Chapter 14 Work, Power and Simple Machines

1 / 102

# Chapter 14 Work, Power and Simple Machines - PowerPoint PPT Presentation

Chapter 14 Work, Power and Simple Machines. Questions to think about before…. What does work mean to you???  List some examples of work:. Is this work???. Work & Science. Now...think about work in terms of science...it probably means something very different than what you listed above.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Chapter 14 Work, Power and Simple Machines' - arabela

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
• What does work mean to you???
• List some examples of work:
Work & Science
• Now...think about work in terms of science...it probably means something very different than what you listed above.
14.1: Work and Power
• What is work?
• Recall...From Chapter 12
• Question: How does an stationary object begin moving?
• Answer: When an unbalanced force acts on it.
• Work: the product of force and distance
• Work is done when a force acts on an object in the direction the object moves.
Work Requires Motion

Question: Does a weight lifter do work on the barbell to lift it over his head?

Stationary Objects
• Question: Is the weight lifter doing work while he holds the barbell stationary over his head?
• Answer: NO, the barbell is stationary
• For a force to do work on an object, some of the force must act in the same direction as the object moves.  If there is NO movement, NO work is done!!!
Work Depends on Direction
• The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement.
• A force does not have to act entirely in the direction of movement to do work.
Is work being done????
• The force acts upward and to the right.
• The suitcase only moves to the right.
• Any part of a force that does not act in the direction of motion does NO work on an object
Calculating Work
• Work = Force x Distance
• Units of Work
• SI unit for force is newtons
• SI unit for distance is meters
JOULE
• The SI unit for work is newton-meter or the JOULE (J)
• When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.
Practice Problem
• Imagine the weight lifter. The weight lifter lifts a 1600 newton barbell over his head.  Assume the barbell is lifted to a height of 2.0 meters.  What is the work done?
• Work = Force x Distance

Work = 1600 N x 2.0 m

Work = 3200 N m = 3200 J

What is Power?
• Power: the RATE of doing work
• Doing work at a faster rate requires more power.  To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time
Answer: YES, because the shovel is moving in the same direction as the force being applied
Calculating Power
• Power = Work / Time
• Work is in joules (J)
• Time is in seconds (s)
• The SI unit for POWER is the watt (W) = one joule per second
• Thus, a 40-watt light bulb requires 40 joules each second that it is lit.
Practice Problem
• You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds.  How much power is used to lift   the box?

Power = work / time

OR can be written as:

Power = (Force x Distance) / Time

(72 N x 1.0 m)/ 2.0 s = 36 J/s = 36 Watts

Horsepower
• Horsepower (hp): common unit for power.  One horsepower is equal to about 746 watts.
• FYI...Interesting side note: Horsepower is literally based on the power output of a very strong horse!!!
14.2 Work and Machines
• Machine = a device that changes a force
• Machines make work easier to do. They can:
• Change the size of the force needed
• The direction of a force
• The distance over which the force acts
• However…

They can’t do work for us!

Ex: a car jack
• Each rotation of the jack applies a small force over a large distance and the car is lifted a small distance
• Tradeoff = total distance traveled is much greater
Ex: oars of a boat
• You move oars a small distance and the end in the water moves a large distance
• Tradeoff = increased travel of the oar requires you to exert a greater force
Ex: pulley
• You pull down on the rope and the load moves up
Work Input
• Because of friction, the work done by a machine is always less than the work done on the machine!
• The force you exert on the machine is called input force. The distance the input force acts through is called the input distance.
• Work input = IF X ID
Work Output
• The force exerted by the machine is called the output force. The distance the output force is exerted through is the output distance.
• Work output = OF X OD
Mechanical Advantage = the number of times that the machine increase an input force
• AMA = load force/effort force
• Q: Using a lever, a person is able to lift a 100N object using only 20N of force. Calculate the MA of this machine
A: AMA = 100N/20N = 5
• In other words, this machine has multiplied the effort force 5 times.
Ideal Mechanical Advantage = MA without friction
• IMA = Input Distance/Output Distance
• Q: A woman drives her car onto a ramp. She drives 1.8 meters along the ramp to raise it 0.3m off the ground. Calculate IMA
Efficiency
• Because some of the work input to a machine is always used to overcome friction, the work output is always less.
• Efficiency = the percentage of the work input that becomes work output
• Always less than 100% due to friction
• Efficiency = W.output/W. input X 100%
14.4 Simple Machines
• The six types of simple machines are:
• Lever
• Wheel and axle
• Inclined plane
• Wedge
• Screw
• Pulley
15.1 Energy and Its Forms
• What is Energy?
• Energy- the ability to do work
• Energy is transferred by a force moving an object through a distance
Work & Energy
• Energy is closely related to work
• Work is a transfer of energy
• When work is done on an object, energy is transferred to that object
• Both are typically measured in joules (J)
Types of Energy
• Energy can be classified as two general types:
• kinetic energy
• potential energy.
Kinetic Energy
• Kinetic energy - (KE) the energy of motion
• The kinetic energy of any moving object depends on two things:
• Mass of the object
• Speed of the object
• To calculate the KE of an object, use the following formula:

KE = ½ mv2

KE = ½mv2
• Notice that doubling the mass doubles the KE
• But, if you double the speed you quadruple the KE!
Practice Problem
• A 70kg man is walking at a speed of 2m/s. Calculate his KE.
Practice Problem Solved
• KE = ½ 70kg x (2m/s)2
• KE = 35kg x 4m/s = 140J
Potential Energy
• Potential energy: energy that is stored as a result of position or shape
• Energy that is stored has the ability to do work!
• There are two types of potential energy:
• Gravitational potential energy and
• Elastic potential energy
GPE
• Gravitational potential energy depends on an object’s mass, height, and acceleration due to gravity.
• GPE = m x g x h or GPE = w x h
• m = mass (kg)
• g= acceleration due to gravity (m/s/s)
• h= height (m)
• Remember m x g = w (N)
Practice Problem
• A diver at the top of a 10 m high platform has a weight of 490N. Calculate GPE
Practice Problem Solved
• GPE = 490N x 10m = 4900J
Elastic Potential Energy
• Elastic potential energy – the PE of an object that is stretched or compressed.
• Something is said to be elastic if it springs back to its original shape after being stretched or compressed
Mechanical Energy
• Mechanical energy- the energy associated with the motion and position of everyday objects
• The sum of an object’s PE and KE
Further Classification of Energy
• Energy can be potential or kinetic, but it can be further classified into different types of energy:
• Thermal energy
• Electrical energy
• Nuclear energy
• Chemical Energy
• Electromagnetic Energy
Thermal Energy
• Thermal energy- the total potential and kinetic energy of all the microscopic particles in an object
• When atoms move faster thermal energy increases causing the object to become warmer
• Ex: sun, hot beverage, lava, feverish skin
Chemical Energy
• Chemical energy- energy stored in chemical bonds.
• When the bonds are broken and new bonds form, the released energy can do work
• Examples:
• fuel like gasoline
• Food
• Any chemical fuel stores energy
Electrical Energy
• Electrical energy- energy associated with moving electric charges  (electrons)
• Electric charges exert forces that do work
• Examples:
• electricity
• lightning
• Electric fence
Electromagnetic Energy
• Electromagnetic energy- energy that travels through space in the form of waves
• Can travel long distances through air and space
• Often used for communication
• Examples:
• visible light
• x-rays
Nuclear Energy
• Nuclear energy- energy stored in atomic nuclei
• Fission- release of energy by splitting nuclei
• Fusion- release of energy when less massive nuclei combine to form a more massive nuclei
• Example: heat and light from the sun (fusion), nuclear power (fission), nuclear bombs (both)
Conversion
• Energy can be converted from one form to another
• Energy conversion = the process of changing energy from one form into another
Energy Conservation
• As one form of energy converts into another form the total energy remains the same!!!
• The law of conservation of energy states that energy can NOT be created or destroyed.
Energy Conservation
• Question: Why do you slow down after you stop pedaling your bike?
• Where did the bike’s KE go?
Energy Conservation
• Since we do not live in a frictionless world, we have to take it into consideration…
• The work done by this frictional force changes KE into thermal energy.
• When the energy lost to frictional forces is accounted for all energy is conserved!
GPE to KE

The gravitational PE of an object is converted to the KE of motion as the object falls.

Energy Conversion Calculations
• When friction is small enough to be ignored, an object’s mechanical energy does not change.
• Remember: mechanical energy is the TOTAL KE and TOTAL PE of an object
• Mechanical Energy = KE + PE
Energy is Conserved
• The total mechanical energy at the beginning of the conversion must equal the total mechanical energy at the end!

(KE + PE)beginning = (KE + PE)end

Practice Problem
• At a construction site, a 1.5kg brick is dropped from rest and hits the ground at a speed of 26 m/s. Assuming air resistance can be ignored, calculate the GPE of the brick before it was dropped.