetic energy

K.E. = ½ mv2

K.E. = kinetic energy in kgm2/s2 or Joules (J)

m = mass in kilogramsv= speed of the object in m/s

Tips for Kinetic Energy

The more mass you have, the more K.E. you have.

The faster you move, the more K.E. you have.

If you are not moving, your K.E. = __0 J__ !

Problem for Kinetic EnergyProblem 1: Scooby Doo (mass 46 kg) is trying to

escape the monster that is in the amusement park. If he is traveling at 8 m/s, what is his kinetic energy ? (ans. 1,472 J)

Given:

Unknown:

Equation:

Potential Energy

Potential energy is the energy due to an object’s position.The equation for potential energy:

P.E. = mghP.E. = potential energy in Joules (J) m = mass in kilograms g= the acceleration due to gravity = -9.8 m/s2

(Yes, we call “a” g in this chapter-it’s the same thing.) h = the height in meters

Tips for Potential Energy

The more mass you have, the more P.E. you have.

The higher an object is off the ground, the more P.E. it has.

If an object is on the ground, its P.E. = ____ !

Problem for Potential EnergyProblem 2: A 9.5-pound cat is stuck in a tree 2 meters off the

ground. What is her potential energy? (ans. -84.7 J)

Given:

Unknown:

Equation:

What is the cat’s kinetic energy as she sits there? _________

The Conservation of EnergyEnergy is conserved. This means it changes

forms, but it is never lost. The total amount of energy always remains constant.

The conservation of energy equation

P.E.i + K.E.i = P.E.f+ K.E.f

i = initial f = final

Example for Conservation of EnergyStarting from rest, a child (mass 20 kg) zooms down a slide. If the slide is 3 meters high, what is her speed at the bottom of the slide?

You will solve this on the next slide. First make sure you understand the following:

The total energy in the initial position equals the total energy in the final position.

The initial position is where the problems starts.

The final position is where you are solving the problem.

Given: hi= 3 meters

hf= 0 meters (since the child will end up on the ground)

g = -9.8 m/s2

vi = 0 (since the child starts from rest)

Unknown: vf

Equation: total energy at top of slide = total energy at the bottom of slide P.E.i + K.E.i = P.E.f+ K.E.f

mghi + ½ mvi2 = mghf + ½ mvf

2

The mass cancels out in every term! Everything falls at the same rate, so the mass does not matter. ghi + ½ vi

2 = ghf + ½ vf2

-9.8 m/s2 (3 m) + 0 = -9.8 m/s2 (0 m) + ½ vf2

-9.8 m/s2 (3 m) = ½ vf2

vf = -7.67 m/s

The Kinda Ka at Six Flags in New Jersey is one of the tallest rollercoasters in the world!

Watch the following YouTube clip and solve the problem that follows. (it starts at 56 seconds in… lots of anticipation If it’s loading, do the problem first)

http://www.youtube.com/watch?v=HN8nv4tVFuA

Problem 3: The height of that hill on the Kingda Ka is 456 feet. Using the conservation of energy, calculate the speed at the bottom in miles per hour. (Yes, you can assume vi= 0 m/s)

(Ans. about 116 miles per hour!)