Energy

New Terms

Energy - the ability to do work. Measured in Joules (J).

Kinetic Energy - Energy associated with objects in motion.

Potential Energy - Stored energy due to position (gravity or elasticity), chemical composition, electrical properties, or nuclear properties.

Energy Transformation - Changing from one type of energy to another.

Law of Conservation of Energy - Energy cannot be created or destroyed, it can only change forms.

Kinetic Energy

Formula is KE=1/2*mass*velocity squared

ANYTHING in motion has KE associated with it: water in a river, a car traveling on a road, you walking on a sidewalk, a feather blowing in the breeze, etc.

Example problem: A 1500 kg vehicle is moving at a constant velocity of 10 m/s. What is the KE of this vehicle?

Problem Solution

• Remember, KE = 1/2mv2

• So, m = 1500 kg, v = 10 m/s (v2 = 100)• KE = 0.5*1500*100• KE = 75,000 J

Potential EnergyANYTHING that has the ability to release energy through a change in position, a chemical reaction, an electrical event, or nuclear reactions has PE.

Most commonly talked about form is position above ground. AKA gravitational potential energy (GPE).

Formula: GPE = mass*gravity*height

Example problem: What is the GPE of a 5000 g object that is 10 m above the ground on earth?

Problem Solution

• Remember, PE is equal to mass * gravity * height• So, m = 5000g, g = 9.8 m/s2, h = 10 m. DON’T

FORGET THAT MASS NEEDS TO BE IN KG! 5kg here.• PE = 5*9.8*10• PE = 490 J

Energy Transformations

According to the LAW of Conservation of Energy, energy cannot be created or destroyed, only changed in form.

What happens to the PE of an object as it falls from a height?

The PE continually decreases to zero as the KE increases from zero to conserve energy in the system.

Example: A ball is found to have 20 Joules of GPE. As it falls from a height to the ground, the GPE goes to zero and the KE increases to become 20 J (the very instant before impact).

Therefore, the total energy (TE) in a system is equal to the PE + the KE.TE = PE + KE

Energy Transformation, cont

Example problems:

a)What is the GPE of a 2 kg object that is 5 m from the ground?

b)What is the TE of the ball?

c)What is the GPE of the ball when it is 2.5 m from the ground?

d)What is the KE at this point?

e)What is the KE just before the ball impacts the ground?

Solutions to Problems

• A) GPE = 2*9.8*5, GPE = 98 J• B) At this point TE = GPE due to no motion (KE = 0)• C) GPE = 2*9.8*2.5, GPE = 49 J• D) KE = TE – GPE (from the total energy equation),

KE = 98-49, KE = 49 J• E) 98 J. Same as a and b because all energy in this

system has been converted to KE.

Assessment Page

Directions: I have provided these questions for you to test yourself. Answer the questions and then check your answers. If you want extra problems to try, search the internet. They are all over the place!

1. Which type of energy is exhibited by a soccer ball that has been kicked in the air towards the goal?

2. Which type of energy is exhibited by a soccer ball kicked on the ground towards a teammate?

3. Which type of energy is exhibited by a soccer ball at its highest point when kicked straight up in the air?

4. Calculate a) the TE of a 0.5 kg apple sitting on a shelf that is 3 m off of the ground. b) What type of energy is this? c) What is the GPE after the apple has fallen and is 1 m off of the ground? d) What is the KE at this point in time? e) What is the KE of the apple just prior to impact?

Answer Page

1. Both KE and PE

2. KE

3. PE

4. a) TE = PE+KE (no motion = 0 KE, so TE = PE here). PE = mgh0.5*9.8*3 = 14.7 J

b) PE (or GPE) c) PE = mgh, 0.5*9.8*1 = 4.9 J d) TE = PE + KE, 14.7=4.9+KE, KE = 9.8 J e) KE = 14.7 J Why?

Energy CANNOT be created or destroyed!