Conceptual Physics

http://mrlafazia.com/PHYS111/lessons/201011Fall/PHYS111_12.ppt

THURSDAY September 30th, 2010

LESSON GOALS:• Discuss nature of Forces• Look at history of Galileo and his contributions to Force concepts• Run Interactive Physics simulation 6.3• Complete real-life experience discussion prompt• Review important definitions (with corresponding simulations)• Read Poem Newton Stole the Limelight• Take a quick quiz

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Discussion

• “What is a force”?– Is it a physical thing? – Are there many types? – Is there a single equation that describes forces?

• “How do forces relate to our Energy concepts”?

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Galileo Galilei on Inertiaa MrLaFazia.com original snapshot

SOURCE: http://www.cartoonstock.com/newscartoons/cartoonists/bro/lowres/bron1636l.jpg

NOTE: Galileo lived from 1564 to 1642… NOT 1489!

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Introduction & Overview

• Galileo’s life teaches us many things– (not the least of which is—DON’T STARE AT THE

SUN THROUGH A TELESCOPE!)

• For today, however, we will discuss his ideas on MOTION

1) the Leaning Tower of Pisa2) defining Force and the role of friction3) Inertia (ramp experiments)

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The Leaning Tower of Pisa• This tower portion is considered “legend,” of course…but...• Wherever he dropped the objects, Galileo showed that less

massive objects fall at the same rate as more massive ones– air resistance must either be equal or eliminated

SOURCE: http://improbable.com/airchives/paperair/volume15/v15i3/v15i3-web-images/KIM-Galileo_of_the_lem_opt.jpeg

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Defining “Force”• Forces are most simply defined as

– A PUSH– Or A PULL

• It was once believed that forces were needed to KEEP objects in motion.– People believed this because life taught them that objects slow

down and eventually stop (like your car not moving) unless a force keeps them going.

• Galileo showed that objects need no force to maintain their motion. – What people originally had not realized was the fact that

friction is a FORCE as well!• The fact is: Forces that aren’t balanced out will ALWAYS

cause a change in any object’s motion (whether speeding up or slowing down or changing direction)!

SOURCE: http://goozydumps.files.wordpress.com/2008/09/galileos_experiment.jpg

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Inertia

• Galileo did a number of experiments to test his theories of forces and motion:

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SOURCE: http://www.cartoonstock.com/cartoonview.asp?catref=mhen235 8/19

Inertia

• Imagine if a ball were allowed to roll down a ramp and roll right onto a level ramp.– Without a force to slow it down or speed it up,

what would happen to its motion?• THIS is the concept of “inertia.”

– What would be needed to speed up the ball?– What would be needed to slow down the ball?– ______________ slow down the ball in the real

world. This does NOT violate the concept of Inertia, but rather justifies it!

Frictional Forces

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The Law of Inertia

10/19

Forces…Motion…& Energy?• Do the results of the ramp experiments (see page

32, Figure 3.4 of Conceptual Physics, if applicable) match up with our understanding of the Conservation of Energy (“Total Energy is Boring”) Principle?

• Perhaps you have heard the formation of these concepts attributed to Sir Isaac Newton or another scientist, before. There is a someday-famous poem which describes this misconception quite well. I will read it to you at the end of the class.

• STAY TUNED! 11/19

A Final Bit of Humor

SOURCE: http://emdashes.com/assets_c/2010/04/Galileoroll2-thumb-182x224.png

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Interactive Physics

• Open and view Simulation 6.3….

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Discuss With A Partner

• “Describe an experience you have had in life where you have had the following things happen (or have caused these things to happen)” :

• Had to keep an object moving (how did you do this?)• Had to keep an object FROM moving (how did you do this?)• Had to stop an object while it was already IN motion (again,

how did you do this?)• Try to provide a name (of your own choosing) for each force

involved!

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Definitions (w/ Simulations)– Mass vs. Weight

• PLEASE learn the difference.– European scales?– American vs. European Scales on the Moon?

• Simulation: Interactive Physics 4.1; 4.2.– Inertia

• Note that this is NOT an actual “quantity.”• Simulation: Interactive Physics 6.4.

– Net-Force• Simulation: Interactive Physics 6.5; 6.6.

– the Equilibrium Rule• possible both while moving AND while not moving

– terminal speed (or “terminal velocity”)• Simulation: Interactive Physics 5.1; 5.2.

– support force• also known as the “normal” or “perpendicular” force

– I primarily call it the “Normal” force…just so you know.

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Poem:Newton Stole the Limelight, by D. LaFazia

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Quick Quiz“A parachutist jumps out of an airplane from the usual height (under 3

miles above the Earth’s surface). She is of average size and mass. Describe her descent in the real world (with air resistance) at the following segments of her fall”:

• What happens during the time from when she has just jumped out of the plane to just before she pulls the ripcord to deploy her ‘chute?

• In the first few moments during and after she pulls the ripcord to deploy her ‘chute, does she (choose the appropriate response)– Fall faster– Rise upward suddenly– Fall slower– Experience no change in her motion?

• Would her eventually-reached terminal velocity be more, less, or remain unchanged if she had never opened the ‘chute?

• BONUS) Describe how the parachute would respond and affect her descent if there were no air resistance.

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Recommended Reading:Sections 3.2-3.7

Sections 4.1-4.4

These sections of the text prepare you for the Force concepts from Unit 2. We will look more at the motion concepts in later

lessons within Unit 2.

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Looking Ahead:

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• Next class we will take a closer look at Force concepts by exploring Newton’s Laws of Motion.

• You will also be assigned HMWK #5 next class.