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!