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# Pressure and Forces

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Ms. Parker's notes on Nov 17 - 18, 2011

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### Pressure and Forces

1. 1. Welcome Back!Today, we will:• Test your knowledge of accident statistics• Perform a lab• Take some notes• Work some problemsBefore the Tardy Bell:• Get a clicker and your lab notebook• Pick up the 15 question accident “quiz” and begin working on the questions
2. 2. Warm-UpWe will be taking the accident statistic “quiz” withthe clickers for your official score. The paper copyis for your use.Turn on your clickers in order to register them. Besure to register them with your last name and firstinitial like this: Parker_J
3. 3. Seat BeltsIn your lab book, start a new page. Title it“Newton’s First Law of Motion and Seat Belts” Besure to make an entry in your table of contents aswell.
4. 4. Seat BeltsSuppose you had to design a seat belt for a racecar that can go 300 km/h (about 200 mi/h). Howwould it be different from one available onpassenger automobiles?Make a heading in your lab book that will titleyour thoughts and take five minutes to jot downideas for a safe race car seat belt.
5. 5. Seat BeltsShare your ideas with your table partner. If youthink of other ideas, be sure to add them to yourlab book.Now turn to your group of four and share yourideas – make any additions or changes to yourideas.
6. 6. What Do You See?
7. 7. Seat BeltsTurn in your textbook to page 266.Read the Investigate carefully so that youunderstand how you will conduct this lab.Remember: each place in the text book that has apencil symbol is a place where you must writein your lab notebook!
8. 8. Review! Seat BeltsNewton’s First LawAn object at rest stays at restan object in motion stays in motion in a straightline with constant speedunless acted upon by a net, external force.
9. 9. Law of InertiaAn object at rest stays at rest…Think about the lab you just participated in andwrite an example of an object staying at rest inyour notes.“The part of the law of inertia that says an objectat rest stays at rest is shown by…”
10. 10. Law of Inertia“…an object in motion continues at the same speed ina straight line…”Think about the lab you just participated in and writean example of an object staying at rest in your notes.“The part of the law of inertia that says that an objectin motion continues at the same speed in a straightline…” BE CAREFUL! Gravity is an external forceacting on an object rolling down an incline!
11. 11. Law of Inertia“…unless acted upon by a net, external force.”Think about the lab you just participated in andwrite an example of a net external force.“The part of the law of inertia that says anet, external force will change the motion of anobject at rest OR an object in motion is shown
12. 12. Take Notes! 3 collisions in every accidentIn every car accident, there are three collisions.1. when the car strikes an object – such as another car, a tree, a light-pole2. as the car stops, the bodies in the vehicle continue to move until they collide with the structure of the car (seatbelt, steering column, air bags, windshield)3. the body stops, but the internal organs keep moving until the body wall exerts a force on them and brings them to rest
13. 13. 3 collisions in every accidentWhile safety features such as seat belts and airbags have dramatically reduced fatalities inautomobile accidents, it is still possible to sufferserious or fatal consequences in a car crashbecause of the third collision.
14. 14. 3 collisions in every accidentIn one recorded accident, a single vehicle with onlythe driver inside failed to follow a turn and struck atelephone pole. The seat belt and air bag preventedany serious injuries apart from a few bruises, but thedriver died.The autopsy showed that the driver’s aorta (thelargest blood vessel in the body – it leads out of theheart to the body) tore in the collision of the heartwith the ribs and the man bled to dead internally.
15. 15. What makes a seat belt safe?In the second part of the lab, you investigated theeffect of different types of seat belts.Keep in mind that the stopping force of each typeof seatbelt was the same because you releasedthe cart from the same height each time.Yet, some seat belts did more damage to the claydrivers than others. Why?
16. 16. Take Notes! PressureIn the thin wire seat belt, all of the stopping forcewas concentrated onto a small area. With abroader strip of material, that same stopping forcewas spread out over a larger area of contact.Force that is spread out over a given area is calledpressure.
17. 17. Pressure
18. 18. Take Notes! PressureIn the picture, the pressure exerted by the feet of theboy on the right is greater than the pressure exertedby the feet by the boy on the left.In our lab, the pressure of the wire as a seat belt wasgreater than the pressure exerted by the ribbon.It is the pressure, not force, that determines howmuch damage a seat belt does to the body.
19. 19. Take Notes! Pressure FormulaPressure = Force/areawhere… force is in Newtons area is in square meters (m2) and pressure is in N/m2 which is also called a pascal.
20. 20. Take Notes! Sample ProblemSample ProblemTwo students have the same mass and apply aconstant force on the ground of 450 N whilestanding in the snow.Student X is wearing snowshoes that have a basearea of 2.0 m2. Student Y, without snow shoes, hasa base area of 0.1 m2.Why does Student Y sink into the snow?
21. 21. Take Notes! Sample ProblemsWhat is the pressure exerted when a force of 10 Nis applied to an object with the following areas?a) 1.0 m2b) 0.2 m2c) 15 m2
22. 22. A videoThere is a famous demonstration of pressure usinga bed of nails.Here is one version