PPT
Pressure
Developer Notes
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Goals
Learn concept of pressure
Introduce bones (skeletal system)
Concepts & Skills Introduced
Area
Physics
Physiology
Concept
Pressure
Skeletal system
Time Required
Warm-up Questions
With the force of gravity pushing me down, and the force of the floor equally and oppositely
pushing me up, why am I not flattened like an accordion?
Presentation
We start by continuing to look at the human body in equilibrium.
One function of the skeletal system is to provide structure and support. Bones are solid and
incompressible. Since the strength of bones is related to cross-sectional area, this is a good place
to introduce the concept of pressure. (P=F/A)
Bones can withstand pressure.
Disc- Have the students figure out how much pressure an average femur can withstand.
An average femur can support 55 kg/cm2 before breaking.
P= F/A so…. (55 kg*9.8 m/s2)/cm2 = 539 N/cm2
539 N/cm2 * (100 cm)2/(1 m)2 = 5,390,000 N/m2 (or 5,390,000 Pascals- another unit for
pressure)
An average femur is approximately 2 cm in diameter (1 cm in radius). Students can the use their
own masses to figure out how much pressure is on their femurs when standing.
Students can then figure out how much more force (weight) their femurs can withstand. (Note:
this does not take into account pressures due to walking, running, jumping, etc. and only applies
to strength of one femur).
Demo- glue a thumbtack to one side of a 1 kg mass. Pick a student volunteer and keep the
thumbtack hidden. Place one of the regular sides on the student’s hand and ask how much force
he/she feels. Repeat using another side. Then turn to the thumbtack side. Your volunteer will
not want to feel that side! Pressure!
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Background/History
How much force do you exert on the ground when you stand on two feet? How much force do
you exert on the ground when you stand on one foot? The same! But why does it feel different?
You are exerting the same force but over different sized areas.
Pressure = Force/Area. (P=F/A).
Pressure is expressed in units of N/m2 (why?), or Pascals (Pa). 1 Pa = 1 N/m2
(Note: the Pascal is not a very “large” unit. Think about it! The force of 1 N spread out over a
square meter! Most “real world” pressures are expressed in kilopascals. There are also other
units for pressure).
Problem
Estimate the pressures exerted by a woman wearing high heels, wearing “sensible” shoes, and by
an elephant.
Procedure
Use the following data to answer the questions.
1 High heel Shoe:
Area of front = 62.8 cm2
Area of heel = 0.25 cm2
1 Sensible Shoe:
Area of sole = 168 cm2
Woman:
Mass = 55 kg
Elephant:
Area of one foot = 800 cm2
Mass = 1500 kg
1. Calculate the pressure (in Pascals) if the woman is standing normally, wearing the high
heel shoes.
2. Calculate the pressure (in Pascals) if the woman is standing normally, wearing the
sensible shoes.
3. How many times more pressure is applied due to the high heel than the sensible shoe?
4. Calculate the pressure (in Pascals) exerted by the elephant standing on all four feet.
How does this compare to the pressure of the woman in high heels?
Summary
1. Would the woman’s dancing partner prefer her to wear the high heel shoes or the sensible
shoes? Why?
Exercises
1. Using Newton’s 3rd law, explain why wearing the high heels may be less than desirable
for your feet.
2. When running, the force on your feet can be 4 times that of your body weight. How does
this affect the pressure?
3. What happens to the pressure on your feet if you go from standing on two feet to one?
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4. The atmosphere exerts a pressure of approximately 101 kPa. What is the force (in N) of
the atmosphere acting on the top of a desk that is 1.5 m long and 1 m wide? What is the
force acting upward on the desk?
5. Why does a sharp knife cut better that a dull knife?
6. The dimensions of a 1 kg rectangular mass are 1 cm wide by 2 cm long. How do the
pressures compare when the mass is resting on its square end vs. its long side?
Challenge
Thinking in physics terms, what are some other drawbacks to wearing high heels?
Just out of interest:
The human foot contains 26 bones, 33 joints, 107 ligaments, and 19 muscles.
Women have about 4 times as many foot problems as men. Wearing high heels contributes to
this pattern.
High heels contribute to knee and back problems, shortened calf muscles, Achilles tendonitis,
metatarsalgia (pain in the balls of the feet), and “Pump Bump,” a bone enlargement at the back
of the heel bone.
(Information from The American Podiatric Medical Association <www.apma.org>).
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