39
Work, Power, and Simple Machines
Work, Power, and Simple Machines
Intro: Great Pyramid of Khufu in Egypt
• The Great Pyramid of Khufu stands over 137 meters high ( about 450 ft).
• Its base covers an area large enough to hold 10 football fields!!!
• More than 2 million stone blocks, each weighing about 20,000 Newtons (about the weight of two small SUV’s), make up the structure!!!!
Intro: Great Pyramid of Khufu in Egypt
• It is one of the Seven Wonders of the World!!!• Egyptians were credited with great effort and ingenuity for
this structure!!!• Egyptians chiseled stone blocks from limestone quarries,
had to transport them to the pyramid site, and raise them to the TOP!!!
• Egyptians only had SIMPLE MACHINES!!!• Several thousand people worked for twenty years to build
the Great Pyramid!!• With modern machinery, today it could be done in 1/5th of
the time with only a few hundred workers • Lets make some machines!!!!!!....but first…lets get back to
basics!
I. What does work mean????
A. Work….what is it good for??1. People use the work for many things…for example
when Levonas is sitting in his Life Guard chair…he tells people he is at work.
a. Science disagrees with this statement!!
2. Work is done only when a force moves an object. a. When you push, pull, lift, or throw an object you are doing
work.
3. Science definition: a force acting through a distance is work. a. Work is done whenever something is moved from one
place to another. b. Your catapult did work!!!
I. What does work mean????
A. Work….what is good for
4. Another important requirement for work is that the distance the object moves must be in the same direction as the force applied to the object.
a. If you are holding a 45 lb plate in your arms, and you walk forward have your arms done any work on the weight?
b. The answer is NOOOOOO!!! The direction of the movement of the bag is not the same as the direction of the applied force!!!!
This would not be considered work!!
I. What does work mean????
A. Work….what is good for
5. Since the applied force is upward, whereas the direction of movement is forward, the direction of movement is NOT the same as the direction of force by your arms.
I. What does work mean????
A. Work….what is good for6. The amount of work done in moving an object is
equal to the force applied to the object times the distance through which the force is exerted (the distance the object moves).
a. Work = Force X Distanceb. Force = mass X acceleration (measured in kg/s2) but more
commonly measured in NEWTONS.
c. Distance is measured in meters
7. So work is measured in a NEWTON-meter….but we call it a JOULE!!!!
I. What does work mean????
A. Work….what is good for???
8. If you lifted an object weighing 200 N through a distance of 0.5m how much work would you do?
a. The force needed to lift the object must be equal to the force pulling down on the object.
b. The force must be equal to what the objects weighs.
c. So the force is 200 N, the amount of work is equal to 200 N x 0.5 m, which is 100 J (Joules)
Example Problem: It takes work to catch a flight!!
• A 600 newton woman who was waiting for the flight lifted her 100 newton suitcase a distance of 0.5 meters above the airport floor and ran 25 meters.
• Calculate how much work was done by the woman’s arms in moving the suitcase. Draw a diagram showing the forces involved in this situation.
• Explain how the work would change if she dragged the suitcase along horizontally instead of lifting it. Draw a diagram showing this situation.
ANSWERS TO EXAMPLE
1. The 600 newton woman does work when she is lifts or drags the 100 newton suitcase, but NOT when she carries it!
2. She lifts the suitcase 0.5 meters, so the work is equal to the force needed to pick up the suitcase(100 N) X the distance (0.5m), or a total of 50 J (Joules)
3. When she DRAGS the suitcase 25 meters the work would be equal to 100 N X 25 meters, which is equal to 2500 J.
a. Assuming that all force is exerted in the horizontal direction!!!!
B. Power
1. Power is the rate at which work is done, or the amount of work per unit of time.
a. Power is calculated by dividing the work done by the time it takes to do it:
i. Power = 𝑊𝑜𝑟𝑘
𝑇𝑖𝑚𝑒
ii. Power =𝐹𝑜𝑟𝑐𝑒 𝑋 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑇𝑖𝑚𝑒
b. The unit of power is simply the unit of work divided by a unit of time, or the JOULE per second.
c. This unit is also called a WATT (W).
d. One watt is equal to 1 joule per second. (1 J/sec)
B. POWER
2. Watt and Electric appliancesa. A 50 watt light bulb does work at the rate of 50 joules per
second; in the same time a 110 watt light bulb does 110 joules of work.
b. Kilowatt= 1000 watts
3. Explain why a bulldozer has more power than a person with a shovel??
a. The bulldozer does more work in the same amount of time.
4. Why does it take more power to run up a flight of stairs than it takes to walk up?
a. The same amount of work is done in both cases, but it takes less time to run than to walk.
b. For the same amount of work, as TIME DECREASES, POWER INCREASES.
B. POWER
1. What is power?a. The rate at which work is done, or the amount of work
per unit of time.
2. What is relationship among power, work, and time?a. Power equals work divided by time.
3. What is a watt?a. One unit for power; 1 watt = 1 J/sec or 1 Newton-meter
/sec
4. A small motor does 4000 J of work in 20 sec. What is the power of the motor in watts?
a. 200 watts
B. POWER
5. Suppose you ride in a sleigh being pulled by horses at 16 kilometers per hour. Another sleigh being pulled at 10 kilometers per hour travels the same distance you do. Which horses are more powerful? How is speed related to power?
a. The horses pulling the faster sleigh are using more power; because speed equals distance/time, power can be expressed as force X speed. Therefore the greater the speed, the greater the power if the force remains constant!
C. Machines
1. An instrument that makes work easier is called a machine; a device that helps you to do something.
a. Machines are not always complicated like car engines, computers etc.
b. Some machines are so simple they do not even have moving parts.
C. Machines
4. How do Machines make work Easier???
a. WORK INPUT
i. Work that goes into the machine: WORK INPUT
ii. WORK INPUT comes from the FORCE that is applied to the machine, or the EFFORT FORCE.
b. When you use a machine you supply the effort force; because you exert this force over a distance you put WORK into a machine!!
C. Machines5. Work Output
a. Work done by a machine is called the work output.
b. The force the machine puts out is called the output force.
c. The work output is used to overcome the force that YOU AND THE MACHINE are working against.
d. The force that opposes the effort force is called the resistance force.
i. The resistance force is often the weight of the object being moved.
C. Machines5. Work Output
e. Example of resistance force:
i. When using a shovel to move a rock your EFFORT is opposed by the rock’s weight. The rock’s weight is the resistance force.
C. Machines6. Do Machines increase the amount of work??
a. Machines DO NOT increase the work you put into them.
b. The work that comes out of the machine can NEVER be greater than the work that goes into thmachine.
c. Like momentum, work is CONSERVED!!
D. Why do Machines Make Work Easier?1. Machines make work easier because they
change either the SIZE or the DIRECTION of the force put into a machine.
2. Three ways a machine can make work easier
a. It can multiply the size of the force but decrease the distance over which the force moves.
b. It can multiply the distance over which the force moves, but decrease the size of the force.
c. It can leave both force and distance unchanged, but change the direction which the force moves.
D. Why do Machines Make Work Easier?3. Most machines make work easier by
multiplying either force or distance BUT NEVER BOTH!!!
4. No machine can multiply both force and distance.
E. Determining How Helpful A Machine Is1. Remember…work output (work that comes out of a
machine) can never be greater than work input (amount of work put into a machine).
a. In reality the work output is always less than the work input….do you know why????
i. Friction!!
2. Some of the work the machine does is used to overcome the force of friction.
3. The comparison of work output to work input is called the EFFICIENCY of the machine.
a. The closer the work output is to the work input the more EFFICIENT the machine.
b. The more efficient a machine, the less friction is present.
F. Efficiency of a Machine1. Efficiency is expressed as a percentage
a. Efficiency can never be greater than 100% bcwork output cannot be greater than work input.
b. There are no machines that are 100% efficient.
c. Machines with the smallest amount of friction are the most efficient.
i. This is why we use oil/lubricants to keep a machine in good condition.
F. Efficiency of a Machine1. What do we mean by how helpful a machine
is??a. How many times the machine multiplies the effort
force to overcome the resistance force.
2. The number of times a machine multiplies the effort force is called the MECHANICAL ADVANTAGE of the machine.
a. The mechanical advantage tells you how much force is gained by using the machine.
b. The more times a machine multiplies the effort force, the easier it is to do the job.
c. MECHANICAL ADVANTAGE= RF/EFa. MA= Resistance Force/ Effort Force
TODAY’S ASSIGNMENT!!!Use the internet to answer the following
1. List the six simple machines.
2. List three examples of each simple machine found in every day use.
3. Explain how each simple machines work.
4. Explain the differences among three classes of levers.
5. Describe the difference between fixed and movable pulleys.
6. Explain how the six simple machines are the basis for all machines.
7. What is a compound machine??
G. Simple Machines1. Inclined Plane
a. A ramp is an example of an inclined plane
b. A ramp decreases the amount of force you need to exert, but it increases the distance over which you must exert your force.
c. ***What you gain in force you pay for in distance!!
d. Ramp would not alter the amount of work needed, just the way in which the work is done.
G. Simple Machines1. Inclined Plane
e. Inclined plane is a flat slanted surface with NO moving parts.
f. The less slanted the inclined plane, the longer the distance over which the effort force is exerted and the more the effort is multiplied.
g. The mechanical advantage of an inclined plane increases as the slant decreases.
G. Simple Machines2. Wedge
a. A wedge is an inclined plane that MOVES.
b. Instead of an object moving along the inclined plane, the inclined plane itself moves to raise the object.
c. As a wedge moves a greater distance it raises the object with greater force.
d. The longer and thinner a wedge is, the less the effort force is required to overcome the resistance force.
G. Simple Machines2. Wedge
e. When you sharpen a wedge, you are increasing the mechanical advantage by decreasing the effort force that must be applied in using it.
f. A knife and ax are two examples.
g. Go online and find how a lock depends on the principals of the wedge.
h. The zipper is another important application of the wedge.
G. Simple Machines2. Wedge
i. Zippers join or separate two rows of interlocking teeth.
j. The part of the zipper that you pull up or down contains three small wedges.
k. Have you ever tried to close a zipper with your hands??? It is almost impossible to create enough force.
l. Without these wedges you would not be able to close the zipper; it changes a weak effort force into a strong one.
G. Simple Machines3. Screw
a. A screw is an inclined plane wrapped around a central bar or cylinder to form a spiral.
b. A screw multiplies the effort force by acting through a long distance.
c. The closer together the threads, or ridges, of a screw, the longer the distance over which the effort force is exerted and the more the force is multiplied.
d. The mechanical advantage of a screw increases when the threads are closer together.
e. Screws increase the amount of force applied to them, but decrease the distance over which the force is applied.
G. Simple Machines4. Lever
a. A lever is a rigid bar that is free to pivot, or move about, a FIXED POINT.
b. The fixed point is called the FULCRUM.
c. If you were ever on a see saw, or used a screw driver to pry something open you have used a lever.
d. When a force is applied on a part of the bar by pushing or pulling it, the lever swings about a fulcrum and overcomes resistance force.
G. Simple Machines4. Lever
e. If you are using a crow bar to pry a nail out of a board.
f. When you push down on the end of the crowbar, the nail moves in the opposite direction.
g. The crowbar changes the direction of the force.
h. But the force exerted on the nail by the crowbar moves a shorter distance than the effort force you exert on the crowbar.
G. Simple Machines4. Lever
i. You push down through a longer distance than the nail moves up. Because WORK IS CONSERVED, this must mean that the crowbar multiplies the effort force you apply.
G. Simple Machines5. Classifying Levers
1. The relative positions of the effort force, resistance force, and fulcrum determine the three classes of levers.
2. Levers such as a crowbar, seesaw, and pliers are first class levers.
G. Simple Machines
5. Classifying Levers
1. The relative positions of the effort force, resistance force, and fulcrum determine the three classes of levers.
2. Levers such as a crowbar, seesaw, and pliers are first class levers.
• Immediately you will see that there is always a fulcrum, load and effort positioned somewhere on the lever, yet it may be difficult to notice how the position of each of these relative to one another can change the characteristics of the lever altogether. For this reason, levers are classified into three different types; called first-, second- and third-class levers (see Figure 2).
• The classification of each depends on the position of the fulcrum relative to the effort and load. In a first-class lever, the fulcrum is placed between the effort and load to resemble a seesaw. Examples of this type of lever include a balance scale, crowbar and a pair of scissors.
• A second-class lever is when the load is placed between the fulcrum and effort. This lever type has been used in the design of many devices such as a wheelbarrow, nutcracker, bottle opener and conventional door.
• Lastly, third-class levers operate with the effort applied between the fulcrum and load. These levers can be found in tweezers, fishing rods, hammers, boat oars, and rakes.
