Physics Help Forum

Oscillation Problem

HeatherN — Wed, 09 Dec 2009 15:26:27 GMT

Problem: A mass is oscillating on a spring with a period of 1.85 s. At t = 0 the mass has zero speed and is at x = 7.10 cm. What is the magnitude of the acceleration at t = 3.85 s?

I tried:
w=2pi/T= 3.396 rad/s

a(t)= -w^2*A*cos(wt+phase constant)
since phase constant = 0 because v initial is 0:

a(t)= -(3.396)^2*0.0710*cos(3.396*3.85)= -0.715m/s^2

But that's wrong. I'm assuming I can't just plug the 3.85s into the equation? But I'm not sure what else to do. Where did I go wrong?

simply supported at both ends round beam

JAW — Wed, 09 Dec 2009 15:00:55 GMT

I've been working on this problem for 3 days now and i now pulling my hair out.. Can someone please help..

The question comes in to parts, and i'm resonably happy with part 1.

Part 1. A round beam is simply supported at both ends and carries a unformly distributed load over its entire top surface. Determine the maximum load (N/m) such that the radius of curvature does not exceed 23m. Assume that the diameter is 200mm the length is 4m and the modulus of elasticity of wood is 10GN/m^2.

Answer:- By use of the formula E/R = M/I = O/y

Ixx of a circle = pye x D^4/64
=pye x 200^4/64
=78539816.34mm^4

M =EI/R
= 10x10^9 x 7.854x10^-5/23
= 34.148 kN/m

BM = Wl^2/8

W=Mx8/L^2
= 34.148x10^3 x8/4^2
=17.074 kN/m

i believe this to be correct up to this point. Please advise.

Part 2. It is decided to strengthen the beam by adding two 20mm diameter steel rods, one to the top surface and one to the bottm surfsce, each to run the full length of the beam. Dtermine the new load capacity for the same radius of curvature. Assume that the modulas of elasticity for steel is 210GN/m^2.

SMOA of 1 x steel rod as follows.

Ixx= pye d^4/64
=pye x 20^4/64
7853.982mm^4

using parallel axis therom Ioo = Ixx+ah^2

7853.982 + 2(pye x 10^2 x 110^2)
= 7610508.204mm^4

Where do i now go with the question.. PLEASE HELP!!!

Many thanks in advance.

Repair objects on moleculair level

Charles Dickens — Wed, 09 Dec 2009 13:36:15 GMT

Hello world,

Just a noob question, I'm not much into physics, but always marvel at the wonders of it :)
If you cut or break something (for example a straw) is it possible, theoretically, to attach/repair (rearrange/connect the molecules again as it was before) the two halves together without a trace where it was cut? Maybe something similar happens when you make a cut in your finger, it heals without a scar.
Hope it made some sense what I tried to explain here.

Regards,

C.D.

Satellites and gravity! Urgent! :(

masterice — Wed, 09 Dec 2009 04:09:27 GMT

Your job with NASA is to monitor satellite orbits. One day, during a routine survey,
you find that a 400 kg satellite is going to collide with a smaller 100 kg satellite travelling in the same
orbit but in the opposite direction. Knowing the construction of the two satellites you expect they will
become enmeshed into a single piece of space junk. When you notify your boss of the impending
collision, he asks you to quickly determine whether the space junk will continue to orbit or crash into
Earth.

my work so far....

(Msat � v2) / R = (G � Msat � MCentral ) / R2

Where Mcentral is Mearth
I'm completely lost after this... im not sure where im headed either, I would be eternally grateful if someone could outline what is needed here... thanks!

"spinning pie plate sprouts" experiment

dannyg — Tue, 08 Dec 2009 22:06:32 GMT

This is an experiment my high school physics teacher actually performed in 1974. It was a great experiment, but I can't remember how it turned out.

The Experiment
1) take a phonograph player, set it to 33 rpm. Turn it on, and leave it on for 2 weeks.

2) take a pie plate, fill it with moist earth.

3) plant in the pie plate a lot of seeds of alfalfa or some other fast sprouting plant.

4) place the pie plate on the spinning record player, and water it daily for 2 weeks until the seeds sprout.

Now the physics question that needs an answer: in what direction will the seeds sprout?

I loved this experiment, and it was one of the main influences that led me to become an electrical engineer. But I can't remember which direction the seeds sprouted.

Possible Solution
For what it's worth, here is my theoretical solution, which I am not sure is correct: the seeds will sprout up and pointed slightly towards the center of the pie plate. The reason is that the seeds "assume" they are at rest, and will sprout in the direction of the applied forces. Seeds normally sprout up because it is the direction of the force applied by the ground below the seed (to balance the force of gravity). The spinning motion would add another force vector, centripetal force, pointed towards the center of the pie plate. So the seed should sprout up and towards the center.

Anyone with a solution or comments on my solution, please speak up.

FS: Brand New Apple iphone 3G S 32GB @ Affordable Prices.

bizwilson — Tue, 08 Dec 2009 18:07:40 GMT

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Amount: $350
Condition: Brand New
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Contact Person: Wilson Dot

What's in the Box:

iPhone 3GS 32GB
Apple Earphones with Remote and Mic
USB power adapter
Dock connector to USB cable
Documentation
SIM eject tool
12-month manufacturers warranty

acceleration of electron in between charged plates

jawad khan — Tue, 08 Dec 2009 15:18:44 GMT

dear.if the x-component and y-component of velocity is given i.e Vx &Vy.and E (electric field) is also given. then what equation should i use to find acceleration of the electron.
i only know about E=F/e(relation of electric field and force)
F=ma.if i want to find the "acceleration" if Vx ,Vy and E is given??
anyone like to answer???

Moment of Inertia of a Quarter Disc

utsav55 — Tue, 08 Dec 2009 11:23:14 GMT

1. Find the Moment of Inertia of a Quarter Disc which has mass M and radius R about the axis passing through the center (of original disc) and perpendicular to the plane.
I found the Moment of Inertia (I) of a disc about the axis passing through the center, perpendicular to the plane and divided by 4. It gave the wrong answer.

So, plz tell me that why dividing by 4 is giving me the wrong answer for the quarter disc.

Also, a hint of how should I start the problem.

Energy

INeedHelp — Mon, 07 Dec 2009 22:51:01 GMT

1) A baby carriage is sitting at the top of a hill that is 22.1 m high. The carriage with the baby weighs 15.8 N. What is the Ep for the carriage and baby in relation to the bottom of the hill?

I know the equation for potential energy=m*g*h but what how would you do this if you have newtons rather than kilograms?

2) A roller coaster is at the top of a 98.2 m hill and has a mass of 1831.0 kg. It is traveling at a velocity of 23.7 m/s. What is the coaster car's TOTAL energy?

I used the eg=m*g*h equation and got a value of 1762081.16 .. i then used the ek=1/2mv^2 and got a value of 514227.195 .. for the total energy i added these 2 values and got a total of 6801507.671, is this correct?

3) A roller coaster car is at the top of a 53.8 m hill and has a mass of 1755.9. It is traveling at a velocity of 41.9. If the track is the only thing EXTERNAL to the system and the friction with the track removes 692.1 J of energy as lost energy, what is the system's TOTAL energy?

i used the eg=m*g*h and got a value of 925780.716 .. i then used the ek= 1/2mv^2 and got a value of 1541337.8 .. i added these together and got 2467118.516 and subtracted 692.1 to get a total energy of 2466426.416 .. is this the way to go about it and did i arrive at the right answer?

problem on volume flow rate

Book — Mon, 07 Dec 2009 22:12:41 GMT

A pump is used to empty a 6000 L wading pool. The water exits the 2.5 cm diameter hose at a speed of 2.1 m/s. How long will it take to empty the pool?

First thing I did was convert to meters
6000 L = 6 m^3 (I think LOL)
2.5 cm = .025 m

First equation I used is Av=Q .025*2.1=.0525 m^2/s The problem is that my book says that Q should be measured in m^3/s which will make sense if I continue but I'm getting m^2/s. What am I dong wrong?
A=cross-sectional area of tube
v= velocity
Q= volume flow rate

Next I would use Q=Volume/Time=> Time=volume/Q T=6m^3/.0525m^2/s

resonant frequency

van-hilst — Mon, 07 Dec 2009 22:08:18 GMT

A series RLC circuit with L = 30 mH, C = 5 �F, and R = 150 Ω is driven by a generator with a maximum emf of 60 V and a variable angular frequency ω. What is the rms current when ω = the resonant frequency?

power expended in the resistor

van-hilst — Mon, 07 Dec 2009 22:06:54 GMT

In the circuit shown, let E0 = 12 V, R = 6 Ω, and L = 0.5 H. The switch is closed at time t = 0. At time t = 4 s, what is the power being expended in the resistor

distance between spots

van-hilst — Mon, 07 Dec 2009 22:04:37 GMT

The widest aperture on my camera lens has a diameter of 7 cm. With it, I can just resolve the separation between two yellow spots on the leaves on a tree if the tree is 600 m away. What is the distance between the spots?

smallest overall image

van-hilst — Mon, 07 Dec 2009 22:03:42 GMT

A point source of white light is located 3 meters from the lens in the previous problem, on its axis. The locations of the point images are at different places for different colors because of chromatic aberration so that the combined image is a circle. If the aperture of the lens has a diameter of 2 cm, what is the diameter of the smallest overall image of the point source that the lens creates?

find magnitude of the magnetic field

van-hilst — Mon, 07 Dec 2009 22:02:13 GMT

A current I is uniformly distributed over the cross section of a long, straight wire of radius a = 2 mm. At the surface of the wire, the magnitude of the magnetic field is B = 8 mT. what is The magnitude of the magnetic field 5 mm from the axis ??