Chapter 14Work, 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.

14.1: Work and Power

• What is work?

• Recall...From Chapter 12

• Question: How does an unmoving object begin moving?

Answer…

• 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.

Is work being done?

Work Requires Motion

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

Answer: yes, force is up and barbell moves up

Stationary Objects

• Question: Is the weight lifter doing work while he holds the barbell stationary over his head? 

ANSWER

• 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?

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

Practice Problem Answered

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

Q: Does a person shoveling snow do work?

• Answer: YES, because the shovel is moving in the same direction as the force being applied

Q: Does a snow blower do work?

• Answer: YES, but because the snow blower does the work in less time it has more POWER!!!

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?

Practice Problem Answered

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

James Watt and Horsepower

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!

Increasing a force

• 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

Increasing Distance

• 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

Changing Direction

• Ex: pulley– You pull down on the rope and the load moves

up

14.3 Mechanical Advantage

• Mechanical Advantage = the number of times that the machine increase an input force

• MA = 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 = 100/20 = 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

• A: IMA = 1.8m/0.3m = 6

14.4 Simple Machines

• The six types of simple machines are:– Lever– Wheel and axle– Inclined plane– Wedge– Screw– Pulley

Lever

3 classes of levers

Wheel and axle

Inclined Plane

Wedge

Screw

Pulley

Chapter 15 Energy

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

• 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.

• Show your work!

Practice Problem Solved

• KE = ½ 70kg x (2m/s)2

• KE = 35kg x 4m/s = 140J

Potential energy

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– h= height – Remember m x g = w (N)

GPECalculate the GPE in the picture below

Show your work here:

• 75kg x 9.8 m/s/s x 4m = 2940 J

Practice Problem

• A diver at the top of a 10 m high platform has a mass of 50kg. Calculate GPE

Practice Problem Solved

• GPE = 50kg x 9.8m/s2 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

– Example: rubber band, basketball 

EPE

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

• 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 

Chemical Energy

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

• Electrical energy- energy associated with moving electric charges 

– Electric charges exert forces that do work

– Examples: • electricity • lightning 

Electromagnetic Energy

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 • radio waves

Nuclear Energy

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 

15.2 Conversion and Conservation of Energy

Conversion

• Energy can be converted from one form to another

• Energy conversion = the process of changing energy from one form into another

Example: a wind-up toy converts PE into KE when it unwinds

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

• Answer: Friction! – 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.

Pendulum Conversions

Bouncing ball

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.

Practice Problem Answered

• (KE + PE)beg = (KE + PE)end

• (½ x 1.5kg x (26m/s)2 + 0)end = (0 + PE)beg

• 507 J = PE

Tying it all in to Nuclear Chemistry

• Nuclear Chemistry Connection/Review: – Remember Einstein’s equation? E = mc2

– This equation says that energy and mass are equivalent and can be converted into each other.  

Nuclear Chemistry

• In other words, energy is released as matter is destroyed and matter can be created from energy.

• Remember the law of conservation of mass was modified to account for this, and says that mass and energy together are always conserved.