PHY 121 Blog help

Ch2_1 #2

2011-08-14T18:11:00.001-04:00

Ch4_1 #1

2010-12-31T10:48:00.000-05:00

Ch 14_3 #4

2010-11-26T23:01:00.001-05:00

Use first and second laws of thermodynamics to find out the answer

Ch 14_3 #3

2010-11-26T22:42:00.000-05:00

Use coeficcient of performance for heat pum used for cooling

Ch 14_3 #2

2010-11-26T22:28:00.000-05:00

Use expression for efficiency of heat engine

Ch 14_3 #1

2010-11-26T22:23:00.000-05:00

use expression for maximum efficiency for heat engine

Ch 14_2 #4

2010-11-26T22:21:00.001-05:00

Use heat efficiency equation to find desired quantities

Ch 14_2 #3

2010-11-26T22:16:00.001-05:00

Find out total heat to find the heat efficiency

Ch 14_2 #2

2010-11-26T22:11:00.001-05:00

Use constant pressure and constant volume processes to calculate the heat. Pay attention what does the work since you need to put sign properly. For part c calculate initial and final temperatures to decide how much energy changed

Ch 14_2 #1

2010-11-26T22:01:00.000-05:00

Use expression for work done by a gas in isobaric process

Ch 14_1 #4

2010-11-26T21:59:00.000-05:00

Use conservation of energy/heat law

Ch 14_1 #3

2010-11-26T21:58:00.000-05:00

Use the conservation of heat/energy

Ch 14_1 #2

2010-11-26T21:53:00.002-05:00

Use ideal gas equation to find initial pressure then use isochoric (constant-volume) process to find final pressure

Ch 14_1 #1

2010-11-26T21:50:00.002-05:00

Do determine what process it is look at what stays constant. The use equation for ideal gas to find final temperature and number of moles

Ch 13_3 #4

2010-11-26T21:28:00.002-05:00

Use Stefan's law to calculate power. Calculate the total area of the cube properly and convert everything into SI units.

Ch 13_3 #3

2010-11-26T21:12:00.000-05:00

Use equation for the rate of conduction of heat across a temperature differenceand use for the temperature of the burner

Ch 13_3 #2

2010-11-26T21:07:00.001-05:00

Use equation for the rate of conduction of heat across a temperature difference

Ch 13_3 #1

2010-11-26T21:05:00.001-05:00

Look at the graph and understnad what different segments of the line correspond to. Remember to convert into SI units.

Ch 13_2 #4

2010-11-26T20:49:00.001-05:00

First find how much heat you need to heat ice to melting temperature, then calculate how much heat needed to warm up water. Where does this heat come from?

Ch 13_2 #3

2010-11-26T20:47:00.000-05:00

Problem similar to the problem Ch 13_2 #2

Ch 13_2 #2

2010-11-26T20:38:00.001-05:00

Use conservation of energy/heat law. Wrtie down equation for heat lost for copper and the equation for heat gained by water. Solve for T_f.

Ch 13_2 #1

2010-11-26T20:36:00.001-05:00

Calculate amount of heat transfered to water in one second. How much water needed to absorb this heat for stated temperature difference?

Ch 13_1 #4

2010-11-21T21:02:00.001-05:00

Think where the heat comes from that heats up the thermometer? Calculate the amount of heat needed to heat the thermometer.

Ch 13_1 #3

2010-11-21T12:00:00.001-05:00

Use equation for heat, specific heat and temperature

Ch 13_1 #2

2010-11-21T11:50:00.001-05:00

Use expression for thermal expansion

Ch 13_1 #1

2010-11-21T11:44:00.002-05:00

Use the conversion of Cal to find J. Do not confuse calorie with food calorie

Ch 12_3 #4

2010-11-16T20:38:00.002-05:00

For part a) use equation for ideal gas. Psi is not SI units so you have to convert it into correct units.
For part b) use relationship for energy and temperature and not that you asked to calculate the energy per one atom. Does the number you obtain is very small or big? Does it make sense?

Ch 12_3 #3

2010-11-16T15:39:00.000-05:00

Use the same formula as before, but remember that nitrogen is diatomic gas and thus you have to use correct molar mass.

Ch 12_3 #2

2010-11-16T15:38:00.001-05:00

Recall formula for the rms speed of the atoms or relationship between absolute temperature and rms speed.

Ch 12_3 #1

2010-11-16T15:36:00.001-05:00

This an ideal gas process with constant volume. Note that the pressure in tire is gauge pressure rather than absolute pressure.

Ch 11_3 #3

2010-11-15T21:30:00.005-05:00

In this problem you have to consider water as viscous fluid. You need to maintain pressure in point P for the water to continue to flow at the other end.

3 comments:

Anonymous said...: Can someone help out? I have no idea how to do this problem; November 14, 2010 8:05 PM
matt said...: I cant figure this one out....Help please; November 15, 2010 12:46 PM

By posting answers your setting up your fellow student for failure on the exam. It sounds trivial but it is true. The equation one has to use is one for flow of viscous fluid.

Ch 11_3 #4

2010-11-15T21:20:00.003-05:00

Part A should be straightforward (there was quiz on that in class).
Part B you have to use continuity equation and Bernoulli's principle to find the pressure in the lower pipe. Then calculate the pressure in the water column.

11 comments:

Anonymous said...: What is the continuity equation professor?; November 13, 2010 5:42 PM
Anonymous said...: the continuity equation is A1v1=A2v2
I'm not sure which equations to use for part A?; November 13, 2010 8:01 PM
Anonymous said...: How the hell do you figure out anything for this question? You aren't given any pressures for any part of the pipe, just areas and velocities. How can you figure out the height or even the exiting force of the water?; November 14, 2010 8:06 PM
Erica said...: Part A: pressure of anything exited in air is just atmospheric pressure (Patm)=1.013 kPa.; November 14, 2010 10:30 PM
Anonymous said...: Alright so I used Bernoulli's for part A, got an answer of 145.2 kPa, says incorrect, however when I plug that into part B, and got an answer, THAT was correct?; November 14, 2010 11:15 PM
Anonymous said...: P(atm)=1.013*10^5 Pa; November 15, 2010 8:21 PM
Anonymous said...: i have no idea how to do part b; November 15, 2010 8:52 PM
Anonymous said...: how do we do this. This is frustrating; November 15, 2010 8:53 PM

Anonymous said...: Thank you to all who post answers! These questions are complicated and without help from the professor, or help in the form of really General hints, they suck.; November 15, 2010 9:16 PM
Anonymous said...: i still cant figure out part A..can anyone help with that?; November 15, 2010 9:16 PM

I do not think we can allow straightforward answers - hints and guidance that explain physical meaning on the other hand are welcome. It is easy to put this answer now however it will not help during the exam.

Ch 12_2 #4

2010-11-14T20:44:00.000-05:00

This is a constant temperature process for an ideal gas.

Ch 12_2 #3

2010-11-14T20:40:00.000-05:00

Recall equation for an ideal gas in the sealed cylinder

Ch 12_2 #2

2010-11-14T20:36:00.001-05:00

Find how many moles of water there are and use equation for ideal gas to find volume

Ch 12_2 #1

2010-11-14T20:20:00.002-05:00

Use equation for ideal gas

Ch 12_1 #4

2010-11-14T11:56:00.001-05:00

Calculate molecular mass of H2O2 then calculate number of moles

Ch 12_1 #3

2010-11-14T11:44:00.001-05:00

Recall that oxygen is diatomic gas and water molecule has just one oxygen atom

Ch 12_1 #2

2010-11-14T11:42:00.001-05:00

Use formula to relate Fahrenheit to Celsius

Ch 12_1 #1

2010-11-14T11:41:00.000-05:00

Recall what is spacing for scale in Kelvins and degrees of Celsius

Ch 11_3 #2

2010-11-12T14:59:00.001-05:00

Use a continuity equiation first to find velocity at point 2 (be careful there are 2 outgoing pipes), then use Bernoulli's principle to find pressure.

Ch 11_3 #1

2010-11-12T14:56:00.001-05:00

Recall that a flow given as a volume per unit time. Do not forget to convert flow into SI units.
Then find the speed of the flow.

Ch 11_2 #4

2010-11-07T19:27:00.001-05:00

Use formula for buoyant force

Ch 11_2 #3

2010-11-07T19:26:00.000-05:00

Use formula for buoyant force

Ch 11_2 #2

2010-11-07T19:09:00.001-05:00

It may be easier to calculate the height under water and then find out the desired distance.

Ch 11_2 #1

2010-11-07T18:32:00.001-05:00

Compare buoyancy of the barge in salt and fresh waters.

Ch 11_1 #4

2010-11-07T12:51:00.001-05:00

Think why is mercury higher on the side of the box. Recall hydrostatic pressure in for liquid in equilibrium.

Ch 11_1 #3

2010-11-07T12:44:00.000-05:00

Use formula for liquids to calculate the pressure.

Ch 11_1 #2

2010-11-07T12:07:00.000-05:00

Think what makes the water raise in the straw? What pressure is needed to maintain that?

Ch 11_1 #1

2010-11-07T12:00:00.001-05:00

Use mass density formula to calculate unknown quantity

Ch10 3 #4

2010-11-05T15:18:00.004-04:00

Recall general formula for Dopler effect and think how many times it occurs

Ch10 3 #3

2010-11-05T15:18:00.003-04:00

Recall the formula for beat frequency and find out what frequency actually sound

Ch10 3 #2

2010-11-05T15:18:00.001-04:00

Think what waves lengths are allowed for the standing longitudinal waves in air columns

Ch10 3 #1

2010-11-05T15:17:00.001-04:00

Recall the formulas for the standing longitudinal waves in air columns

Ch10 2 #4

2010-10-29T19:16:00.001-04:00

How does the frequency depend on length?

Ch10 2 #3

2010-10-29T19:14:00.001-04:00

You can figure out the answer for the fundamental mode, and then figure out which overtone is shown

Ch10 2 #2

2010-10-29T19:13:00.000-04:00

This uses the Doppler Effect to shift the frequency

Ch10 2 #1

2010-10-29T19:11:00.001-04:00

See sheet 20

Ch10 1 #4

2010-10-27T16:57:00.000-04:00

See the sheets on intensity, especially sheet 9.2

Ch10 1 #3

2010-10-27T16:55:00.000-04:00

See sheet 9.5 for the definition of dB

Ch10 1 #2

2010-10-27T16:54:00.001-04:00

See sheet 4

Ch10 1 #1

2010-10-27T16:52:00.001-04:00

Use the speed of sound, and compute the times from the different distances the sound waves travel. Why are the distances different?

Ch9 3 #4

2010-10-24T13:43:00.001-04:00

This a resonance question, and you are tapping at the resonance frequency. Why must this be the resonance frequency?

Ch9 3 #3

2010-10-24T13:41:00.001-04:00

This is most easily done using conservation of energy

Ch9 3 #2

2010-10-24T13:40:00.001-04:00

This is essentially the generalization of the previous problem.

Ch9 3 #1

2010-10-24T13:39:00.002-04:00

Remember that total energy is conserved, and figure out when either KE or PE is zero. When KE is zero PE must be its maximum and vice versa

Ch9 2 #4

2010-10-20T15:52:00.001-04:00

CH9 2 #3

2010-10-20T15:49:00.001-04:00

See section 9.2.3

Ch9 2 #2

2010-10-20T15:47:00.002-04:00

The intermediate step in this problem involves finding the period of the oscillation from the information given.

Ch9 2 #1

2010-10-20T15:43:00.001-04:00

See sheet 14 and use Newton's 2nd law, F = ma.

Ch9 1 #4

2010-10-18T15:06:00.000-04:00

Use the definitions on sheets 4 - 6.

Ch9 1 #3

2010-10-18T15:05:00.001-04:00

Use the basic definitions.

Ch9 1 #2

2010-10-18T15:00:00.004-04:00

Ch9 1 #1

2010-10-18T15:00:00.001-04:00

Remember that a positive value means "up".

Ch8 3 #4

2010-10-12T20:00:00.001-04:00

Ch8 3 #3

2010-10-12T19:58:00.003-04:00

The stick isn't moving, so the sums of all forces and all torques must both be zero.

Ch8 3 #2

2010-10-12T19:58:00.001-04:00

Ch8 3 #1

2010-10-12T19:54:00.003-04:00

This problem is like the conservation of energy problems in Ch6, but now there is also kinetic energy of rotation, equation 8.15

Ch8 2 #4

2010-10-12T19:54:00.001-04:00

Ch8 2 #3

2010-10-12T19:53:00.001-04:00

Use equation 8.12. What is the moment of inertia for a cylinder?

Ch8 2 #2

2010-10-12T19:50:00.001-04:00

The "net torque" is the sum of all torques. Remember the opposite rotation directions have opposite signs.

Ch8 2 #1

2010-10-12T19:49:00.000-04:00

Use equation 8.10. Make sure to use the perpendicular distance between force and pivot.

Ch8 1 #4

2010-10-12T19:47:00.000-04:00

This problem is very much like the previous problem and uses the same basic ideas. Do you see why?

Ch8 1 #3

2010-10-12T19:45:00.001-04:00

Use the rotational analogies for linear motion in constant acceleration. The rotational forms are equations 8.7 - 8.9

Ch8 1 #2

2010-10-12T19:43:00.001-04:00

Decide if any of the quantities, angular or linear, must be the same for both people by using the picture without any math needed. Then use definitions like equation 8.3.

Ch8 1 #1

2010-10-12T19:41:00.000-04:00

Ch7 3 #4

2010-10-12T10:16:00.001-04:00

Ch7 3 #3

2010-10-12T10:15:00.000-04:00

Watch out to not confuse "elastic" and "inelastic"

Ch7 3 #2

2010-10-12T10:14:00.001-04:00

CH7_3 #1

2010-10-12T10:12:00.001-04:00

Ch7 2 #4

2010-10-06T16:18:00.003-04:00

In addition to Ch7 momentum conservation, you'll need the work-energy theorem from Ch 6. The friction does work to slow the block down. Do this problem in two pieces: (1) immediately before and immediately after the collision, and (2) while the block slows down.

Ch7 2 #3

2010-10-06T16:17:00.007-04:00

Ch7 2 #2

2010-10-06T16:17:00.006-04:00

Ch7 2 #1

2010-10-06T16:17:00.005-04:00

See Ch7 example 7.2

Ch7 1 #4

2010-10-06T16:11:00.007-04:00

See Ch7 sheet 11

Ch7 1 #3

2010-10-06T16:11:00.006-04:00

Break these problems into parts corresponding to each value of the force. (Right is moving in the positive x direction.)

Ch7 1 #2

2010-10-06T16:11:00.005-04:00

The change in momentum is the difference between final and initial momentum

Ch7 1 #1

2010-10-06T16:10:00.002-04:00

See Ch 7 sheet 3

Ch6_3 #2

2010-09-28T20:41:00.002-04:00

See Ch 4 Sheet 24, Ch 6 Sheet 19' and 28.

Ch6_3 #4

2010-09-28T15:31:00.003-04:00

Se Ch 6 Sheet 15. Here in addition to gravity the frictional force in the atmosphere does work Wfr and equation (6.4) on Sheet 15 holds. See on Ch 6 Sheet 18 how you can write the same equation in terms of the total mechanical energy E = KE + PE with the conservation of E being E2 - E1 = 0. With friction the energy loss is Wfr = E2 - E1.