15
Jul
09

Friction (problem)

ABFrictionIn the figure, we have block B (with weight Wb) hanging from the pulley connected by a wire to block A (with weight Wa). If we were to let it start moving with constant speed, block B would go down and block A will move to the right. Friction is present between block A and the surface. We are tasked to find the coefficient of friction between block A and the surface.

(NOTE: We may isolate the system by parts. Here, I am taking the system as a whole)

First, we look at the forces involved.

  1. Weight of block B
  2. Tension (above block B)
  3. Tension (beside block A)
  4. Friction (on block A and surface)

Equations:

  • F = ma = 0 (Since we have constant velocity, our acceleration is zero.)

We have the weight of block B (assumed positive) which goes downward and tension going upward, tension going the opposite direction of the weight of block B, and friction going against the weight of block B. So our equation becomes

  • (mass of block B * gravity) – Tension(B) – Tension(A) – friction = 0

But wait! We lack tension. Recall that If we isolate block B and its tension, we can find for tension.

  • (mass of block B * gravity) – Tension(B) = 0
  • Or we can write this as Tension(B) = Weight(B)

Mass of block B times the gravity (or weight of block B) equals the tension of the string above block B. Also, Tension B equals Tension A (assuming the pulley is frictionless).

So our equation becomes

  • (weight of block B) – Tension (B) – Tension (B) – friction = 0
  • OR: weight of block B – weight of block B – weight of block B – friction = 0
  • But in this example, let us use (weight of block B) – (2 * Tension) – friction = 0

Friction equals  u(mew) times the normal force of block A. Thus,

  • (weight of block B) – (2 * Tension) – u*normal force of block A = 0

The coefficient of friction (u) can be taken by the equation

  • u = [(weight of block B) - (2*Tension)] / normal force of block A

By now, you should be able to get the coefficient of friction. Simply substitute the weights and tensions in the equations and you will arrive to your final answer.

—————-

If you are to accelerate the system (say block B going downward), you will need Newton’s Second Law of Motion. The acceleration for block B is similar to the acceleration of block A because we assume a frictionless pulley (but not a frictionless surface).

  • F = ma

In terms of the y-component (which only block B has),

  • (mass of block B * g ) – Tension = mass of block B * acceleration

In the x-component (block A),

  • Tension – friction = mass of block A * acceleration

The tension and friction are subtracted.  If they are added, then the motion will be going to the left instead of the right. Also, if we add them, the wire (tension A) will break.

Using the equation Fnet = ma = Fa + Fb

  1. Fnet = (mass of block A *acceleration) + (mass of block B *acceleration)
  2. Fb = (mass of block B * gravity) – Tension = mass of block B * acceleration
  3. Fa = Tension – friction = mass of block A * acceleration

As mentioned earlier, their acceleration is the same. By combining equations, we get

(mass of block A * acceleration) + (mass of block B * acceleration) = [Tension - friction] +
[(mass of block B * gravity) - Tension]

Or we can factor out acceleration

a (mass of block A + mass of block B) = T – f + weight of block B – T (where tension =0)

In getting acceleration,

a = [weight of block B - friction] / [mass of block A  +mass of block B]

By now, you should arrive at the proper value of acceleration. If you get a negative answer, it just means that it is decelerating.

12
Jul
09

RA3 (camstudio test)

played RA3 in window mode (-win)

original size: 157mb (.avi)
around 9+ minutes using MS MPEG-4 Video Codec V2
20 frames, 50 ms, 20 frames/second
key frame: 8
smooth-crisp: 70
data rate: 3000

audio off sync at the last part

conversion using AVC (Xvid): 27.3 mb (.mp4)

12
Jul
09

RA3 (fraps test)

I am trying out fraps, an on-screen video capturing software, where I am playing Red Alert 3 (attempting to rush). Video size was around 1.1 GB for 4 minutes (800×600)… converted using AVC to .mp4 (320×480, x264, 256 bit rate) and it became 10.8mb.

11
Jul
09

Headlines (Popsci.com)

04
Jul
09

chapter 3 (ip)

12 font size
Times New Roman (please use this font)
double spaced
3-5 pages (short bond paper)

may contain pictures, flowcharts, or diagrams in your theory & methodology (see sample below)

DEADLINE: Monday, July 6, 2009

04
Jul
09

Protected: ip chapter 3 sample

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01
Jul
09

Protected: Sample Footnoting

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27
Jun
09

Chapter 1 (IP)

12 font size, Times New Roman, double spaced, 2-5 pages (short bond paper)
  • Introduction
  • Statement of the Problem
  • Significant of the Study
  • Scope and Limitations
  • Definition of Terms

DEADLINE: Monday, June 29, 2009

27
Jun
09

PopSci Headlines

26
Jun
09

Hammock in the Philippines by Steven Cravis

Steven Cravis. I have been admiring his music for a while now (2 years?) and I still recall hearing his song for the first time, Hammock in the Philippines. It never fails to give me that calming effect I need everyday especially in my line of work. The song reminds me of my threshold in being patient… that I should give chances everyday to everybody. It also shows me a darker side of myself… a side I may not be able to hold out much longer. But the song helps keep me together. It helps me stay in control of what I do and what I say.

If you would like to listen or download some of his FREE tracks, you may do so here or visit his official site where you can also hear his music here.

23
Jun
09

Protected: p6 ip ideas; get inspired! (#05)

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Bryan King is a graduate of the undergraduate BS Physics program in Silliman University and was a member of the Junior Philippine Physics Society. His interests once lied in anime, online gaming, alternative music, and heavy dinners but now they're in science and technlogy, logic games & puzzles, classical & rock music, motorcycles, and side dishes. His thesis was on radiation of mangoes and is currently a teacher somewhere. He believes he was once a lost sound wave in the milky way.

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