PHYSICS 1AL
FLUIDS
Winter 09
Introduction
In this lab you will investigate several properties of fluids both at rest and in motion.
Pre-lab Questions:
1. What is the “specific gravity” of a substance? If a substance has a specific gravity of
7.5 what is its density in kg/m3? What is its density in grams/cc?
2. Ice is less dense than water so it floats in water. But the densities of ice and water are
close, so most of an iceberg is submerged, even in salt water which is denser than
pure water.
a. For the specific gravities of ice and water given in the table on the next page what
fraction of the volume of the ice cube in a glass of pure water will be above the
surface? If you used salt water of specific gravity from the same table what
fraction of the volume of the ice cube would be above the surface?
b. If the ice cube’s volume is (1.0 cm)3 what is the weight of the cube?
c. What is the weight of the part of the cube above water level in pure water?
d. What is the weight of the part below water level in pure water?
3. Suppose you were on top of a cliff of height h, and threw a ball horizontally with
speed v. How far would it go before it hits the ground? (Ignore air resistance) (ie,
what is the formula that describes range in terms of horizontal speed and height?
4. Examine the schematic of the tower. Water flows in at the top to balance the water
flowing out through the holes, so the height of the water in the tower stays fixed.
Take state 1 at the top of the water in the tower and state 2 in the water just as it
flows out of the hole.
a. What is the pressure at states 1 and 2?
Explain.
b. Review section 9.7 on the Bernoulli
Principle. Write down Bernoulli’s
equation (be sure you understand every
term in the equation before you come to
class).
c. If the holes are at heights of 3.0 cm, 13
cm and 23 cm above the surface of the
water in the lower basin and the top
hole is 3.0 cm below the top of the
water in the tower, predict which one
you believe will go farthest (top, middle
or bottom). Explain your reasoning.
5. Write “I checked my grades online”.
(Please, actually check them on
webct.ucsd.edu. And if you have some problems then it would be better to solve them
with your TAs during the last lab instead of emailing them during finals week.)
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PHYSICS 1AL
FLUIDS
Winter 09
Group Activity:
A planetary scientist wants to measure the density of an odd shaped meteorite that has been
found. She weighs the meteorite and finds the weight is 120 N. Next she submerges it
completely in pure water, and she finds its weight is 100 N when underwater. You may
assume g = 10 m/s2.
Substance
Specific gravity
• What is the density of the rock?
• If the rock were a cube what size would it be?
• If the specific gravity of typical earth rocks is 3.0 and
of iron is 8.0 and the meteorite is made of a mix of
earth type rock and iron, what percentage is iron and
what percentage is rock?
Experiment A: Measuring Specific Gravity
Ice
0.92
Water
1.00
Salt water
1.03
Porcelain
approx 2
Aluminum
2.7
• You are stranded on a remote island. Through a series
Iron
of fortunate events you become scientific advisor to
Copper/brass
the local tribal chief. You devise a way to measure
Silver
lengths (in units of the chief’s foot), and force in units
you name FUs (force units). Also by fortunate
Lead
circumstance you have a copy of on old physics text
with you. Predictably, the chief want you to tell him if
Gold
his crown is made of copper (which is much valued)
or of iron colored to look like copper. Can you do this
with the equipment available? If so describe each step of the measurements and
calculations you need to make.
7.9
8.9
10.5
11.3
19.3
• In reality you are in a physics 1A lab and have to measure the specific gravity of the
crown provided given the excellent equipment provided. Describe the measurements
you need to make and why you need to make them. Then do the experiment and report
the density of the crown.
Experiment B: Bernoulli and the Tower
In the first few weeks of Physics 1A you learned that all objects fall with the same
acceleration and that if you know a projectile's initial position and velocity, you can predict
its trajectory. In this section of the lab, we will see that fluids behave in the same way.
Specifically, we will examine water shooting out the side of tower at 3 different heights. We
will see that water molecules behave exactly the same way as footballs flying through the air,
and verify Torricelli's Result, which derives from the Bernoulli Principle.
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PHYSICS 1AL
FLUIDS
Winter 09
Bernoulli’s Principle relates the work done on a
volume of a fluid (the pressure terms of the
equation) with the kinetic energy of that volume
(the 1/2ρv2 terms) and the potential energy (the ρgy
terms). Bernoulli's equation is usually written like
this:
2
P1 + 1 2 !v1 + !gy1 = P2 + 1 2 !v22 + !gy2
•
•
•
The water speed at state 1 can be taken to
be zero as the level of water in the tower
is constant. Using Bernoulli's equation
find the speed of the fluid at state 2 as it
comes out of the hole (this is the speed of
efflux).
If the holes are at heights of 3.0 cm, 13
cm and 23 cm above the surface of the water in the lower basin, and the top hole is
3.0 cm below the top of the water in the tower, calculate the speed of efflux for
each hole.
For each hole calculate the expected range for each stream of water. From your
calculations, which one goes farthest (top, middle or bottom)?
When you get to lab look at the actual tower you will use. Measure the heights of each of the
three holes below the overflow pipe and above the base of the tower. Record your
measurements and predictions in a table in your lab book. Include space in your table to
record your observations of the range.
• Place the pump in the plastic basin, and fill the basin with enough water to submerge
the pump.
• Place a lab-jack at one end of the basin, put the tower on the lab-jack, and insert the
pump's hose into the tower. Adjust the lab jack so that the base of the tower is at the
water level of the basin. Make sure the tower is vertical.
• Plug in the pump, and fill the tower with water. Make sure the water flowing out of the
tower stays in the basin.
• Measure where each stream of water enters the water in the basin. (Follow the stream
down into the water with your finger.) Enter your measurements in the table next to the
calculated values.
• Comment on how well you can measure the range, and on how well the observations
agree with the calculations. Do you have evidence in your results for any of the
following effects:
o a tilt from vertical of the tower
o a reduction of the speed of efflux from the ideal case
o any other possible systematic effect
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