Physics Forums - Introductory Physics http://www.physicsforums.com Algebra- and calculus-based general physics homework en Wed, 16 May 2012 22:11:54 GMT vBulletin 60 http://www.physicsforums.com/Nexus/misc/rss.jpg Physics Forums - Introductory Physics http://www.physicsforums.com Electric field by infinite line of charge http://www.physicsforums.com/showthread.php?t=606567&goto=newpost Wed, 16 May 2012 22:02:00 GMT 1. The problem statement, all variables and given/known data
Given an infinitely long line of charge density λ extended along the x-axis, what is the electric field at a point X = x(x')+y(y')+z(z') (in space)?


2. Relevant equations

E = kq / r^2, dq = (lamda)dx

3. The attempt at a solution

dE = kλ ∫ [x(x')+y(y')+z(z')- x(x')] dx / [(y^2) +(z^2)]^(3/2) (integral from -∞ to +∞)

I end up getting 0 because -∞ + ∞ equals 0? ]]> Introductory Physics Sam99 http://www.physicsforums.com/showthread.php?t=606567 Thermodynamics: Pouring Cream into Coffee http://www.physicsforums.com/showthread.php?t=606566&goto=newpost Wed, 16 May 2012 22:00:23 GMT I did a general problem of the perfect time to pour cream into coffee.
I was hoping someone could check my work

It's in the pdf below:

docs(dot)google(dot)com/open?id=0Bx8zdkF5WYAbMl80UjI5U2lmdlU

Thanks ]]> Introductory Physics AvgStudent http://www.physicsforums.com/showthread.php?t=606566 Compton scattering problem - how much did wavelength change by? http://www.physicsforums.com/showthread.php?t=606546&goto=newpost Wed, 16 May 2012 20:10:42 GMT *1. The problem statement, all variables and given/known data* How much will the wavelength of the incident X-ray photon change by if it is scattered by 30 degrees when it interacts with an... 1. The problem statement, all variables and given/known data

How much will the wavelength of the incident X-ray photon change by if it is scattered by 30 degrees when it interacts with an electron?

2. Relevant equations

This is Compton scattering.

This is the equation I used:

lambda ' - lambda (0) = (h)/(m(e)c) (1-cos theta)

where h is planck's constant, me is the mass of the electron, c is speed of light.

3. The attempt at a solution

Basically I just subbed in everything into the equation above, and got 2.424 x 10^-12 m.

The thing I'm not sure about is whether I'm finished or not.

Does lambda ' minus lambda (0) give me the difference? Did I use the right form of the equation here? Do I need to do anything else?

Thank you :) ]]> Introductory Physics daleklama http://www.physicsforums.com/showthread.php?t=606546 Kinematics problem with constant acceleration http://www.physicsforums.com/showthread.php?t=606540&goto=newpost Wed, 16 May 2012 19:37:55 GMT A ball is thrown downwards from a top of a 20m building with an initial velocity of 15m/s. What equation could be used to model its height??? My answer: h (t) = -4.9t2-15t+20 A ball is thrown downwards from a top of a 20m building with an initial velocity of 15m/s. What equation could be used to model its height???

My answer: h (t) = -4.9t2-15t+20 ]]> Introductory Physics livestrong136 http://www.physicsforums.com/showthread.php?t=606540 Simple kinematics problem http://www.physicsforums.com/showthread.php?t=606534&goto=newpost Wed, 16 May 2012 19:27:12 GMT *1. The problem statement, all variables and given/known data* Block A weighs 75lbf and the coefficients of static and kinematic friction between A and the plane are both 0.50. Determine the... 1. The problem statement, all variables and given/known data

Block A weighs 75lbf and the coefficients of static and kinematic friction between A and the plane are both 0.50. Determine the acceleration of A under the action of the 25 lbf force when the velocity at A is (a) 8fps up the plane, (b) 15 fps down the plane. (c) Zero.

2. Relevant equations

Sum of all forces = mass * acceleration

3. The attempt at a solution

I dont know where to start... the box is doing up a plane and it's cosine is 4/5 and its sine is 3/5. ]]> Introductory Physics jean28 http://www.physicsforums.com/showthread.php?t=606534 SHM Vertical http://www.physicsforums.com/showthread.php?t=606493&goto=newpost Wed, 16 May 2012 16:41:53 GMT SHM - VERTICAL Surely whether you consider the motion used to establish the SHM system, or the motion as soon as the system is released you should derive the same equation. DISPLACEMENT BELOW... SHM - VERTICAL

Surely whether you consider the motion used to establish the SHM system, or the motion as soon as the system is released you should derive the same equation.

DISPLACEMENT BELOW EQUILLBRIUM.
However, consider a verticle spring with particle attached hanging in equilibrium initially, displaced downward and subject to resistance.

-Considering intial conditions, acceleration is down => resistance is up, T is up (as spring extended), and mg down.
- However considering the moment the system is released, acceleration is up => resistance is down, T is up, mg is down.

The correct solution is the equation derived following the initial conditions.

(I note that for the conditions after, I have taken T as up, but this must only hold until length l is again reached, where compression may take over depending upon the relative elastic and gpe potential models).
(However I still do not understand why exactly conditions 2 would not mean in line to establish a valid shm equation, as some motion upward should still occur).

DISPLACEMENT ABOVE EQUILIBIRUM:

A system is initially in equilbrium it is the displaced in the direction upward from equilibrium - the displacement is such that the length of the spring is less than l. (for this example, l + e = 60 + 11.8cm) and displacement is 15cm.

- This time, considering conditions after release, leads me to dervie the correct equation. The particle will accelerate downward, mg acts downward and T acts upward.
- However, considering intial conditions, acceleration is upward, T would be downward (spring is now compressed) and mg would be downward.

Any assitance, immensely appreciated, thank you. =] ]]> Introductory Physics binbagsss http://www.physicsforums.com/showthread.php?t=606493 Differentiate De Broglies Theorem http://www.physicsforums.com/showthread.php?t=606490&goto=newpost Wed, 16 May 2012 16:35:31 GMT 1. The problem statement, all variables and given/known data

Someone has written down in some notes about differentiating DeBroglies Theorem.

They've started with Px =h/λ
(That's P sub x)

Then they've simply said, this differentiated is:

ΔPx = [-h/(λ^2)]Δλ

How have they done this? What method? What have they differentiated with respect to?

Thank you!

2. Relevant equations



3. The attempt at a solution ]]> Introductory Physics ZedCar http://www.physicsforums.com/showthread.php?t=606490 Determining the Radius of the Moon (Ratio and shadow measurements) http://www.physicsforums.com/showthread.php?t=606483&goto=newpost Wed, 16 May 2012 16:06:50 GMT Hello. I have been going through some old OU books on the sciences and started working through an excercise when I came to a stumbling block. I have come here in the hope that someone may be able... Hello.

I have been going through some old OU books on the sciences and started working through an excercise when I came to a stumbling block. I have come here in the hope that someone may be able to shed some light on my problem. This is all for personal study as I am not enrolled on any courses in the subject and just wish to expand my mind.

1. The problem statement, all variables and given/known data

The exercise I am going through is part of an old OU science module that belonged to my mother. Unfortunantely she no longer has the videos which I need to explain to me what I need to do next to correct my results.

Basically the exercise makes the assumption that the shadow cast by the earth on to the surface of the moon during a solar eclipse is the same size as the earth itself. Now whilst I have an idea why this is not the case the assumption stands through most of the exercise in order to show a method of calculation to the student.

Through the use of the photos of the moon showing different phases of a partial eclipse I am asked to make measurements to estimate the radius of the earth in the photograph.

Before I explain any more I would like to show everything I have calculated thus far in the exercise.

(Exact correct figures may not be used during the exercise as the learning experience is more to do with the approach than the results)

2. Relevant equations


3. The attempt at a solution

The radius of the Earth's shadow in the photograph is:

less than 7.2 cm (upper limit)
more than 6.4 cm (lower limit)

Radius of Earth's shadow in the photograph (RE) = 6.80cm +/- 0.40cm

Radius of the Moon in the photograph = (RM) 2.75cm +/- 0.1cm

Therefore the Radius of the Earth is 2.47 times bigger than the radius of the moon.

RE = 2.47 RM

Limits of Uncertainty

Max Ratio = 7.2/2.65 = 2.72
Min Ratio = 6.4/2.85 = 2.25

Therefore RE = (2.47 +/- 0.22) RM

Now RE = 2.47 RM (Text book uses a figure of 6200km for earths radius)

So 6200/2.47 = RM

RM = 2510

I then calculate the limits of uncertainty for distance using the earlier numbers and get:

RM = 2510 +/- 231 km

This is the extent of my working out

Having gotten this far through the exercise I read the following:

"One word of caution about this calculation of RM: throughout Section 3 [the above] you make the assumption that the shadow of the Earth at the Moon is the same size as the Earth itself. As you will see in the TV programme, this is an unjustified assumption that leads to a considerable error in the value obtained for RM. However, the programme also shows how to correct this error, so be prepared to adjust your value of RM after viewing."

Unfortunately I don't have this TV program and have been unable to obtain a copy or find anything similar avaliable. So I have come here in the hope that one of you brilliant minds can assist me in completeing this exercise by explaining to me what I am missing and how to correct my inital results. ]]> Introductory Physics BenMur http://www.physicsforums.com/showthread.php?t=606483 Electron in infinite well equation http://www.physicsforums.com/showthread.php?t=606482&goto=newpost Wed, 16 May 2012 16:05:44 GMT *1. The problem statement, all variables and given/known data* An electron is confined in an infinitely deep well of width 0.1nm, about the size of an atom. Estimate the energy of the ground state... 1. The problem statement, all variables and given/known data

An electron is confined in an infinitely deep well of width 0.1nm, about the size of an atom. Estimate the energy of the ground state in eV.

2. Relevant equations

Is this the equation I should be using?

E=(n^2 hbar^2 ∏^2)/(2m L^2)



3. The attempt at a solution ]]> Introductory Physics ZedCar http://www.physicsforums.com/showthread.php?t=606482 SUVAT Question http://www.physicsforums.com/showthread.php?t=606459&goto=newpost Wed, 16 May 2012 13:37:06 GMT Hi, i'm just practicing some SUVAT style questions as part of my revision and have a quick question.

Here's an example, a ball is catapulted vertically upwards at 10m/s, find the time it takes to hit the ground.(Discount air resistance)

I used S =0 U=10 m/s V=? A=-9.8 T=?

Using v =u +at gave me a time of 1.02s however this is wrong.

Is this because i've done this as one motion? Should I split it into to, ie find highest position then journey back down?

Thanks

Just to reiterate this isn't a homework question. ]]> Introductory Physics jendrix http://www.physicsforums.com/showthread.php?t=606459 Pressure at the bottom of cylinder immersed in two liquids http://www.physicsforums.com/showthread.php?t=606442&goto=newpost Wed, 16 May 2012 12:17:29 GMT *1. The problem statement, all variables and given/known data* A solid cylinder has base area A, height 20 cm and density 0.8 g/cm3, floats in the boundary of oil and water. If the density of oil... 1. The problem statement, all variables and given/known data
A solid cylinder has base area A, height 20 cm and density 0.8 g/cm3, floats in the boundary of oil and water. If the density of oil 0.6 g/cm3, find
a. the height of cylinder that immersed in oil and water
b. the hydrostatic pressure at the bottom of the cylinder if the height of oil = 3 times height of cylinder immersed in water


2. Relevant equations
P = ρgh
W = mg
Fa = ρgV


3. The attempt at a solution
a. I am able to do this one. I got the height that immersed in each liquid = 10 cm

b. So the height of oil = 30 cm. How to find the hydrostatic pressure at the bottom of cylinder? Does the oil give pressure for the bottom part?

Water will gives pressure at the bottom which is equal to ρgh = 1000 x 10 x 0.1 = 1000 Pa

I do not know what to do next...I even don't know whether 1000 Pa is the final answer or not...

Thanks ]]> Introductory Physics songoku http://www.physicsforums.com/showthread.php?t=606442 Displacement of an object in circular motion http://www.physicsforums.com/showthread.php?t=606432&goto=newpost Wed, 16 May 2012 11:38:40 GMT *1. The problem statement, all variables and given/known data* A1. An object undergoing circular motion is pictured below at time t = 0 s. (a) What is the angular frequency of the object? (b) What... 1. The problem statement, all variables and given/known data
A1. An object undergoing circular motion is pictured below at time t = 0 s.
(a) What is the angular frequency of the object?
(b) What is the equation of the displacement of the object in the x-direction?
(c) What is the equation of the displacement of the object in the y-direction?
(d) What is the y-displacement of the object at t = 0 s?
(e) What is the first time after t = 0 s that y = 0?

I cant put the diagram on here but its a circle on x/y axis with the object in the lower right quadrant where θ=45° and v=2m/s and the r=2m

2. Relevant equations
T=2∏r/v
ω=2∏/T
x=Acos(ωT+δ)

3. The attempt at a solution
Ive managed to do part (a) using the equations above and got 1rad/s
However im not sure on the rest, i was thinking of using x=Acos(ωT+δ) but im not sure how

cheers for any advice ]]> Introductory Physics jami8337 http://www.physicsforums.com/showthread.php?t=606432 Water Evaporation (Thermodynamics) http://www.physicsforums.com/showthread.php?t=606426&goto=newpost Wed, 16 May 2012 11:12:07 GMT *1. The problem statement, all variables and given/known data* If the air is at100% relative humidity and a water surface is at the same temperature as that of the air above it, do any of the... 1. The problem statement, all variables and given/known data

If the air is at100% relative humidity and a water surface is at the same temperature as that of the air above it, do any of the water molecules leave their liquid state to become vapour?

2. Relevant equations

[itex]r=\frac{e}{e_s}[/itex]

Clausius-Clapeyron equation:

[itex]\frac{de_s}{dT}= \frac{1}{T} \frac{L_v}{V_v-V_w}[/itex]

3. The attempt at a solution

How can I use the Clausius-Clapeyron relation (or any other method) to determine whether the net evaporation is positive or negative?

If I rewrite the equation in terms of saturation vapor pressure, I don't know the values for volumes. So how will I be able to solve it?

There are other versions of this problem where the water surface may be warmer/colder or the relative humidity may be different. So, how can I approach such problems?
I could not see any worked examples anywhere online. So any help is greatly appreciated. ]]> Introductory Physics roam http://www.physicsforums.com/showthread.php?t=606426 Electromagnetic induction question http://www.physicsforums.com/showthread.php?t=606425&goto=newpost Wed, 16 May 2012 11:00:10 GMT *1. The problem statement, all variables and given/known data* img341.imageshack.us/img341/1338/scanpic0001a.jpg This is the picture. Sorry i cant add a link cause i do not have 10 posts. The... 1. The problem statement, all variables and given/known data

img341.imageshack.us/img341/1338/scanpic0001a.jpg
This is the picture. Sorry i cant add a link cause i do not have 10 posts.

The part that i dont understand is why the aluminium ring takes a longer time to fall when an alternating current is used

3. The attempt at a solution

Take this scenario. Lets say the alternating current is flowing in the coil, and a North pole is induced at the top of the coil and South pole induced at the bottom of the coil.

Since it is alternating, when the current in the coil decreases, the magnetic field and hence magnetic flux through the aluminium ring decreases since it is linked with a soft iron rod.

By lenz's law, the aluminium ring would want to oppose the change, thus a north pole is induced at the top and south pole induced at the bottom of the ring to increase the flux through the ring.

Since unlike poles attract, why does the ring take a longer time to fall?? I know when the current increases the aluminium ring will be repelled but since it is attracted when the current decreases why does it still take a longer time to fall?

Your help is greatly appreciated. Thanks! ]]> Introductory Physics aznking1 http://www.physicsforums.com/showthread.php?t=606425 Recession Speed - maximum http://www.physicsforums.com/showthread.php?t=606419&goto=newpost Wed, 16 May 2012 10:05:59 GMT *1. The problem statement, all variables and given/known data* *see attachment* *2. Relevant equations* *3. The attempt at a solution* 1. The problem statement, all variables and given/known data


see attachment
2. Relevant equations



3. The attempt at a solution

Now the question did not say what the wavelength was which is annoying.
BUT would this be a good way to work it out?

[itex] \frac {v}{c} = \frac{Δλ}{λ}[/itex]
Attached Images
File Type: png PhysicQ.png (14.9 KB)
]]> Introductory Physics jsmith613 http://www.physicsforums.com/showthread.php?t=606419