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Newton’s 2 nd Law: Pressure and Free Fall Explained Physics October 27/28

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Objectives 1.distinguish between force and pressure 2.explain why the acceleration of an object in free fall does not depend upon the masses of the object 3.describe the effect air resistance on a falling object

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What is pressure? Topic Pressure Facts The amount of force per unit of area When the force is perpendicular to the surface area, then: Pressure = force_____ Area of application P = F/A Units: pascals (Pa) = N/m 2

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Pressure vs. Force Topic What makes pressure different from force? Facts Force is any push or pull Pressure is the force applied in a certain area Pressure and force are not the same!

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More about pressure Topic What does the pressure equation tell us? Facts P = F/A More area means less pressure Less area means more pressure Therefore, pressure and area are inversely proportional

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Think Critically Topic What happens to force and pressure when you stand on scale with two feet vs. one foot? Facts The force does not change because the weight stays the same whether one foot or two feet. The pressure increases on one foot because there is the same force (weight) applied to less area

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Think critically Topic If you are on thin ice, how should you get back to land without breaking the ice? Facts A.Tip-toe B.Crawl on all fours C.Lie down flat and push Answer: C

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Think Critically Topic Why should you lay down on thin ice? Facts When you lie flat, you increase your area which in turn decreases your pressure (P and A are inversely proportional). By decreasing the pressure it reduces the odds that the ice will crack.

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Practice using the pressure equation Topic Michelle has a mass of 75 kg and sits at her desk with a pressure of 9000 Pa. What area does she cover on the chair? Facts Known m = 75kg P = 9000 Pa Unknown A Equation P=F/A Solve 1.w = (75kg)(10m/s 2 ) = 750N 2. P=F/A 9000 Pa = 750N A (9000 Pa)(A) = 750N (A) (A) (9000Pa)(A) = 750N (9000Pa) (9000 Pa) A = 0.083 m 2

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Practice using the pressure equation Topic Brooke comes home from school and puts her books down on the kitchen table while she goes to grab a snack. The books have a combined weight of 25 N and the area of contact is 0.19 m by 0.24 m. What pressure do the books apply on the table? Facts Known F = 25N A = (0.19m)(0.24m) = 0.046m 2 Unknown P Equation P=F/A Solve P = (25N)/ (0.046m 2 ) = 540 Pa or 540 N/m 2

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Class work Practice solving for Pressure with handout

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Free Fall Revisited Topic Free fall Facts all free falling objects undergo the same acceleration at the same place on Earth (10ms/ 2 ) air resistance causes the exceptions

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Do you want to jump out of a plane? Topic Terminal speed Facts The speed at which the acceleration of a falling object is zero because friction balances the weight.

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Why terminal speed happens http://www.youtube.com/watch?v=_nOxqdSssJU&feature=related

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Closing Why does the floor squeak when someone tip-toes on a wooden floor?

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Homework and Announcements Study for the quiz- Newton’s 1 st and 2 nd Law! A DAY ONLY Next class you will –Get 30 minutes to complete a review packet to prepare for the quiz –Get 20 minutes to take the quiz –With the remainder of class you are to start reading and taking notes on chapter 6, Newton’s 3 rd law I will check that you completed the notes for chapter six Wednesday(A) and Thursday(B)

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