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Chapter 22

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Team Exercise (5 minutes)

- Write down the UAE for Energy.
- What are the units for energy in metric and English units?
- List 10 types of energy. Each team will be asked for a different type.

Energy (review)

- Energy has several forms:
- Kinetic (linear and rotational)
- Potential (gravitational, magnetic)
- Electrical
- Heat
- Nuclear
- Radiative

- What are the units for Energy?
- Joules or ergs
- Btu
- ft·lbf

Kinetic Energy

- Kinetic Energy = ½ mass ´ speed2

Power

- Power is equal to the amount of work done or energy used per unit time.
- P = W/t or P = E/t
- The unit for power is the Joule/second which is also called a Watt.

Potential Energy

- The energy that is stored is called potential energy.
- Examples:
- Rubber bands
- Springs
- Bows
- Batteries
- Gravity?

Questions:

- How much work is done when a weight lifter lifts a barbell weighing 1000 Newtons 1.5 meters above the ground?
- How much work is done when a weight lifter pushes on a stationary wall with a force of 1000 Newtons for 15 seconds?

Light Bulbs and Appliances

- How much energy does a 100 Watt light bulb use in one hour?
- How about a 40 Watt light bulb?
- Which bulbs shines brighter?

Mechanical Energy

- When work is done on an object, the object generally has acquired the ability to do work.
- This "ability to do work" is called energy and it has the same units as work….Joules.
- Two Types of Mechanical Energy
- Potential Energy and Kinetic Energy

Work/Energy Relationship

- If you want to move something, you have to do work.
- The work done is equal to the change in kinetic energy.
- Work = DKE

Example Question

- When the brakes of a car going 90 km/h are locked, how much farther will it skid than if the brakes lock at 30 km/h?
- Answer: 9 times

Conservation of Energy

- Energy cannot be created or destroyed...
- ...it may be transformed from one form into another...
- ...but the total amount of energy never changes.
- Demos
- Galileo's incline
- Bowling ball pendulum

Pendulum Example Problem

- At what point is the kinetic energy the highest?
- Answer: At the bottom of its path

- At what point is the potential energy the highest?
- Answer: At the ends of its path

Example Problem

- A 100 kg mass is dropped from rest from a height of 1 meter.
- How much potential energy does it have when it is released?
- How much kinetic energy does it have just before it hits the ground?
- What is its speed just before impact?
- How much work could it do if it were to strike a nail before hitting the ground?

Machines - An Application of Energy Conservation

- If there is no mechanical energy losses then for a simple machine...
- work input = work output
- (Fd)input = (Fd)output
- Examples - levers and tire jacks

Efficiency

- Useful energy becomes wasted energy with inefficiency.
- Heat is the graveyard of useful energy.

Comparison of Kinetic Energy and Momentum

- Kinetic energy is a scalar quantity.
- Momentum is a vector quantity.

Example Questions

A 10 lb weight is lifted 5 ft. A 20 lb weight is lifted 2.5 ft. Which lifting required the most work?

(a) 10 lb weight

(b) 20 lb weight

(c) same work for each lifting

(d) not enough information is given to work the problem

(c) same work for each lifting

An object of mass 6 kg is traveling at a velocity of 30 m/s. How much total work was required to obtain this velocity starting from a position of rest?

(a) 180 Joules

(b) 2700 Joules

(c) 36 Joules

(d) 5 Joules

(e) 180 N

(b) 2700 Joules

A 20 Newton weight is lifted 4 meters. The change in potential energy of the weight in Newton.meters is

(a) 20

(b) 24

(c) 16

(d) 80

(e) 5

(d) 80

End of Chapter 6 potential energy of the weight in Newton