Chapter 12 Work and Machines

8th Grade

Section 1- What is Work?

• What example of work can you think of?– Lifting your backpack– Taking your books out of your backpack– Pushing a child on a swing

• Work– When you exert a force on an

object that causes the object to

move some distance.– Work is done on an object the object MOVES in the

same direction in which the force is exerted.

• 2 Requirements for Work:– 1) motion– 2) distance

• There is no work without motion!

• Ex. You may exert a force on a car stuck in the snow but unless the force you exert makes the car move then you have done no work.

• What does direction have to do with work?

• Lifting books out of a backpack is work, but carrying them to class is not.

• Why didn’t you do any work when you carried your books to school??

• Answer: To do work on an object, the force you exert must be in the same DIRECTION as the objects MOTION.

How is Work Calculated??

• The amount of work done on an object can be determined by multiplying force times distance.

• Work = Force X Distance• Joules (J)

– The amount of work you

do when you exert a force of 1

Newton to move an object a

distance of 1 meter.

• Let’s work a few examples…..

Calculating Work Examples• How much work does it take to lift a 50

newton potted plant 0.5 meters off the ground onto a table?

• Work = Force X Distance

• Work = 50 Newton X 0.5 Meters

• Work = 25 J

Calculating Work Examples

• How much work does it take to lift a 100 newton plant onto a table that is 0.5 meters off the ground?

• Work = Force X Distance

• Work = 100 X 0.5

• Work = 50 J

Calculating Work Examples

• Is more work done when a force of 2 N moves an object 3m or when a force of 3N moves an object 2m?

• 2N X 3 = 6J

• 3N X 2 = 6J

• The work done in the two situations is equal!

Power• The rate at which work is done.• Power equals the amount of work done on

an object in a unit of time. • Factor that affects power?

– Time

• Ex. You need more power to run up the stairs when your backpack than to walk because it takes you less TIME.• Ex. Applying more power to an object means

doing more work in the same time.

Calculating Power• Power = Work / Time

• Units:– (J/s) Joules per second or Watts

(W)

• What if you aren’t given work?

• Power = (Force X Distance) / Time

Calculating Power Examples• A small motor does 5000J of work in 30

seconds. What is the power of the motor in watts?

• Power = Work / Time

• P= 5000 J / 30 s

• P = 166.66 Watts

Calculating Power Examples• A tow truck exerts a force of 11,000 N to pull

a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck?

• Power = (Force X Distance) / Time

• Power = (11,000 N X 5.0 m) / 25 s

• Power = 2,200 Watts or 2,200 J/s

• Work and Power Practice Problems- Handout

• Copy Questions and Work out Solutions on your own paper.

Section 2- How Machines Do Work• Machine

– A device that allows you to do work in a way that is easier or more effective.

– Changes the amount of force you exert or the distance over which you exert the force.

– Ex. Shovel- makes moving soil easier

• Machines make work EASIER by changing at least 1 of 3 factors:– 1) amount of force you exert– 2) distance over which you exert the force– 3) direction in which you exert the force

Input and Output Forces• When you use a machine, you exert a

force.– Ex. You exert a force on the shovel when you

use it to lift soil

• Input Force– Force applied to a machine.– Ex. Exerting a force on the shovel moves it a

certain distance.

• Output force– The force the machine exerts on an object. – Ex. Shovel exerts a force over another distance

Input and Output Work• Refer to page 413

• Input Work– Input force X input distance– The gardener exerts a large input force over a

small input distance

• Output Work– Output force X output distance– The shovel exerts a small output force over a

large output distance.

Mechanical Advantage• A machines mechanical advantage is the

number of times a machine increases a force exerted on it.

• Compares the input and output FORCE of a machine.

• Formula:– Mechanical Advantage = (Output) / Input

Practice Problems• You exert an input force of 10 newtons on a

hand-held can opener, and the opener exerts an output force of 30 newtons on a can. What is the mechanical advantage of the can opener?

• Output Force / Input Force

• 30 N / 10 N = 3

• *Increasing Force

Practice Problems• Suppose your input force is 20 newtons and

the machine’s output force is 10 newtons. What is the machine mechanical advantage?

• Output force / Input force

• 10 N / 20 N = 0.5

• * Increasing Distance

• If only the direction change, the input force will be the same as the output force. The mechanical advantage = 1.

Practice Problems

• If a machines has an input force of 40N and an output force of 80N, what is its mechanical advantage?

• Output force/ Input force

• 80N / 40 N

• = 2

• Refer to handout given during class for more examples of mechanical advantage problems.

Efficiency of Machines• ***IDEAL SITUATION ***

– Work you put into a machine is EXACTLY equal to the work done by the machine.

– In REAL situations the output work is ALWAYS less than the input work.

– WHY???

• In every machine, some work is wasted overcoming the force of FRICTION!– Without friction there would be equal input and output

work• An ideal machine would have an efficiency of 100%.

• All machines have an efficiency < 100%

Efficiency

• Compares output and input WORK of a machine expressed as a percentage.

• Formula:

• Efficiency = (Output work / Input work) X 100%

• Let’s Practice!

Efficiency Calculations• You do 250,000 J of work to cut a lawn with a

hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower?

• (Output work / Input work) X 100%

• (200,000 / 250,000) X 100%

• The efficiency of the lawn mower is 80%

Efficiency Calculations

• The input work you do on a can opener is 12 J. The output work the can opener does is 6 J. What is the efficiency of the can opener?

• (Output work/ Input work) X 100%

• (6 J) / 12J = .5 J X 100%

• = 50%

Section 3 – Simple Machines• 6 Types of Simple Machines:

– 1) Inclined Plane– 2) Wedge– 3) Screw– 4) Lever

– 5) Wheel and Axle– 6) Pulley

Inclined Plane

• Simple Machine

• Flat, sloped surface

• Allows you to exert your input force over a longer distance.

• Ex. A ramp in a parking garage

• Lengthening the ramp will INCREASE its ideal mechanical advantage.

Wedge

• Simple Machine

• Thick at one end and tapers to a thin edge at the other end.

• Thought of as an inclined plane that moves.

• Ex. As you bite into a peach, your front teeth act as a wedge.

• Incisors are shaped like wedges!

Screw

• Simple Machine

• Can be thought of as an inclined plane wrapped around a cylinder.

Levers• A rigid bar that is free to pivot, or

rotate, on a fixed point.– The fixed point that the lever rotates around is called

the fulcrum.

• Most of the machines in your body consist of bones and muscles are levers.

• Ex. Waving your hand at the wrist.• Joints act as the fulcrums of levers in your body.• Ex. When you raise your leg, your hip acts as a

fulcrum for the upper leg.• Ex. Using a paint opener to open a can of paint

3 Classes of Levers

• Pg. 427

• A second class lever always multiplies force.

• A hockey puck is considered a machine because it multiplies distance.

Wheel and Axle

• Simple Machine

• Made of two circular or cylindrical objects fastened together that rotate about a common axis.

• Ex. Screwdriver

Pulley

• A simple machine made of a grooved wheel with a rope or cable wrapped around it.

• Ex. Raise and lowering a flagpole

Compound Machines

• Utilizes two or more simple machines

• Ex. Bicycles