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Work, Power, & Machines

Chapter 14 Integrated Chemistry and Physics

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What is work?

• The product of the force applied to an object and the distance through which that force is applied.

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What is work?• According to the physics

definition, you are NOT doing work if you are just holding the weight above your head.

• You are doing work only while you are lifting the weight above your head.

• No movement : No work

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For work to be done on an object, the object must ___________?____________.

• move in the direction of the force.

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Work Requires Motion• If the wall doesn't move, the prisoner

does no work.

• No movement : No work

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Work Depends on Direction

• 1) Work must have a force• 2) The force must be in the

direction of the motion

Force, F

distance, d

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Calculating Work

• To do work on an object you have to push the object a certain distance in the direction that you are pushing

• Work = force x distance = F x d• If I carry a box across the room I do

not do work on it (the box) because the force is not in the direction of the motion. Was any work done?

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Is work being done or not?

Mowing the lawn Weight-lifting Carrying groceries Moving furniture

up a flight of stairs Pushing against a

locked door Swinging a golf

club

• YES• YES• NO• YES

• NO

• YES

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Is work being done or not?

• Climbing stairs?• Lifting a book?• Pushing a shopping cart?• Carrying a football?

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Calculating Work

All or part of the force must act in the direction of the movement.

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Units of Work: The Joule

• 1 newton-meter is a quantity known as a joule (J).

• Named after British physicist James Prescott Joule.

•(1818-1889)

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What is the SI unit of work?

Duh!!!!!• The joule! • Or 1 NM!

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Using the Work Formula

• Work = Force x DistanceF = 500 pounds (2000 N)D = 8 feet (2.5 meters)

• W = 2000 N  x  2.5 m = 5000 N-m

= 5000 J

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Do you do more work when you finish a job quickly?

•NO• Work does NOT involve time, only

force and distance.

•NO

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Bell Work

• Do you do more work carrying your book bag upstairs or when you walk to the cafeteria from this room?

• What are the units for work?

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• How quickly work is done.• Amount of work done per unit time.• If two people mow two lawns of

equal size and one does the job in half the time, who did more work?

• Same work. Different power exerted.

• POWER = WORK / TIME

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James Watt

• A unit named after Scottish inventor James Watt.

• Invented the steam engine.

• P = Work/time – Joules/second– 1 watt = 1 J/s

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watts

• Used to measure power of light bulbs and small appliances

• An electric bill is measured in kW/hrs.

• 1 kilowatt = 1000 W

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Horsepower (hp) = about 746 watts

• Traditionally associated with engines. (car,motorcycle,lawn-mower)

• The term horsepower was developed to quantify power. A strong horse could move a 746 N object one meter in one second.

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What does power measure?

• The rate of doing work!!!!!!• How fast the work is done!• Work/time

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Calculating Power: Page 415

1.0 m

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You row a boat across a pond. You do 3600 J of work on the oars in 60 seconds. How much power did you use?

• 3600 J /60 sec = 60 J/sec = 60 W

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What is the SI unit of power?

• Watt

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Machines Do Work• A device that makes work easier.• A machine can change the size,

the direction, or the distance over which a force acts.

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Ramps are useful machines!

Increasing Distance

Reduces Force

• It makes it easier to move.

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Increasing ForceA ramp can reduce the force

WORK DONElittle force big distance

WORK DONEbig force little distance

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Two forces, thus two types of work

• Work InputWork done on a

machine=Input force x the

distance through which that force acts (input distance)

• Work OutputWork done by a

machine=Output force x

the distance through which the resistance moves (output distance)

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Figure 7 page 419

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Can you get more work out than you put in?

Work output can never be greater than work input.

•NO•NO

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End of Section 2

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How Does Input Force Location Affect a Machine?A nutcracker is a machine used to make cracking nuts easier. As shown below, use a nutcracker to crack three nuts, each time squeezing the nutcracker’s handles at a different location.

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Applying force at which handle location resulted in the nutcracker cracking the nuts the most easily? The nutcracker worked best when force was applied at location 1.How does the distance from the nutcracker’s pivot point to the point where the force is applied affect the nutcracker’s ability to crack nuts?The greater the distance between the pivot and the force, the better the nutcracker was at breaking nuts.

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Mechanical Advantage (MA)

•The number of times a machine multiplies the input force.

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Actual Mechanical Advantage

• ACTUAL• Involves friction.• Calculated the same for all

machines

• Actual Mechanical Advantage = Output force/Input force

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Ideal Mechanical Advantage

• IDEAL• Involves no friction.• Is calculated differently for different

machines• Usually input distance/output

distance– Actual mechanical advantage is always

less than ideal mechanical advantage.

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Calculating Mechanical Advantages:

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Calculating Mechanical Advantages:

• MA equal to one. (output force = input force)

• Change the direction of the applied force only.

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Calculating Mechanical Advantages:

• Mechanical advantage less than one

• An increase in the distance an object is moved (do)

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Efficiency

• Efficiency can never be greater than 100 %. Why?

• Some work is always needed to overcome friction.

• A percentage comparison of work output to work input.– work output (WO) / work input (WI)

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End of Section 3

Thank you!

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1. The Lever

• A bar that is free to pivot, or move about a fixed point when an input force is applied.

• Fulcrum = the pivot point of a lever.• There are three classes of levers

based on the positioning of the input force, output force, and fulcrum.

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First Class Levers• Fulcrum is located

between the effort and resistance.

• Makes work easier by multiplying the effort force AND changing direction.

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First Class Levers• Work Out = Work In• Small force applied over large

distance is the same as large force applied over a small distance.

F d = F d

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Second Class Levers

• Resistance is found between the fulcrum and input force.

• Makes work easier by multiplying the input force, but NOT changing direction.

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Third Class Levers• Input force is

located between the output force and the fulcrum.

• Does NOT multiply the input force, only multiplies the distance.

• Examples:

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Mechanical advantage of levers.• Ideal = input arm

length/output arm length

• input arm = distance from input force to the fulcrum

• output arm = distance from output force to the fulcrum

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Mechanical advantage of levers.  

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2. The Wheel and Axle• A lever that

rotates in a circle.• A combination of

two wheels of different sizes.

• Smaller wheel is termed the axle.

• IMA = radius of wheel/radius of axle.

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3. The Inclined Plane• A slanted surface

used to raise an object.

• Examples: ramps, stairs, ladders

• IMA = length of ramp/height of rampCan never be less than one.

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Bell Work

• Give an Example for each of the following simple machines- Lever- Wheel and axel- Inclined plane

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4. The Wedge

• An inclined plane that moves.

• Examples: knife, axe, razor blade

• Mechanical advantage is increased by sharpening it.

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5. The Screw• An inclined plane

wrapped around a cylinder.

• The closer the threads, the greater the mechanical advantage

• Examples: bolts, augers, drill bits

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6. The Pulley• A chain, belt , or

rope wrapped around a wheel.

• Can either change the direction or the amount of effort force

• Ex. Flag pole, blinds, stage curtain

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Pulley types

• FIXED• Can only change

the direction of a force.

• MA = 1

• MOVABLE• Can multiply an effort

force, but cannot change direction.

• MA > 1

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Page 432 Figure 19

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• A combination of two or more simple machines.

• Cannot get more work out of a compound machine than is put in.

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Assignment:

•Pages 441-442•1-11, 13, 14, 15, 17, 19, 22,

26, 27, 28, 29•WB Section 4

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14.2 Work• 5. A woman lifts her 100-newton child

up one meter and carries her for a distance of 50 meters to the child’s bedroom. How much work does the woman do?

100 N X 1 m = 100 N·m or 100 joulesNote: No work is done on the child when

she carries it.

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14.2 Power

5. A horse moves a sleigh 1.00 kilometer by applying a horizontal 2,000-newton force on its harness for 45 minutes. What is the power of the horse? (Hint: Change Km’s to m’s and convert time to seconds.)

45 min = 2700 s2000 n X 1000 m / 2700 s740.74 watts

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14.3 Mechanical Advantage

5. A machine with a mechanical advantage of 2.5 requires an input force of 120 newtons. What output force is produced by this machine?

2.5 = x / 120 n X = 2.5 x 120 n

X = 300 newtons