HPP Activity 41v1
Is blood pressure the same everywhere in your body?
Exploration
Let's explore what happens to pressure in a tube that changes cross-sectional area.
Equipment:
1 Venturi tube
Have your instructor demonstrate a Venturi tube. It consists of a horizontal tube that is wide at
each end and narrow in the middle. There are vertical tubes attached to the narrow and wider
sections of the horizontal tube attached to a differential pressure gauge.
GE 1.
Flow air through the Venturi tube and observe the pressure in the different
sections of the Venturi tube. What do your observations indicate about the
pressure in the each part of the horizontal tube?
2. Is this the pressure outward on the side of the tube, inward (from the outside
in), or in the direction of flow? Explain your reasoning.
3. Where is the pressure greater, where is it less? How do the pressures inside
the tube compare to the pressure outside the tube?
4. Is the velocity of the fluid the same in each part of the tube? Explain why
you think so.
GE 2.
1. Take a sheet of paper about 4 6 in. and blow across the top of it. Which
direction does the paper move?
2. Is the force per unit area (pressure) greater on top of the paper or below the
paper?
3. Is the velocity of air greater above the paper or below the paper? Explain
why you think so.
Activity Guide
2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 41v1
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4. How is the pressure related to the velocity of the fluid? (Is the pressure
higher where the velocity is higher? Qualitatively, what is the relationship
between velocity and pressure?) Explain.
Invention
Part of Bernoulli’s Theorem states that
P1 + 1/2 v12 = P2 + 1/2 v22
(5)
where P1 is the pressure on the side of the tube in the part of the tube with area A1, P2 is the
pressure on the side of the tube in the part of the tube with area A2, v1 is the velocity in the part
of the tube with area A1, v2 is the pressure in the part of the tube with area A2 and is the density
of the fluid. This is only true if both parts of the pipe are at the same level. A more general
definition of Bernoulli’s Theorem will be given later.
Application
GE 3.
1. Is the equation (5) consistent with what you observed in GE 1 and GE 2
above?
Application
Equipment:
1 Hydraulic lift pump
1 force pump
In order to get fluid to flow, you have to push on it. A pump is a mechanism for doing just this.
GE 4.
1. Examine a hydraulic lift pump. Explain what happens when the piston is
raised and lowered. Examine the valves. Explain how the pump works in
terms of pressure and other concepts you have learned.
2. The heart works like two force pumps. Examine a force pump. Explain
what happens when the piston is raised and lowered. Examine the valves.
Explain how the pump works in terms of pressure and other concepts you
have learned.
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2010 The Humanized Physics Project
HPP Activity 41v1
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Application
Equipment:
1 waterflow system or a computer simulation of it
Obtain the setup shown below.
GE 5.
1. If the system is horizontal (all pipes are at the same height), predict whether
the pressure is the same at points A – E. Rank the pressure at points A - E.
Explain your reasoning.
2. If the system is horizontal (all pipes are at the same height), predict whether
the flow rate is the same at points A – E. Rank the flow rate at points A – E.
Explain your reasoning.
3. Turn the pump on and observe the flowmeters. The flow meters measure
the volume flow rate. The volume flow rate is the volume of fluid crossing a
cross-sectional area, A, in a time t. The volume flow rate is the velocity
times the area
FV = vA
It is related to the flow rate given above (sometimes called the mass flow rate)
as follows
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HPP Activity 41v1
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Fm = FV
Where Fm = m/ t = A v is the (mass) flow rate.
How do the flow rates through the pipes at points A –E compare?
4. The diameter of the large pipes in the system is 1 inch and the diameter of
the small pipe is ½ inch. Find the velocity of the water through the pipes at
points A - E.
5. Use Bernoulli’s Theorem for pipes at the same level h,
p1 + ½ v12 = p2 + ½ v22,
to determine the pressure difference between
points A and B
points A and C
points A and D
points A and E
Discuss your answers with an instructor.
Obtain the setup shown below.
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2010 The Humanized Physics Project
HPP Activity 41v1
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GE 6.
1. If the system is horizontal (all pipes are at the same height), predict whether
the pressure is the same at points A – E. Rank the pressure at points A - E.
Explain your reasoning.
2. If the system is horizontal (all pipes are at the same height), predict whether
the flow rate is the same at points A – E. Rank the flow rate at points A – E.
Explain your reasoning.
3. Turn the pump on and observe the flowmeters. How do the flow rates
through the pipes at points A –E compare?
4. The diameter of the large pipes in the system is 1 inch and the diameter of
the small pipe is 3/4 inch. Find the velocity of the water through the pipes at
points A - E.
5. Use Bernoulli’s Theorem for pipes at the same level h,
p1 + ½ v12 = p2 + ½ v22,
to determine the pressure difference between
points A and B
points A and C
points A and D
points A and E
Equipment:
1 waterflow system
Bernoulli’s Theorem for flowing fluids actually has another part to it that has to do with the
height of the fluid above a certain level. Bernoulli’s Theorem is:
p1 + ½ v12 + gh1 = p2 + ½ v22 + gh2.
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2010 The Humanized Physics Project
HPP Activity 41v1
6
This is actually a statement of the conservation of energy per unit volume. The term ½ v2 the
kinetic energy per unit volume and the term gh is the potential energy per unit volume of a
small mass of fluid m, at point 1 and point 2. See Reading 9b for a derivation of Bernoulli’s
equation and an understanding of the equation in terms of work and potential and kinetic energy.
GE 7.
Suppose the system in GE 5 were rotated so that it is vertical.
1. If the system is vertical, is the pressure the same at points A – E? Rank the
pressure at points A - E. Explain your reasoning.
2. If the system is vertical, is the flow rate the same at points A – E? Rank the
flow rate at points A – E. Explain your reasoning.
3. Turn the pump on and observe the flowmeters. How were your predictions?
4. The diameter of the large pipes in the system is 1 inch and the diameter of
the small pipe is 1/2 inch. Find the velocity of the water through the pipes at
points A - E.
5. Use Bernoulli’s Theorem
p1 + ½ v12 + gh1 = p2 + ½ v22 + gh2,
to determine the pressure difference between
points A and B
points A and C
points A and D
points A and E
Discuss your answers with an instructor.
6. Do you think your blood pressure is the same in all parts of your body?
Explain.
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