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Work, Energy and Power

- Chapter 5 Section 4

Wagon Example

- Push a wagon on a sidewalk and it starts to roll

down the sidewalk. - The wagon eventually comes to a stop shortly

after the push. - Friction slows the wagon down.
- Mechanical Energy is not conserved in the wagon

since there is a change in kinetic energy.

Work-Kinetic Energy Theorem

- Work-Kinetic Energy Theorem The net work done

on an object is equal to the change in the

kinetic energy of the object.

Work-Kinetic Energy Theorem Equation

- Wnet ?KE
- Wnet Net Work
- ?KE Change in kinetic Energy
- Force is not required and applies to all objects

universally.

Friction

- When dealing with the work done by friction, the

Work-Kinetic Energy Theorem can be put into an

alternative form. - Wfriction ?ME

Frictionless

- When a problem deals with frictionless objects or

where friction is neglected. - Wfriction 0
- ?ME 0
- MEi MEf
- This is the Conservation of Mechanical Energy

Work-Kinetic Energy Theorem Work

- It doesnt matter if friction is present or its

frictionless, the Theorem demonstrates that work

is a method of transferring energy. - Perpendicular forces to the displacement cause no

work, cause the energy is not transferred.

Distinction Between W and Wnet

- Its important to make the distinction between the

two expressions - W Fd(cos?)
- This expression applies to the work done on an

object due to another object - Definition of work
- Wnet ?KE
- Shows only the NET FORCE on an object
- Relates to the net work done on an object to

change the kinetic energy of an object

Example Problem

- A 10.0 kg shopping cart is pushed from rest by a

250.0 N force against a 50.0 N friction force

over a 10.0 meter distance. - How much work is done by each force on the cart?
- How much kinetic energy has the cart gained?
- What is the carts final speed?

Example Problem Answers

- 2500 J
- 2000 J
- 20 m/s

Everyday Power

- What is power?
- A few everyday uses of power.
- Electricity
- Engines
- Etc
- Basically any time work is done, power is

generated.

Power

- Power The rate at which energy is transferred.
- In other words, power is the rate at which energy

is transferred.

What is Power?

- Power is the amount of work done over a certain

time interval. - P W/?t
- Power Work / time

Alternative Power Form

- Power can also be described through forces and

the speed of the object. - P Fv
- Power Force Speed

SI Units of Power

- The SI units for Power is the watt
- Variable for a watt is a capital letter W
- A watt is equal to one joule per second
- Horsepower is often used with power when dealing

with mechanical devices such as engines. - 1 horsepower 746 watts

Road Design

- Why are many mountain roads built so that they

zigzag up the mountain rather than straight up?

The Physics Behind Road Design

- The same energy is needed to reach the top of the

mountain regardless of the path. - Therefore the work is the same.
- The zigzag path has a longer distance and takes

more time to reach the top - Therefore less power is needed on the zigzag path

vs. straight up.

Machine Power

- Machines with different power ratings do the same

work, but do so over different time intervals. - The only main difference between different power

motors is that more powerful motors can do the

work in a shorter time interval.

Example Problem

- Two horses pull a cart. Each horse exerts a

250.0 N force at a 2 m/s speed for 10.0 minutes. - Calculate the power delivered by the horses.
- How much work is done by the two horses?

Example Problem Answers

- 1000 watts
- 600,000 Joules

Light Bulbs

- A common everyday thing that you take for granite

is artificial light. - A light bulb usually has marked on it the wattage

it uses. - Example 60 watt light bulb (most common)
- A 60 watt light bulb will use 60 joules of energy

over the course of 1 second. - Where does the energy come from?

From Sunlight to Artificial Light

- Sunlight ? Plants ? Fossil Fuel (coal) ? Steam ?

Turbine ? Electricity ? Light - Whenever energy is transferred, heat is produced.
- 2nd Law of Thermodynamics
- So it takes light to produce light and its very

inefficient.