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\ \D4,`Calculating Surface Area Under Pressure!
Activity Summary:
The primary focus of this lesson is to introduce the basic qualitative concepts and associated mathematical formulation for pressure. The motivation is to introduce the students to a physical application involving the measurement and/or computation of surface area, which is treated as a secondary objective herein. This lesson begins with a short presentation that qualitatively defines pressure through examples, in order to make it more tangible to the students, and then gives the mathematical formulation. There are then two quick computations practicing the use of the pressure formula, followed by two short activities that first require the measurement and computation of surface area in order to calculate either force or pressure given the other.
Subject:
Math or science: surface area, solving 1variable equations, and physical science
Grade Level:
Target Grade: 8
Upper Bound: 10
Lower Bound: 7
Time Required: This presentation and activity can easily consume an entire 60 minute class period and could be extended into a second class period if desired.
Activity Team/Group Size: The presentation and two quick computations are to be given on an individual basis, and then the class should be split into groups of 4 for the two activities.
Activity Cost Per Group [in dollars]:
1 box found around the house, $0.00
2 tape measures $5.00 ea. (total $10.00)
2 nails  $0.10
Authors:
Graduate Fellow Name: Ryan Pedrigi*
Teacher Mentor Name: Jessica Belcher
Date Submitted: 4/11/06
Date Last Edited: 4/11/06
*This powerpoint presentation was created by Ryan Pedrigi
Parent Lesson Plan(s): There are no parent lessons, but it is recommended that students be very familiar with both measuring and calculating surface area (particularly of a circle and parallelepiped).
Activity Introduction / Motivation:
This activity was motivated by the lead teacher wanting a creative way to practice measuring and calculating surface area. This activity accomplishes this, while simultaneously introducing the students to pressure, which is a commonly used physical principle in science, technology, and everyday life.
Activity Setup:
The students are given the attached worksheet and asked to fill in the answers to the questions as they are brought up in the presentation (encouraging them to follow along). The lesson then begins with the teacher presenting the powerpoint file, which has imbedded within two example problems that are to be performed individually by the students. Finally, there are two small activities to be performed by the students in groups of 4 (note that the activities are also imbedded as deeper questions in the powerpoint file).
Activity Plan:
The power point presentation begins with some qualitative examples of pressure. The first being of two different types of pushups that could be illustrated visually by the teacher calling a student to the front of the class and performing both a regular and fingertip pushup and then having the student describe which was harder (the fingertip should be because of the significantly lesser surface area over which the student exerts a force to undergo the motion, leading to a larger pressure). The next four slides look at random examples, like inflation of a balloon, a bike pump, the heart (another type of pump), etc., some of which could be explained with actual props brought into class.
The mathematical formulation of pressure is then given, which should be written on the board with very simple examples of its use (i.e. given a force of 5 pounds acting over a surface thats 5 in.2, what is the pressure? (a: 1 psi)). Next the students are given 2 problems that reinforce their mathematical understanding of pressure (imbedded within the powerpoint slides 8 and 9), which should be worked out individually but with the teacher leading the solution execution.
Finally, two small activity questions are given (stated in slides 10 and 11) to allow students to work a pressure computation that first requires the measurement of surface area (using the measuring tape provided). The first activity asks why is it harder to hammer the tip of a nail into wood versus the head, the teacher can demonstrate this phenomenon for the class by attempting to strike a nail into a block of wood (trying each end separately). In order to show that the answer has to do with the pressure exerted by each end of the nail, the students will be asked to compute the pressure created by a hammer exerting a force of 200 pounds for both the tip and head. To accomplish this, they will need to be given a nail and tape measure to measure the radius of both the tip and head of the nail to compute surface area. The students will find through their calculations that the tip of the nail exerts a much larger pressure than the head (for a given force of 200 pounds) because of its smaller surface area. The second question simply asks for the total force acting on a threedimensional object (provided to the students by the teacher  1 box, cooler, etc. for each group), pretending that the object is a submarine at a depth of 1000 feet (corresponding to a pressure of 445 psi). The students will again need to measure (with the measuring tape) the dimensions of the object and calculate surface area, and then calculate force with the same pressure equation used throughout this lesson and first introduced in slide 7.
Activity Closure:
The lesson is done when the final activity problem has been solved. The solutions to these activities (group problems) can be discussed as a class, if desired by the teacher.
Assessment:
The students can turn in their answers to the worksheet along with either or both the two individual computations and the two activity problems.
Learning Objectives:
Introduce pressure both qualitatively and quantitatively
Practice calculating surface area (from measurements) and give an example of its application
Practice solving 1variable, algebraic equations
Practice measurement with a tape measure
Prerequisites for this Activity:
This lesson gives the teacher an opportunity to branch into a slight aside, giving the students a tangible application to their everyday mathematics; in this case solving 1variable equations and calculating surface area. Prior to giving this lesson, students should be refreshed on how to calculate surface area, particularly of a circle and parallelepiped (note that this activity can easily be changed to allow for the practice of measuring and computing surface area of most any shape particularly for the last problem stated on slide 11). In addition, they should be fairly proficient at solving 1variable equations. Thus, this lesson provides them with a practical application with which to practice using these mathematical tools.
Background & Concepts for Teachers:
Pressure is defined as the ratio of an applied force to the area of the surface of a body over which that force is acting. [Strictly speaking (i.e. thermodynamically), the term pressure is only used when referring to a fluid, however, for the purposes of this audience it can generally be thought of as simply a force acting over an area (allowing the introduction of stress also force over area  to be neglected)]. The following are some ideas that underlie selected slides (denoted s#) in the presentation
S3: A balloon classically illustrates pressure. As air is forced into the balloon, the air molecules are pushed together, but since they dont like to be so close to each other they press against the inside of the balloon with a force. Since the balloon is rubber, it distends outwards (i.e. inflation); the more air that is forced in, the closer the molecules are, the more they dont like it and the harder they push against the inside surface of the balloon (i.e. increased pressure). The far right image was taken at the instant that a water balloon exploded.
S4: A bike pump similarly forces air into a closed container, creating pressure. In particular, note that the gauge pictured has the letters psi on it. Ask the students if they have ever seen this before and what those letters mean (answer: units of pressure psi: pounds per square inch).
S5: The heart is a pump that creates pressure by squeezing down (contracting remember the heart is a muscle) on its chambers once they fill with blood. The contraction reduces the inner surface area of the chamber and thus increases the pressure on the fluid, which causes it to push out into the blood vessels and nourish the body. The far right image is of a fully implantable, artificial heart (replaces the ventricles of a person with heart failure).
S6: Water pressure is more or less the weight of the water pushing in on a submerged object (it can be calculated through the equation: pressure = density (water)*acceleration of gravity*depth). The illustration on the right gives a great visual of how water pressure might act on an object: the diver inflates a balloon at the surface and then ties it off so no air can escape, as he dives the water pressure compresses the balloon and consequently the air in the balloon until at a depth of about 260 feet the balloon is no longer distended because the pressure is so great that all of the air molecules are completely compacted together. Also make a note that pressure acts on all surfaces of a submerged object (as shown by the submarine diagram).
S8S9: The two individual problems are worked out; the teacher can go through the answer if desired.
S10: The reason the tip of the nail is easily hammered into the wood in comparison to the head is because the (surface) area of the tip is so much smaller than that of the head (approximately *(1/32)2 (conservative) versus *(1/8)2 ); this difference translates into a required force needed to hammer in the head that is at least 16 times larger than that needed for the tip, in order to produce the same pressure.
S11: The students will need to measure (with measuring tape) the dimensions of the 3D object given to them and then calculate the surface area of that object. Knowing the pressure that is being exerted on that object, the total force acting over the objects surface (area) can simply be calculated as (f = p*a).
Materials List:
Computer and connected projector with Microsoft Powerpoint
Worksheet for each student (attached)
One 3D object (some type of parallelpiped, such as a box) per group
2 nails (for measuring/calculating the area of the tip versus head) per group
2 tape measures per group
Balloons (optional demonstration)
Hammer and wood (optional demonstration)
Activity Extensions:
Additional problems and/or activities involving the measurement of surface area of any object for the calculation of force based on imaginary pressures could be implemented by the teacher as needed.
Attachments:
Pressure.doc
GK12_A2_Pressure.ppt
Pressure worksheet.doc
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