Work And Energy

Work And Energy

4.30 Review

Review Note

• Front Page - Divide page as shown

• Back Page - use for examples

SECTION I: DEFINITIONS

Types of Energy

• Type = Measurable clue used to track amounts of energy.

• Potential Energy - a vague, redundant term you will find in different places, but we will not use (so don't put for an answer)

KINETIC ENERGY

• Kinetic Energy - energy that is tracked by observing changes in velocity

• Moving Objects HAVE kinetic energy (they are not using or requiring kinetic energy)

• Objects that are speeding up are increasing their kinetic energy

• Objects that are slowing down are decreasing their kinetic energy

GRAVITATIONAL ENERGY

• Gravitational Energy - energy that is tracked by observing changes in height/elevation

• Elevated Objects HAVE gravitational energy (they are not using or requiring gravitational energy)

• Objects that are getting higher are increasing gravitational energy

• Objects that are getting lower are decreasing gravitational energy

ELASTIC ENERGY

• ELASTIC Energy - energy that is tracked by observing changes in the amount of stretch

• stretched Objects HAVE elastic energy (they are not using or requiring elastic energy)

• Objects that are getting more stretched are increasing elastic energy

• Objects that are less stretched are decreasing elastic energy

Other Energies

• Thermal Energy - energy that is tracked by observing temperature.

• Chemical Energy - energy that is tracked by observing if chemical reactions occur/ chemical bonds are rearranged

• Nuclear Energy energy that is tracked by observing if nuclear reactions occur

• Sound Energy energy that is tracked by observing if sound is created or gets louder and softer

• Light Energy energy that is tracked by observing if light is created or gets brighter or dimmer

LAW OF CONSERVATION OF ENERGY

• Energy can NEVER be created or destroyed. It can be transformed from one type to another or transferred from ne object to another - but the total amount of energy never changes.

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SECTION II: types of energy tracking

Tracking Energy

• Start by tracking the changes in the clues. Then determine energy transformation.

• Example

• Beginning: Girl holds bow and arrow (bow not stretched)

• Middle: bow string is stretched as she aims

• End: Arrow is flying through the air after she lets go

Multiple Choice

How did the energy clues change from beginning to middle?

• stretch of string increased, speed decreased

• speed increased, height decreased

• stretch of string increased, reactions in girls muscles

• temperature increased, speed decreased

Multiple Choice

What was the energy transformation from beginning to middle

• kinetic energy turned into gravitational energy

• chemical energy turned into elastic energy

• elastic energy turned into kinetic energy

• elastic energy turned into gravitational energy

Multiple Choice

What was the energy transformation from middle to the end

• kinetic energy turned into gravitational energy

• chemical energy turned into elastic energy

• elastic energy turned into kinetic energy

• elastic energy turned into gravitational energy

Example 2

Point A: person is high in the air at highest point (stopped for an instant)

Point B: person has landed on trampoline and stopped

Point C: person is heading upwards and feet just leave trampoline.

Multiple Choice

Change in clues from Point A (highest point) to Point B (lowest point)

• Speed increased, Height decreased

• Speed decreased, stretch increased

• Height decreased, stretch increased

• nuclear reaction happened temperature changed

Multiple Choice

Energy Transformation from Point A (highest point) to Point B (lowest point)

• gravitational energy transformed into kinetic energy

• kinetic energy transformed into elastic energy

• gravitational energy transformed into elastic energy

• elastic energy transformed into kinetic energy

Multiple Choice

Energy Transformation from Point B (lowest point) to Point C ((feet just leaving trampoline on the way up)

• gravitational energy transformed into kinetic energy

• kinetic energy transformed into elastic energy

• gravitational energy transformed into elastic energy

• elastic energy transformed into kinetic energy

Poll

How are you feeling about definitions and energy transformations

• I got this.

• Still shaky, but better than before

• Still shaky, no change

• even more confused now than before

SECTION III: ENERGY PROBLEM SOLVING

3 equations

Can be used to solve for Work, Kinetic Energy, Gravitational Energy, Speed, Height, Mass.

Sample Problem #1

• A 10,000 kg house is 1000 meters high in the air and descending at a rate of 2 m/s. What is the total of its kinetic and gravitational energies.

• E_g = mgh = (10,000)(9.8)(1,000) = 98,000,000 J

• E_k = 1/2 mv² = .5(10,000)(2²) = 20,000 J

• Total = 98,020,000 J

Sample Problem #2

• Energy at Top of Hill = Energy at Bottom of Hill

• E_{g at top} = E_{k at bottom}

• mgh _{(at top)} = 1/2 mv² _{(at bottom)}

• (90)(9.8)(25) = 0.5(90)v²

• 22050 = 45v²

• 490 = v²

• 22.1 m/s = v