Work

• Work =

• Work happens when _____________

• If nothing is moved (no distance) then there is no work.

• If the force is perpendicular (not in the direction of movement), there is no work

Work

• Newtons (N) are a measure of force and meters (m) are a measure of distance.

• Work is measured in Newton-meters (Nm), {force x distance}

• Newton-meters are also known as joules (J).

Work

• When finding the amount of work done you multiply the amount of force used by the distance the force was used over.– Ex: If you lift a box using 250 N of force 2 m in

to the air,

• you have done 500 Nm of work.

• Force x distance = Work

• 250 N x 2 m = 500 Nm

Work Sample Problems – record in journal

1. Calculate the work done on your 80 N backpack to lift it from the ground to your back, a distance of 1.5 meters.

2. Calculate the work done on your sister to push her 10 meters across the floor at a constant speed. Force of friction = 150N.

3. Calculate the work done on your 20 N lunch tray as you carry it from your lunch table to the kitchen, a distance of 5 meters. Neglect air resistance.

Potential & Kinetic Energy

Potential & Kinetic Energy Sample Problems – record in journal

1. Identical twins Pat and Chris are painting a house. Pat is standing on the scaffolding 5 meters above the ground. Chris is standing on the scaffolding 5 meters above Pat. Who has more potential energy? Explain.

2. Jared and Clay are climbing the stairs. Jared gets tired and stops halfway to the fourth floor. Clay makes it to the fourth floor without a problem. If Jared is twice as heavy as Clay, who has more potential energy? Explain.

Potential & Kinetic Energy Sample Problems – record in journal

3. A person weighing 630 N climbs up a ladder to a height of 5 meters.

a. How much work does the person do?

b. How much potential energy does he have?

c. Where does the energy come from to cause this increase in PE?

Work

• You can complete the same amount of work with less force (easier) by increasing the distance over which the work is done.

• For example, if you needed to move six chairs from the downstairs porch to the upstairs deck for a party,

Work

• you could stack the chairs up and carry them all in one trip. You would not have to walk very far (distance) but the load would be very heavy and require a lot of effort (force)

• Six chairs / One trip (d x F)

•

Work

• or you could take one chair at a time. You would have to walk much farther (distance) but the load would be very light and not require much effort (force)

• One chair / Six trips (d x F)

Work & Levers

• Either way the same Work is done; (six chairs are moved from the dining room to the back yard).

• This idea relates to levers – they help us use less force to do the same amount of work.

• The work becomes easier for us with the use of a lever.

Levers

• Parts of the lever are: – effort, fulcrum, resistance

EffortEffort

Effort

Resistance

ResistanceResistance

Levers

• Catapults are levers!

• Draw a quick sketch of your stage 2 catapult.

• Label the effort, resistance, and fulcrum of your catapult design.

Levers

• Move the effort or the resistance in a lever to use less force

• Levers work because work = f x d.

• a small force acting over a long distance can be transformed into a large force acting over a short distance

Levers

Work & Levers• “Input force” and “output force” are the

same thing as work.

• Input force = Output force

• To calculate the input force (work) use the distance from the fulcrum to the effort force.

• To calculate the output force (work) use the distance from the fulcrum to the resistance force.

Levers

Archimedes and the Law of the Lever

"Give me a place to stand on, and I will move the earth."

quoted by Pappus of Alexandria in Synagoge, Book VIII, c. AD 340

Levers

the 140 lb boy 2 feet from the fulcrum (center of gravity) balances his 70 pound sister 4 feet from the fulcrum

2 x 140 = 4 x 70

Mechanical Advantage

• Man first started using machines to make work easier and faster.

• How much easier and faster a machine makes your work is the mechanical advantage of that machine.

• In science terms, the mechanical advantage is the number of times a machine multiplies your effort force.

Mechanical Advantage

• Mechanical Advantage of the Levers

• To find the MA of a lever, divide the effort arm length by the resistance arm length.

• MA = effort arm length / resistance arm length

Mechanical Advantage

• Interpret the diagram to answer the questions.

1. What is the length of the resistance arm? 2. What is the length of the effort arm?3. What is MA of the see saw above?4. What is the resistance force in the diagram?5. How much effort force would be needed to overcome the

resistance force?