Chapter 5 Section 4


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 – The net work done on an object is equal to the change in the kinetic energy of the object.

Work-Kinetic Energy Theorem
Equation
W net
= ΔKE

W net
= Net Work

ΔKE = Change in kinetic Energy

Force is not required and applies to all objects universally.


When dealing with the work done by friction, the Work-Kinetic Energy Theorem can be put into an alternative form.
W friction
= ΔME



When a problem deals with frictionless objects or where friction is neglected.

W friction
= 0

ΔME = 0

ME i
= ME f
This is the Conservation of Mechanical
Energy

Work-Kinetic Energy Theorem &
Work

It doesn’t 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.

net

Its important to make the distinction between the two expressions:

W = Fd(cosθ)

This expression applies to the work done on an object due to another object

Definition of work

W net
= ΔKE

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


1.
2.
3.
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 cart’s final speed?

1.
2.
3.
2500 J
2000 J
20 m/s

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

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


Power is the amount of work done over a certain time interval.
P = W/t
P = Power (watt)
W = Work (J) t = Time (s)


Power can also be described through forces and the speed of the object.
P = Fv
Power = Force • Speed


The SI units for Power is the “watt”

A watt is equal to one joule or energy per second

Horsepower is often used with power when dealing with mechanical devices such as engines.

1 horsepower = 746 watts


Why are many mountain roads built so that they zigzag up the mountain rather than straight up?


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.


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.


1.
2.
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?

1.
2.
1000W or 1kW
600,000 Joules or 0.60MJ


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?


Sunlight  Plants  Fossil Fuel (coal) 
Steam  Turbine  Electricity  Light

Whenever energy is transferred, heat is produced.

2 nd Law of Thermodynamics

So it takes light to produce light and its very inefficient.