Mechanical advantage

That rock is much too big… Why I will lift that smaller rock over there.

Back to work…

• If you have to move 100 bricks. Does the work change based on HOW you do the work???

• Nope…• The work is the same work… regardless to

how you do the work…• THE WORK IN = THE WORK OUT• yarp

• Calculation of work??? (draw it!!!)

• Dare to compare… (same length = same work)• Work =

• Work =

• Work =

• yarp

We are not interested in the work.

• We are getting the mechanical advantage with the force we need to use.

• Force = push or pull…• The less push you need to use, the better!!!!• You better have the 3 levers set up for class!• yarp

How about the lever???• Teeter totter lever… identify the parts…• Class 1 lever (see saw, pry bars)

• Class 2 lever (wheel barrow, doors)

• Class 3 lever (fishing pole, broom, rake)

Levers all over the place… identify em…

• Can you think of any???• Yarp?

Pulleys…. A bit more difficult…

• Pulleys use the same rule…• Work in = work out

• So string the pulleys to increase the distance on the work out side!!!!

• Look at the demo set up!!!!

Efficiency

Last in a long run of a unit!!!

Well… in theory…

• Theoretically… work in = work out…• Sigh… but all the work/energy does not go

where you want it.• You have to keep in mind the other energy

transfers that are going on…• So… TOTAL WORK IN = TOTAL WORK OUT…• yarp

• Efficiency : the % of your work in used for the desired work out.

• Inefficient things have energy transfers you do not want…

• In a car… the work in/energy is gasoline• What is the desired work out of a car engine?• Physical energy to move the car.• What other work out energies exist you may

not want?• These lower the efficiency of a car• yarp

Calculation of efficiency• % Efficiency = work energy out x 100

work energy in• you pedal a bike with 1500 J of energy… the

moving of the bike (work out) is 1425 J. What is the efficiency?

• 95%• yarp

• You push a sled with a force of 50 N and a distance of 5 meters. What is the work done?

• 250 J• The above 250 J was the work energy out.

What is the efficiency if you had 1000 J of work in energy?

• 25% efficient• yarp

• You push the same sled, but now it has wheels. The 50 N of force moves it 15 meters. What is the work?

• 750 J• The 750 J of work out energy came from the

1000 J of work in energy. What is the efficiency?

• 75%• yarp

Factors that affect efficiency

• Resistance forces:– Air resistance– friction

• Energy conversions you do NOT want as your work out– Heat, sound

• That is why we modify things to be efficient.• yarp

Remember…

• Low efficient things are NOT destroying energy• It is changed into forms NOT useful for the

work out.• Not what we want..• yarp

Sometimes you can get vector advantage… for efficiency

• Compare the below… what are you working against???

Lift this box work out

Force for work in…

Lift this box work out

Force for work in…

Elevators do this

A counterweight to ease the pull

The counterweight

to help ease the needed

force.Here’s where the people go

What is wrong???

We be done…

• Yes… done we are• Are we done?• We are…• yarp