Physical Science

Ch 5

(Part I)

: Simple Machines

• With your partner (on the back of your study guide), identify the 6 different simple machines and give 1 example of each which can be seen in this classroom or somewhere in the school.

.......

• A simple machine is a device which does work (w = f x d) with only 1 movement.

• Simple machines make work easier by changing either the

size or direction

of the force.

• Does the teeter-totter shown change the size or direction of the force applied?

• There are 6 different simple machines.

1. Lever 2. Pulley 3. Wheel & Axle 4. Incline Plane 5. Wedge 6. Screw

• If 2 people are pushing identical boxes the exact same distance, then they are doing the same amount of work (F x D).

However, if one is pushing the box faster than the other, then that person is generating more

power

.

Power is the rate at which work is done.

• The formula for power is:

Power = Work / Time The SI unit for power is the watt (W)

• Let’s say Gary is lifting weights at the gym. It takes him 2 sec. to lift a 120 N weight over his head (0.5 m).

• How much work did Gary do?

• How much power did he generate?

• A compound machine is a combination of 2 or more simple machines working together to accomplish a task.

• The force put into a machine to do the work is called the effort force. That is what you apply.

• The force then produced by the machine is called the resistance force.

• So the hand is applying the effort force, and the resulting force which lifts the rock is the resistance force.

• Sometimes, a machine can actually produce more force than you put into it. That’s how you might be able to lift a person on the other side of the teeter totter, even though they may be heavier than you.

• The mechanical advantage is the number of times a machine multiplies the effort force.

M.A. = F

r

/ F

e •

“ Give me a lever long enough and I could move the world”

Archimedes

• For example: Bud weighs 100 N. So when he sits on the teeter totter he is applying a force of 100 N (F e ). He is able to lift Mike and Gary who have a combined weight of 150 N. Therefore, 150N would be the resistance force produced by the teeter totter (F r ). The mechanical advantage of the teeter totter would be, 150 N / 100 N = 1.5

• Randy is using a pulley with a mechanical advantage of 3 to lift a 5 gallon bucket of cement which weighs 240 N. How much force will Randy have to apply?

• Answer: 240 N / 3 =

80 N

• A lever is a rigid bar which pivots about a fixed point, called a fulcrum or axis.

• There are 3 parts to a lever: 1. Axis 2. Effort arm - where the F e is applied 3. Resistance arm - where the F r is applied f Effort Arm Resistance Arm g

• There are also 3 different classes of levers. 1st class, 2nd class, and 3rd class.

• The class is determined by how the 3 different parts are arranged in relation to each other.

• 1st class levers have the fulcrum in the middle.

• 2nd class levers have the resistance force in the middle.

• 3rd class levers have the effort force in the middle.

Work

in

& Work

out

• Work is F x D. The amount of work you put into a machine (work in ) is determined by the effort force which you apply, and the distance over which you apply it.

Work in = F E x D E

• For example, if the lever below is pushed downward 2 m with 50 N of force, then 100 J of work is put into the machine.

That’s work in .

• Likewise, work

out

is equal to the resistance force times the distance which it is moved (resistance distance).

Work

out

= F

R

x D

R

• So in the same lever below, Work out = 25 N x 4 m = 100 J

• If the amount of work put into a machine is equal to the amount of work which comes out of the machine, then it is said to be an ideal machine.

Work

in

= Work

out

• However, when a simple machine is used, often times friction will come into effect. • For example, if you are using a wheel and axle which is very rusty, then not only are you having to overcome the resistance force, but also the friction created by the rust.

• As a result, you may need to put more work into the machine to get a certain amount out.

• The ratio of work in to work out is called efficiency.

• The formula for efficiency is W out / W in

• Larry is using a pulley to put some hay up in the barn. The pulley has an efficiency of .60 (60%), and Larry has to do 150 J of work to get the hay up in the loft. How much work is actually being produced by the pulley?

• Answer: Efficiency = W out So, W out / W in = Efficiency x W in = .60 x 150 J =

90 J of work

• A pulley does 500 J of work to lift a 125 N weight. How high was the weight lifted?

Answers: • Work = Force x Distance so, Distance = Work / Force = 500 J / 125 N =

4 m

1. A rusty pulley has an efficiency of .25. If a person does 400 J of work to lift an object, how much work will be done by the machine?

2. If the person in question #1 does the work in 8 sec., how much power did the person produce?

3.

As a car travels down the highway, it’s engine does 186,000 J of work over a period of 1 min. How much power was produced by the engine?

4. A kid on a teeter-totter does 800 J of work as he lifts another kid 1 m upward. If the other kid had a weight (Fr) of 600 N, what is the efficiency of the teeter-totter?

5. A crane lifts a 35,000 N steel girder a distance of 25 m in 45 sec. How much work was done, and how much power did the crane require to lift the girder?

6. How many kilowatts of power were produced by the crane in #5?

Kim and Chris want to teeter totter together. However, Chris weighs 25 pounds more than Kim. If Chris sits 2 m away from the axis on her side of the teeter totter, will Kim need to sit the same distance away, closer to the axis, or further back from the axis? Why?

•

On a typical trebuchet, the resistance arm (projectile side) is longer than the effort arm (counter-weight side). However, if the effort arm were made longer and the resistance arm shorter, that would increase the amount of effort force put into the machine. Would this result in the projectile flying further? Explain.

• Identify 4 simple machines used in the Goldberg Device below, and tell why it would probably not work.

• Come up with 1 completely new example of each of the 3 different classes of levers.

http://www.sciencenetlinks.com/interactives/powerplay.html

Subject # Weight (lbs) Weight (N) Distance Traveled Time (1) Time (2) Average Time 1 2 3 1. How much work was done by each of the subjects? (Show work!) Subject 1 _________ Subject 2 _________ Subject 3 _________ 2. How much power was produced by each of the subjects? (Show work!) Subject 1 _________ Subject 2 _________ Subject 3 _________ 3. Did the person who made it up the stairs the quickest produce the most power?

Why or why not?

4. Do you see any relationship between the size of the person and the amount of work done? If so, what is that relationship?

5. Do you see any relationship between the size of the person and the amount of power produced? If so, what is that relationship?