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Unformatted text preview: Physics 1101 Unit 5
UNIT 5: FLUIDS Self—Demos SelfDemonstration 5.1: The Bernoulli Effect
IMPORTANT: Read text Sections 9.79.8 ﬁrst! Key point: Bernoulli’s principle: Where the pressure is higher, ﬂow'speed is lower; where the
pressure is lower, ﬂow speed is higher. A Bernoulli effect demonstration located in the Learning Center involves a ping—pong ball and a
funnel. Play with this and see if you can explain what happens to the ball within the funnel. The. pressure is smaller
ﬂf C where the
ﬂow 1'5 faster Streamlines qfuir around hall inﬁmm’! Figure © The Nufﬁeld Foundation 2004, from http://www.practicalphysics.org/go/Experimeng405.html Observation 1: With the hair dryer off, hold it pointing up, with the funnel at the top (left
picture in the ﬁgure above). Place the pingpong ball in the funnel. Turn on the hair dryer.
Explain in your notebook why the fast stream of air doesn't blow the ping—pong ball out. Observation 2: Keep the hair dryer on, and rotate it slowly so the nozzle is horizontal.
Explain in your notebook Why the fast stream of air doesn't blow the ping—pong ball out. Observation 3: Continue and rotate the hair dryer until it is pointing down, with the funnel at
the bottom (see the ﬁgure above). Figure out what keeps the ping pong ball from falling out,
and explain in your notebook be ready to explain. Hint: Consider the ﬁgure above, which shows some streamlines for the air ﬂow around the ball.
Compare the flow at points A, B, C, and D. Consider Where the air speed is high and low and
what this means about where the pressure is high and low. 59 Physics 11 01 Unit 5 with the vernier caliper don't hesitate to ask a TA or look for instructions posted near the
lab.) Record the result. (b) Submerge the solid cylinder in the graduate cylinder containing water and measure
the volume of water displaced. Record the result. The agreement between these values
conﬁrms that the volume of liquid displaced by an object actually does equal the Volume of
the object. Decide which method, (a) or (b), is more accurate and record your conclusion. Prepare
to discuss your conclusion. Irregular solid:
(b) Determine the volume of the irregular solid by method (b) only. Record the result. (2) Weight’and Buoyant force in water Beam Balance Hook undre
balance pan Support Rod Top shelf of Carrel To find weight of object in liquid, raise
the graduated cylinder until the hanging
object is submerged. (Use the bottle provided instead of the graduated cylinder for this part.) For both the solid cylinder and irregular solid, hang each under the balance pan of the balance
pan using the small hook shown in the ﬁgure above to determine the weight in air, and record
the results as the mass of each object. Then, being careful to not let the objects touch the
bottom or sides, use the bottle provided (not the graduated cylinder) and weigh each object
While suspended in water. Using the bottle provided will help you to avoid having the objects
touch the sides or bottom. Finally, record the difference between the weight in air and the
weight in water as the buoyant force BF for each object. The agreement between the BF
(measured in grams) and the displacement volumes (in cm3) conﬁrms Archimedes’Principle. (3) Repeat step (2) for the solid cylinder using alcohol as the ﬂuid instead of water. 61 Physics 1101 Unit 5 Check that your result is close to what you had in (3), and enter the average of your results to (3) and (5). ANALYSIS The speciﬁc gravity of an object is the ratio of its density p = m/ V to the density of water: SG = p/pw=(m/V)/pw. Rearranging this slightly we see 86 = m/(pwV) = m/mw. From the latter
we see that is it the same as the ratio of the mass of the object to the mass of an equivalent
volume of water (the water which the object displaces). Multiplying by g on top and bottom to
think about weights instead of masses, this gives SG = (mg)/(mwg) = W/FB, where W = mg is
the weight of the object, and mwg is the weight of the displaced water, which, according to
Archimedes, is none other than the buoyant force F B. Use the above result that SG = W/FB to compute the speciﬁc gravity of both the solid cylinder
and the irregular solid, and enter your result. Comparing your result to the table of densities
below, identify the material composition of each object and enter your conclusions. Copper 28.92 x 103 “ {Gold i193 x 103 WW"
Lead 2113 x 103 ‘ LNickel is 8 x 103
Platinum ‘ 110.5 x 103
Tungsten E 19.3 X 103 .Uranium ? 18.7 X 103
?Zinc $6.9 x 103 iIron (steel) 57.86 x 103
EAluminum {3.7 x 103 gBrass $8.5 x 103 i Finally, to determine the speciﬁc gravity of alcohol note that F B,water = pw Vg and FB,a1c0h01= palcoholVg, Where pw is the density of water and palcohol is the density of alcohol, and
think about what the ratio F B,a1cohol/F BMater will give you. Use this to calculate the specific
gravity of alcohol and record your result. 63 Physics 1101 Unit 5 average fall time from your consistent data and, from this value, compute the sedimentation
velocity Vs in units of cm/s, compute the radius R of the ball bearing in units of cm
(remember R = D/2 and 1 inch = 2.54 cm), record your results in a table. (4) Repeat part 3, using the two tubes containing the 1/16" and 3/16"
bearings. Measure each fall time three or four times to get an
average value. Record your results in a table. From the trend in
your observations imagine what velocity you would expect for a tiny
ball bearing of diameter zero and use this to fill in the ﬁrst row of the
data table. Hint: The 1/16" ball moves very slowly and you may have to wait
quite a while for it to appear from the top of the tube. Once you see it,
you may wish to tilt the tube a bit to get it to moVe away from the wall
of the tube before the ball gets to the starting point. Be sure to have
the tube vertical for the measured part of the fall! Figure 1 (5) Take the 1/8" bearing tube out of the ice bath (making sure that it was in there at least 15
minutes). Measure the time for the 30 cm fall time a couple of times for this tube which is now at 0°C and determine the average of your consistent values. Compute the fall velocity and
record your result. Write whether you think the glycerol is now more or less Viscous. T heoly and Analysis The equation for sedimentation velocity is v. = 2g(p—pi) R2
5 977 where p = 7.8 g/cm3 is the density of stainless steel, pL= 1.25 g/cm3 is the density of glycerol, 71
is the viscosity of glycerol (an unknown at this point), and g = 980 cm/s2 is the acceleration of
gravity. Note that it is most convenient here to express g in units of cm to be able to combine it
well with all of the other units used in this lab. Summarize the sedimentation velocity measurements in a table. Make a plot of vs vs. R2. Note
that because the theoretical formula gives v5=0 for R=0, it is legitimate to make your ﬁt go
through the origin. Find the viscosity of glycerol from the fact that the slope of your plot must
be slope = 2g (p — pL)/(9n). Record your result for the viscosity. Finally, use Eq. (1) above to ﬁnd the viscosity of glycerol at 0°C from the velocity you found for
the 1/8" ball. . 65 ...
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