query 6

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course Phy 232

7/13 12

006. `query 6

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Question: query introset change in pressure from velocity change.

Explain how to get the change in fluid pressure given the initial and final fluid velocities, assuming constant altitude

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Your Solution:

Using Bernoulli's equation, we omit altitude since it is constant, and get: 0.5rho*v1^2 + P1=0.5rho*v2^2 + P2

Then solve for the change in fluid pressure after plugging in the values given.

confidence rating #$&*:

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Given Solution:

Bernoulli's Equation can be written

1/2 rho v1^2 + rho g y1 + P1 = 1/2 rho v2^2 + rho g y2 + P2

If altitude is constant then y1 = y2, so that rho g y1 is the same as rho g y2. Subtracting this quantity from both sides, these terms disappear, leaving us

1/2 rho v1^2 + P1 = 1/2 rho v2^2 + P2.

The difference in pressure is P2 - P1. If we subtract P1 from both sides, as well as 1/2 rho v2^2, we get

1/2 rho v1^2 - 1/2 rho v2^2 = P2 - P1.

Thus

change in pressure = P2 - P1 = 1/2 rho ( v1^2 - v2^2 ).

Caution: (v1^2 - v2^2) is not the same as (v1 - v2)^2. Convince yourself of that by just picking two unequal and nonzero numbers for v1 and v2, and evaluating both sides.

ALTERNATIVE FORMULATION

Assuming constant rho, Bernoulli's Equation can be written

1/2 rho `d(v^2) + rho g `dy + `dP = 0.

If altitude is constant, then `dy = 0 so that

1/2 rho `d(v^2) + `dP = 0

so that

`dP = - 1/2 rho `d(v^2).

Caution: `d(v^2) means change in v^2, not the square of the change in v. So `d(v^2) = v2^2 - v1^2, not (v2 - v1)^2.

STUDENT SOLUTION: The equation for this situation is Bernoulli's Equation, which as you note is a modified KE+PE equation. Considering ideal conditions with no losses

(rho*gy)+(0.5*rho*v^2)+(P) = 0

g= acceleration due to gravity

y=altitude

rho=density of fluid

v=velocity

P= pressure

Constant altitude causes the first term to go to 0 and disappear.

(0.5*rho*v^2)+(P) = constant

So here is where we are:

Since the altitude h is constant, the two quantities .5 rho v^2 and P are the only things that can change. The sum 1/2 `rho v^2 + P must remain constant. Since fluid velocity v changes, it therefore follows that P must change by a quantity equal and opposite to the change in 1/2 `rho v^2.

MORE FORMAL SOLUTION:

More formally we could write

· 1/2 `rho v1^2 + P1 = 1/2 `rho v2^2 + P2

and rearrange to see that the change in pressure, P2 - P1, must be equal to the change 1/2 `rho v2^2 - 1/2 `rho v1^2 in .5 rho v^2:

· P2 - P1 = 1/2 `rho v2^2 - 1/2 `rho v1^2 = 1/2 rho (v2^2 - v1^2). **

Your Self-Critique:

OK

Your Self-Critique Rating:

OK

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Question: query billiard experiment

Do you think that on the average there is a significant difference between the total KE in the x direction and that in the y direction? Support your answer.

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Your Solution:

I think on average that the total KE in the x direction was larger than KEy, but I'm not sure it's a very significant

difference because it did not vary by much. I would say that it is not significant.

confidence rating #$&*:

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Given Solution:

** In almost every case the average of 30 KE readings in the x and in the y direction differs between the two directions by less than 10% of either KE. This difference is not statistically significant, so we conclude that the total KE is statistically the same in bot directions. **

Your Self-Critique:

OK

Your Self-Critique Rating:

OK

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Question: What do you think are the average velocities of the 'red' and the 'blue' particles and what do you think it is about the 'blue' particle that makes is so?

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Your Solution:

I would say the average velocities of the blue particles are about 6 or 7 and the average velocity of the red was about

4. I think their mass made it this way.

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Given Solution:

** Student answer with good analogy: I did not actually measure the velocities. the red were much faster. I would assume that the blue particle has much more mass a high velocity impact from the other particles made very little change in the blue particles velocity. Similar to a bycycle running into a Mack Truck.

INSTRUCTOR NOTE: : It turns out that average kinetic energies of red and blue particles are equal, but the greater mass of the blue particle implies that it needs less v to get the same KE (which is .5 mv^2) **

Your Self-Critique:

I didn't talk about average kinetic energy because it asked about velocity but that makes sense as well.

Your Self-Critique Rating: 3

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Question: What do you think is the most likely velocity of the 'red' particle?

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Your Solution:

I think the most likely velocity would be around 4.

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Given Solution:

** If you watch the velocity display you will see that the red particles seem to average somewhere around 4 or 5 **

Your Self-Critique:

OK

Your Self-Critique Rating:

OK

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Question: If the simulation had 100 particles, how long do you think you would have to watch the simulation before a screen with all the particles on the left-hand side of the screen would occur?

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Your Solution:

Since they all hit off of each other, and they are much like gas particles in a room; this would likely never happen

because of diffusion properties.

confidence rating #$&*:

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Given Solution:

** STUDENT ANSWER: Considering the random motion at various angles of impact.It would likely be a very rare event.

INSTRUCTOR COMMENT

This question requires a little fundamental probability but isn't too difficult to understand:

If particle position is regarded as random the probability of a particle being on one given side of the screen is 1/2. The probability of 2 particles both being on a given side is 1/2 * 1/2. For 3 particles the probability is 1/2 * 1/2 * 1/2 = 1/8. For 100 particlles the probability is 1 / 2^100, meaning that you would expect to see this phenomenon once in 2^100 screens. If you saw 10 screens per second this would take about 4 * 10^21 years, or just about a trillion times the age of the Earth.

In practical terms, then, you just wouldn't expect to see it, ever. **

Your Self-Critique:

OK

Your Self-Critique Rating:

OK

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Question: prin phy and gen phy problem 10.36 15 cm radius duct replentishes air in 9.2 m x 5.0 m x 4.5 m room every 16 minutes; how fast is air flowing in the duct?

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Your Solution:

N/A univ phy

confidence rating #$&*:

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Given Solution:

The volume of the room is 9.2 m * 5.0 m * 4.5 m = 210 m^3.

This air is replentished every 16 minutes, or at a rate of 210 m^3 / (16 min * 60 sec/min) = 210 m^3 / (960 sec) = .22 m^3 / second.

The cross-sectional area of the duct is pi r^2 = pi * (.15 m)^2 = .071 m^2.

The speed of the air flow and the velocity of the air flow are related by

rate of volume flow = cross-sectional area * speed of flow, so

speed of flow = rate of volume flow / cross-sectional area = .22 m^3 / s / (.071 m^2) = 3.1 m/s, approx.

Your Self-Critique:

`

Your Self-Critique Rating:

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Question: prin phy and gen phy problem 10.40 What gauge pressure is necessary to maintain a firehose stream at an altitude of 15 m?

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Your Solution:

N/A univ phys

confidence rating #$&*:

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Given Solution:

** We use Bernoulli's equation. Between the water in the hose before it narrows to the nozzle and the 15m altitude there is a vertical change in position of 15 m.

Between the water in the hose before it narrows to the nozzle and the 15 m altitude there is a vertical change in position of 15 m.

Assuming the water doesn't move all that fast before the nozzle narrows the flow, and noting that the water at the top of the stream has finally stopped moving for an instant before falling back down, we see that we know the two vertical positions and the velocities (both zero, or very nearly so) at the two points.

All that is left is to calculate the pressure difference. The pressure of the water after its exit is simply atmospheric pressure, so it is fairly straightforward to calculate the pressure inside the hose using Bernoulli's equation.

Assuming negligible velocity inside the hose we have

change in rho g h from inside the hose to 15 m height: `d(rho g h) = 1000 kg/m^3 * 9.8 m/s^2 * 15 m = 147,000 N / m^2, approx.

Noting that the velocity term .5 `rho v^2 is zero at both points, the change in pressure is `dP = - `d(rho g h) = -147,000 N/m^2.

Since the pressure at the 15 m height is atmospheric, the pressure inside the hose must be 147,000 N/m^2 higher than atmospheric. **

Your Self-Critique:

Your Self-Critique Rating:

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Question: Gen phy: Assuming that the water in the hose is moving much more slowly than the exiting water, so that the water in the hose is essentially moving at 0 velocity, what quantity is constant between the inside of the hose and the top of the stream? what term therefore cancels out of Bernoulli's equation?

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Your Solution:

N/A univ phys

confidence rating #$&*:

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Given Solution:

** Velocity is 0 at top and bottom; pressure at top is atmospheric, and if pressure in the hose was the same the water wouldn't experience any net force and would therefore remain in the hose **

Your Self-Critique:

Your Self-Critique Rating:

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Question: query gen phy problem 10.43 net force on 240m^2 roof from 35 m/s wind.

What is the net force on the roof?

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Your Solution:

N/A univ phys

confidence rating #$&*:

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Given Solution:

** air with density around 1.29 kg/m^3 moves with one velocity above the roof and essentially of 0 velocity below the roof. Thus there is a difference between the two sides of Bernoulli's equation in the quantity 1/2 `rho v^2. At the density of air `rho g h isn't going to amount to anything significant between the inside and outside of the roof. So the difference in pressure is equal and opposite to the change in 1/2 `rho v^2.

On one side v = 0, on the other v = 35 m/s, so the difference in .5 rho v^2 from inside to out is

`d(.5 rho v^2) = 0.5(1.29kg/m^3)*(35m/s)^2 - 0 = 790 N/m^2.

The difference in the altitude term is, as mentioned above, negligible so the difference in pressure from inside to out is

`dP = - `d(.5 rho v^2) = -790 N/m^2.

The associated force is 790 N/m^2 * 240 m^2 = 190,000 N, approx. **

Your Self-Critique:

Your Self-Critique Rating:

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Question: gen phy which term 'cancels out' of Bernoulli's equation and why?

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Your Solution:

N/A univ phys

confidence rating #$&*:

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Given Solution:

** because of the small density of air and the small change in y, `rho g y exhibits practically no change. **

Your Self-Critique:

Your Self-Critique Rating:

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Question: `q001. Explain how to get the change in velocity from a change in pressure, given density and initial velocity, in a situation where altitude does not change.

Can you tell from your expressions whether the change in velocity, for a given pressure change, is greater, less or equal when the initial velocity is greater?

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Your Solution:

You can use Bernoulli's equation.

.5 *rho*v ^2 + rho * g *h + P

Pressures must be constant, and if altitude doesnt change, then you have

.5 * rho *v^2 = constant

The change in velocity would be equal to the pressure change, and the velocity would be equal and opposite.

@&

Not quite, but close.

Using mostly your words:

The change in 1/2 rho v^2 would be equal and opposite to the pressure change.

*@

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Self-Critique Rating:OK

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Question: `q002. Water is moving inside a garden hose at 4 meters / second, and it exits the nozzle at 8 meters / second. What is the change in pressure as the water moves from the end of the hose out into the surrounding air?

Neglecting the effect of air resistance:

How high would the stream be expected to rise if the hose was pointed straight upward?

How far would the stream travel in the horizontal direction before falling back to the level of the nozzle, if directed at 45 degrees above horizontal?

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Your Solution:

.5 * rho * v^2

.5 * 1ooo * (64-16)=24,000

change in velocity=change in pressure

change in pressure is = -24000 N/m^2

H= (v0^2sin^2theta)/2g

H= 3.27 m

vf^2=vi^2 + 2*a*d

0=64 + 2*9.8* d

d=3.27 m

confidence rating #$&*:

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Self-Critique Rating:

I think I calculated the pressure change from one end of the hose to the tip, not the tip to the outside, though

I'm not sure what I could've done to correct this, or if I did calculate the right pressure change.

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Question: univ phy problem 12.77 / 14.75 (11th edition 14.67): prove that if weight in water if f w then density of gold is 1 / (1-f). Meaning as f -> 0, 1, infinity. Weight of gold in water if 12.9 N in air. What if nearly all lead and 12.9 N in air?

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Your Solution:

Tension=f*w

Tension and buoyant force are both opposite to gravity but equal in size.

T+dw* vol = w

which gives f*dg*vol + dw * vol = dg * vol

You can take out volume because it's a constant.

f *dg +dw = dg

Solve for dg

dg=dw/(1-f)

This is relative density.

19.3=1/(1-f)

f=.9481

The weight of the gold in the water is

T=f*w

T=.9481*12.9 N

T=12.23 N

confidence rating #$&*:

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Given Solution:

** The tension in the rope supporting the crown in water is T = f w.

Tension and buoyant force are equal and opposite to the force of gravity so

T + dw * vol = w or f * dg * vol + dw * vol = dg * vol.

Dividing through by vol we have

f * dg + dw = dg, which we solve for dg to obtain

dg = dw / (1 - f).

Relative density is density as a proportion of density of water, so

relative density is 1 / (1-f).

For gold relative density is 19.3 so we have

1 / (1-f) = 19.3, which we solve for f to obtain

f = 18.3 / 19.3.

The weight of the 12.9 N gold crown in water will thus be

T = f w = 18.3 / 19.3 * 12.9 N = 12.2 N.

STUDENT SOLUTION:

After drawing a free body diagram we can see that these equations are true:

Sum of Fy =m*ay ,

T+B-w=0,

T=fw,

B=(density of water)(Volume of crown)(gravity).

Then

fw+(density of water)(Volume of crown)(gravity)-w=0.

(1-f)w=(density of water)(Volume of crown)(gravity).

Use

w==(density of crown)(Volume of crown)(gravity).

(1-f)(density of crown)(Volume of crown)(gravity) =(density of water)(Volume of crown)(gravity).

Thus, (density of crown)/(density of water)=1/(1-f). **

Your Self-Critique:

OK

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Self-Critique Rating:

OK

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Question: univ phy What are the meanings of the limits as f approaches 0 and 1?

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Your Solution:

As f approaches 0, the density of the crown starts to become less than that of the water, and the crown floats. As

f approaches 1, the density of the crown is greater than that of the water and it sinks.

confidence rating #$&*:

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Given Solution:

** GOOD STUDENT ANSWER: f-> 0 gives (density of crown)/(density of water) = 1 and T=0. If the density of the crown equals the density of the water, the crown just floats, fully submerged, and the tension should be zero. When f-> 1, density of crown >> density of water and T=w. If density of crown >> density of water then B is negligible relative to the weight w of the crown and T should equal w. **

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Self-Critique Rating:OK

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Question: `q003. Water exits a large tank through a hole in the side of a cylindrical container with vertical walls. The water stream falls to the level surface on which the tank is resting. The tank is filled with water to depth y_max. The water stream reaches the level surface at a distance x from the side of the container.

Without doing any calculations, explain why there must be at least one vertical position at which the hole could be placed to maximize the distance x. Explain also why there must be distances x that could be achieved by at least two different vertical positions for the hole.

Give all the possible vertical levels of the hole.

What is the maximum possible distance x at which it is possible for a water stream to reach the level surface, and where would the hole have to be to achieve this?

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Your Solution:

The must be at least one vertical position the maximize the distance x because there is once spot that will give the

most KE when the water is exiting at it's max speed. There also must be distances x that could be achieved by two vertical positions for the hole

becuase the range is a function of sin and there are two angles or spots that equal the same thing when put into

the sin function.The maximum possible distance x at which it is possible for a water stream to reach the level surface would be maybe

the same height that it drops from.

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As the hole gets lower what happens to the velocity of the stream and how would this tend to affect the distance x?

As the hole gets lower what happens to the distance the water falls after exiting the container, and how does the distance of fall tend to affect the distance x?

If the hole is at the level of the water surface what would be the distance x?

If the hole was at the level of the surface on which the cylinder is resting what would be the distance x?

How does this explain why there must be at least one vertical position at which the hole could be placed to maximize the distance x?

In terms of x and y_max, what is the expression for the velocity of the exiting water?

In terms of x and y_max, what is the horizontal range of the water stream?

Apply either a first-derivative or second-derivative test to maximize the horizontal range.

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Self-Critique Rating: I'm not too sure about this one. I couldnt picture it very well and I got confused."

Self-critique (if necessary):

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Self-critique rating:

Self-critique (if necessary):

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Self-critique rating:

#*&!

@&

Good for the most part.

Check my notes.

Except for `q003, which I do want you to revise, revisions are optional.

&#Please see my notes and submit a copy of this document with revisions, comments and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).

Be sure to include the entire document, including my notes.

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