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# Pressure and Forces

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Ms. Parker's notes for January 3 - 6, 2011. Covers topics of pressure, force, and surface area.

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### Pressure and Forces

1. 1. Welcome Back!<br />Today, we will:<br /><ul><li>Test your knowledge of accident statistics
2. 2. Perform a lab
3. 3. Take some notes
4. 4. Work some problems</li></ul>Before the Tardy Bell:<br /><ul><li>Get a clicker and your lab notebook
5. 5. Pick up the 15 question accident “quiz” and begin working on the questions</li></li></ul><li>Warm-Up<br />We will be taking the accident statistic “quiz” with the clickers for your official score. The paper copy is for your use.<br />Turn on your clickers in order to register them. Be sure to register them with your last name and first initial like this:<br />Parker_J<br />
6. 6. Seat Belts<br />In your lab book, start a new page. Title it “Newton’s First Law of Motion and Seat Belts” Be sure to make an entry in your table of contents as well.<br />
7. 7. Seat Belts<br />Suppose you had to design a seat belt for a race car that can go 300 km/h (about 200 mi/h). How would it be different from one available on passenger automobiles?<br />Make a heading in your lab book that will title your thoughts and take five minutes to jot down ideas for a safe race car seat belt.<br />
8. 8. Seat Belts<br />Share your ideas with your table partner. If you think of other ideas, be sure to add them to your lab book.<br />Now turn to your group of four and share your ideas – make any additions or changes to your ideas.<br />
9. 9. What Do You See?<br />
10. 10. Seat Belts<br />Turn in your textbook to page 266.<br />Read the Investigate carefully so that you understand how you will conduct this lab.<br />Remember: each place in the text book that has a pencil symbol is a place where you must write in your lab notebook!<br />
11. 11. Seat Belts<br />Newton’s First Law<br />An object at rest stays at rest<br />an object in motion stays in motion in a straight line with constant speed<br />unless acted upon by a net, external force.<br />Review!<br />
12. 12. Law of Inertia<br />An object at rest stays at rest…<br />Think about the lab you just participated in and write an example of an object staying at rest in your notes.<br />“The part of the law of inertia that says an object at rest stays at rest is shown by…”<br />
13. 13. Law of Inertia<br />“…an object in motion continues at the same speed in a straight line…”<br />Think about the lab you just participated in and write an example of an object staying at rest in your notes.<br />“The part of the law of inertia that says that an object in motion continues at the same speed in a straight line…”<br />BE CAREFUL! Gravity is an external force acting on an object rolling down an incline! <br />
14. 14. Law of Inertia<br />“…unless acted upon by a net, external force.”<br />Think about the lab you just participated in and write an example of a net external force. <br />“The part of the law of inertia that says a net, external force will change the motion of an object at rest OR an object in motion is shown by…”<br />
15. 15. 3 collisions in every accident<br />In every car accident, there are three collisions.<br />when the car strikes an object – such as another car, a tree, a light-pole<br />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)<br />the body stops, but the internal organs keep moving until the body wall exerts a force on them and brings them to rest<br />Take<br />Notes!<br />
16. 16. 3 collisions in every accident<br />While safety features such as seat belts and air bags have dramatically reduced fatalities in automobile accidents, it is still possible to suffer serious or fatal consequences in a car crash because of the third collision.<br />
17. 17. 3 collisions in every accident<br />In one recorded accident, a single vehicle with only the driver inside failed to follow a turn and struck a telephone pole. The seat belt and air bag prevented any serious injuries apart from a few bruises, but the driver died.<br />The autopsy showed that the driver’s aorta (the largest blood vessel in the body – it leads out of the heart to the body) tore in the collision of the heart with the ribs and the man bled to dead internally.<br />
18. 18. What makes a seat belt safe?<br />In the second part of the lab, you investigated the effect of different types of seat belts.<br />Keep in mind that the stopping force of each type of seatbelt was the same because you released the cart from the same height each time.<br />Yet, some seat belts did more damage to the clay drivers than others. Why?<br />
19. 19. Pressure<br />In the thin wire seat belt, all of the stopping force was concentrated onto a small area. With a broader strip of material, that same stopping force was spread out over a larger area of contact.<br />Force that is spread out over a given area is called pressure.<br />Take<br />Notes!<br />
20. 20. Pressure<br />
21. 21. Pressure<br />In the picture, the pressure exerted by the feet of the boy on the right is greater than the pressure exerted by the feet by the boy on the left. <br />In our lab, the pressure of the wire as a seat belt was greater than the pressure exerted by the ribbon.<br />It is the pressure, not force, that determines how much damage a seat belt does to the body.<br />Take<br />Notes!<br />
22. 22. Pressure Formula<br />Pressure = Force/area<br />where… <br /> force is in Newtons<br /> area is in square meters (m2)<br /> and pressure is in N/m2 which is also called a pascal.<br />Take<br />Notes!<br />
23. 23. Sample Problem<br />Sample Problem<br />Two students have the same mass and apply a constant force on the ground of 450 N while standing in the snow.<br />Student X is wearing snowshoes that have a base area of 2.0 m2. Student Y, without snow shoes, has a base area of 0.1 m2.<br />Why does Student Y sink into the snow?<br />Take<br />Notes!<br />
24. 24. Sample Problems<br />What is the pressure exerted when a force of 10 N is applied to an object with the following areas?<br />1.0 m2<br />0.2 m2<br />15 m2<br />Take<br />Notes!<br />
25. 25. A video<br />There is a famous demonstration of pressure using a bed of nails.<br />Here is one version<br />