Work & Power!!

transcript

Work & Power!!Chapter 14

ObjectivesDefine work and power.

Calculate the work done on an object and the rate at which work is done.

Use the concept of mechanical advantage to explain how machines make doing work easier.

Calculate the mechanical advantage of various machines.

Can You Define Work In Terms of Physics??

1. Yes2. No

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What is Work?

What Is Work?

1. Manual Labor2. Burger King3. Lifting

Weights

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Work…

A force causing a change in the motion of an object Work measures the effect of a force over a distance. Object must be moved (distance) for work to have occurred

Work = Force x distance W = F x d

When the force is opposing the direction of movement = - work

Work = visual concept

How is Work Measured?W=FdSI Units

Force = NewtonsDistance = metersWork = Newton x meters

Work = N x m1 N*m = 1 Joule

What the Heck is a Joule?Joules

Historical perspective:Named in honor of physicists James Prescott Joule (1818-1889). He discovered work and non-mechanical energy could be interchangeable, which was the foundation of the law of conservation!!

Important Stuff!!W = Fd

So… 1 J = 1 N * m

1 Joule = 1 kilogram * meter 2/ second2

1 N*m = 1 J = 1 kg*m2/s2

What is 1 Joule of Work?1 joule of work is done when you lift an apple from your waist to the top of your head!!

Example Problems!!!Imagine a father playing with his daughter by lifting her over his head. How much work does he do each lift, assuming he lifts her 2.0 m and exerts a force of 190 N?

Is the following an example of work? A waiter carries a tray full of meals above his head by one arm across the room.

Yes No

70%1. Yes2. No

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This is not an example of work. The force is the waiter pushing up on the tray (vertical force) and the displacement (horizontal motion) is across the room. The force is not causing the displacement so there is no work.

A tired squirrel (mass of 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm. Determine the number of push-ups which a tired squirrel must do in order to do a mere 5.0 Joules of work. Hint! ___ J = 1 Push up

27% 1. 1 E1 Push ups2. 10.0 Push ups3. 11.0 Push ups4. 1.1 E1 Push ups

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While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.8 m. How much work does he do in 30 strokes?

A mechanic uses a hydraulic lift to raise a 1200 kg car 0.5 m off the ground. How much work does the lift do on the car?

A teacher applies a force to a wall and becomes exhausted. Work was not

accomplished? True or False.

1. True2. False

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A book falls off a table and free falls to the ground. Is this an example of

Yes No

1. Yes2. No

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This is an example of work because of gravity. The force of gravity acts on the book which displaces the book towards the ground.

A rocket accelerates through space. Is this an example of work?

Yes No

1. Yes2. No

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Yes, this is an example because the force of the released fuel, pushes the rocket, which causes the rocket to be displaced through space.

What is Power?

PowerWhy is it more exhausting to run up stairs instead of walk? What’s the difference between running and walking up stairs?

PowerQuantity that measures the rate at which work is done.

Power = Work / timeP = W / tUnits for Power?

How did they come up with a Watt???

P = W / tWork = JoulesTime = secondsPower = Joule / second

ThereforeWatts = 1 Joule / 1 second1 W = 1 J/s

How does your family pay for their Power???

You pay for energy based on the amount of power that you use. The Power company charges you so much per kilowatt that you use.

Power = visual concept

Example Problems:While rowing in a race, John does 3960 J of work on the oars in 60.0 seconds. What is his power output in watts?

Using a jack, a mechanic does 5350 J of work to lift a car 0.5 m in 50.0 s.

What is the mechanic's power output?

Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Explain your answers.

Which student does the most work? Which student delivers the most power?

Ben and Will do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads.Yet, Ben is the “most powerful”, since he does the same work in the least amount of time. Time and Power are inversely proportional.

During the Personal Power lab, Jack and Jill ran up the hill. Jack is twice as massive as Jill; yet Jill ascended the same distance in half the time. Explain your answers.

Who did the most work? Who delivered the most power?

Jack does more work than Jill. Jack must apply twice the amount of force to lift his twice as massive body up the same flight of stairs.

Yet, Jill is “more powerful” than Jack. Jill does ½ the work yet does it in ½ the time. The reduction in work is compensated for by the reduction in time.

Your monthly electric bill is expressed in kilowatt-hours, a unit of energy delivered by the flow of l kilowatt of electricity for one hour. Use conversion factors to show how many joules of energy you get when you buy 1 kilowatt-hour of electricity.

What is a Machine???

More Machines???

MachinesWhich is easier, lifting a car yourself or using a jack?Which requires more work?

How do Machines Do Work???

They redistribute the work that we put into them!!

They change the direction of an input forceThey can increase an output force by changing the distance over which the force is applied.

Mechanical AdvantageQuantity that measures how much a machine multiplies force or increases a distance.Scientists or engineers use mechanical advantage to determine how to build machines. Mechanicaladvantage

Output Force

Input Force= =

Input DistanceOutput Distance

Mechanical Advantage!!Machines with an mechanical advantage greater than 1 multiply forces and are used to lift heavy objects. Machines with an mechanical advantage less than 1 do not multiply force, but increase distance and speed.

Your arms and a bat together form a machine that increases speed without multiplying force.

To Restate the Last SlideMechanical Advantage Bigger Than 1

Multiply ForcesUsed to Lift Heavy Objects

Mechanical Advantage Smaller Than 1

Don’t Multiply ForcesDo increase distance and speed

Force and Work

Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high?

Does the ramp multiply force or increase distance and speed?

Remember!!!Mechanical

advantage

Output Force

Input Force= =

Input Distance

Output Distance

Mechanical AdvantageTells how much a machine multiplies force or decreases distance

Mechanical Advantage Calculate the mechanical

advantage of a ramp that is 5.0 m long and 1.5 m high.

List the given and unknown values.Given: input distance = 5.0 m

output distance = 1.5 mUnknown: mechanical advantage = ?

2. Write the equation for mechanical advantage.

Because the information we are given involves only distance, we only need part of the full equation:

3. Insert the known values into the equation, and solve.

mechanical advantage

5.0 m1.5 m

mechanical advantage

input distanceoutput distance

Work & Power!!

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