# Newton’s 2 nd Law: Pressure vs. Force Physics October 24/25.

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Newton’s 2 nd Law: Pressure vs. Force Physics October 24/25

Objectives 1.distinguish between force and pressure 2.Solve for pressure

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

Pressure vs. Force Pressure a push or pull (including weight) in a certain area P = F/A Units: Pa or N/m 2 Force any push or pull (including weight) F = ma Units: N

What is pressure? Topic Pressure Facts Pressure = force_______ Area of application P = F A F = force (N) A = Area (m 2 ) P= Pressure (pascals- Pa) (N/m 2 )

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

Pressure vs. Force Topic Why do the boxes have the same force? Facts They have the same weight (weight is a force)

Pressure vs. Force Topic Why do the boxes have different pressures? Facts The standing book covers less area; therefore, has more pressure

Think Critically Topic What happens to force and pressure when you stand on scale with two feet vs. one foot? Facts force stays same b/c weight stays the same The pressure increases on one foot b/c same force (weight) applied to less area

Reach for the Stars- For Everyone 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

Reach for the Stars 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.

Another Demo Pressure vs. Force

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

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 ) = 543 Pa or 543 N/m 2

Class work Practice solving for Pressure with handout

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

Homework Practice a study strategy to review notes so far on Newton’s laws 1.Write math problems from examples previously completed in class and solve them 2.For each topic, write a real world scenario for how the physics is present 3.Attend office hours, to check the accuracy of your answers and get clarification when needed 4.Take your notes, fold them in half and answer the questions without looking at the notes 5.Rewrite questions from class work and quizzes and answer them 6.Write tricks that can be confusing- come up with a song or slogan to distinguish subtly different concepts

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