Thursday

October 23, 2014

October 23, 2014

Total # Posts: 292

**Materials Science**

A hollow cylinder with outer radius r0 and inner radius ri is loaded by a uniformly distributed load q across its length L. The beam is supported by a roller at its left end and a pin L5 from its right end. What is the maximum bending moment in the beam? Express your answer in...
*October 17, 2014*

**Materials Science**

A 4-point bending test is performed on the same beam, with symmetric loads of 5kN separated by 2m. Draw the bending moment and shear force diagrams for the beam. Then answer the following questions about the shear force V and bending moment M. What is the shear force in the ...
*October 17, 2014*

**Materials Science**

A 3-point bending test is performed on a simply supported beam of length L=4m and of rectangular cross-section, with width b and height h. For a central load of 10kN, draw the bending moment and shear force diagrams for the beam. Then answer the following questions about the ...
*October 17, 2014*

**Materials Science**

Calculate the ratio of the moments of inertia I1I2 of two beams of rectangular cross section of the same width, one with a height h and the other, made up of three thinner beams of height h/3. I1/I2 = ?
*October 16, 2014*

**Materials Science**

A 3m long balance beam is simply supported with an overhang at each end of a=0.5m. A gymnast weighing 50kg is standing a distance c to the right of support A and a distance b to the left of support B. How does the maximum bending moment in the beam depend on the distance c? ...
*October 16, 2014*

**Materials Science**

A thin-wall cylindrical tank has radius R, wall thickness t≪R and is subjected to an internal pressure p. The cylindrical wall of the tank is constructed by welding steel plates along 45∘ seams, Consider a small element of the cylindrical wall, and define local ...
*October 10, 2014*

**Materials Science**

At a point in a plate subjected to plane stress loading along its surface, the cartesian stress components acting on the x-face of an element oriented along the (x,y) frame are σx=40MPa and τxy=70MPa, The maximum allowable shear stress, to prevent yielding of the ...
*October 10, 2014*

**Materials Science**

In this problem, you will construct the 2D Mohr's circle for two plane stress states defined in terms of the Cartesian components of the stress tensor in the x,y reference frame: [ σx,σy,τxy]. For each of these stress states, you must construct and use the ...
*October 10, 2014*

**Materials Science**

A compression bar, having a square cross section of side length b, must support a compressive load of magnitude W, uniformly distributed over the bar cross section. The bar is made of two segments connected by a glued joint along plane pq, which is at an angle α to the ...
*October 10, 2014*

**Algorithms**

Arrange the following functions in increasing order of growth rate (with g(n) following f(n) in your list if and only if f(n)=O(g(n))). a)2^2^n b)2^n^2 c)n^2 * log(n) d)n e)n^2^n
*October 9, 2014*

**Networks**

Consider the "Hybrid Model" described in lecture 3.4, with m=10. Let us compare the cases of a=0.8 and a=0.2. At date t=10,000, consider a node born at date i=20 and a node born at date i=9999. Recall that the approximation of the expected degree at time t of a node ...
*October 9, 2014*

**Networks**

Consider the "Preferential Attachment Model" described in lecture 3.3, with m=10. At date t=40, what are: (1) the expected degree for node 15 (born at time i=15), and (2) the expected degree for node 30 (born at time i=30)? [Hint: Use the approximation for the ...
*October 9, 2014*

**Networks**

Consider the model of "Growing Random Networks" described in lecture 3.1 (so the first model where links are added to existing nodes uniformly at random), with m=10. At date t=40, what are: (1) the expected degree for node 15 born at i=15, and (2) the expected degree...
*October 9, 2014*

**Physics**

Which of the following processes is used in order to fabricate monocrystalline silicon? Note that more than one correct answer is possible. The Czochralski process Silicon ribbon process The float-zone process The silicon casting process
*October 8, 2014*

**Physics**

The width of the space charge region at a p-n junction is reduced by applying a voltage bias over the p-n junction. What is the correct statement on both the bias voltage and the p-n junction? The voltage is a forward bias and the diffusion of the majority charge carriers ...
*October 8, 2014*

**Logic**

Consider a propositional language with three propositional constants - mushroom, purple, and poisonous - each indicating the property suggested by its spelling. Using these propositional constants, encode the following English sentences as Propositional Logic sentences. Use ~ ...
*October 6, 2014*

**Big Data**

Suppose ABC is a frequent itemset and BCDE is NOT a frequent itemset. Given this information, we can be sure that certain other itemsets are frequent and sure that certain itemsets are NOT frequent. Other itemsets may be either frequent or not. Which of the following is a ...
*October 6, 2014*

**Big Data**

Imagine there are 100 baskets, numbered 1,2,...,100, and 100 items, similarly numbered. Item i is in basket j if and only if i divides j evenly. For example, basket 24 is the set of items {1,2,3,4,6,8,12,24}. Describe all the association rules that have 100% confidence. Which ...
*October 6, 2014*

**Big Data**

Suppose we want to assign points to whichever of the points (0,0) or (100,40) is nearer. Depending on whether we use the L1 or L2 norm, a point (x,y) could be clustered with a different one of these two points. For this problem, you should work out the conditions under which a...
*October 6, 2014*

**Big Data**

The edit distance is the minimum number of character insertions and character deletions required to turn one string into another. Compute the edit distance between each pair of the strings he, she, his, and hers. Then, identify which of the following is a true statement about ...
*October 6, 2014*

**Big Data**

Suppose our input data to a map-reduce operation consists of integer values (the keys are not important). The map function takes an integer i and produces the list of pairs (p,i) such that p is a prime divisor of i. For example, map(12) = [(2,12), (3,12)]. The reduce function ...
*October 6, 2014*

**Physics**

Let's assume that solar light reaches a silicon solar cell with an angle of incidence of θi=0o. For simplicity, let's consider the refractive index of silicon to be nSi=3.5. The refractive index of air is nair=1. What percentage of light would be lost due to ...
*October 3, 2014*

**Physics**

A solar cell with dimensions 12cm x 12cm is illuminated at standard test conditions. The cell presents the following external parameters: Voc=0.8V,Isc=3A,Vmp=0.75V,Imp=2.5A Calculate the fill factor in percentage [%]: Calculate the efficiency (in %):
*October 3, 2014*

**Physics**

The current density of an ideal p-n junction under illumination can be described by: J(V)=Jph−J0(e^qV/kT−1) where Jph is the photocurrent density, J0 the saturation current density, q the elementary charge, V the voltage, k the Boltzmann's constant, and T the ...
*October 3, 2014*

**Physics**

The width of the space charge region at a p-n junction is reduced by applying a voltage bias over the p-n junction. What is the correct statement on both the bias voltage and the p-n junction? The voltage is a forward bias and the diffusion of the majority charge carriers ...
*October 3, 2014*

**Materials Science**

εx= εy= εz= γyz= γxz= γxy
*October 1, 2014*

**Materials Science**

A cube of the isotropic, linear elastic epoxy resin E=2GPa, ν=0.3, α=50×10−6K−1 at 20∘C is unloaded. It is then heated to 60∘C. What is the corresponding strain matrix? The temperature is held constant at 60∘C. What stress is ...
*October 1, 2014*

**Materials Science**

An isotropic epoxy resin (E=2GPa, ν=0.3) is reinforced by unidirectional glass fibers (Eglass=70GPa), aligned in the 2 direction, such that the fiber composite is transversely isotropic, with the 1-3 plane being the plane of isotropy. The elastic constants of the fiber ...
*October 1, 2014*

**Materials Science**

εx= εy= εz= γyz= γxz= γxy
*October 1, 2014*

**Materials Science**

A linear elastic, isotropic epoxy resin with elastic modulus, E=2GPa, and Poisson’s ratio, ν=0.3, is loaded by the following stress state: σ=⎡⎣10 5 0 5 −20 0 0 0 0⎤⎦MPa Give numerical values for all strain components in the ...
*October 1, 2014*

**Materials Science**

Rubber particles, roughly 1mm in diameter, have been added to an epoxy resin to increase its resistance to crack growth. Estimate the Young's modulus of the composite, given that the Young's moduli of the epoxy and the rubber are, respectively, 2GPa and 10MPa, and that...
*October 1, 2014*

**Materials Science**

Shear modulus G (in GPa):???
*October 1, 2014*

**Materials Science**

Aluminum is a cubic crystal, with S11=1.59×10−11 m2/N S12=−0.58×10−11 m2/N S44=3.52×10−11 m2/N What are the Young's modulus, Poisson's ratio and shear modulus, corresponding to these values? Young's modulus E (in GPa): ...
*October 1, 2014*

**Materials Science**

thx
*October 1, 2014*

**Materials Science**

-8.4 *10^-5 wrong!
*October 1, 2014*

**Materials Science**

A cylindrical pressure vessel, with a diameter of 40cm and a wall thickness of 10mm is made of steel (isotropic, with Young’s modulus = 200GPa; Poisson’s ratio = 0.28) and is pressurized to 20MPa. The longitudinal and hoop stresses in the wall, σl and σh, ...
*October 1, 2014*

**Material Sciences**

The molar crosslink density of a rubber is measured to be 468mol/m3 for a temperature T=300K. What is the Young's modulus of the rubber in MPa? What happens to the modulus of the rubber when the temperature is increased? Young's modulus decreases Young's modulus ...
*September 28, 2014*

**Material Sciences**

Imagine a container is removed from a hot oven and plunged into cold water. Use your answer from Question 3 to justify which material would be more likely to have better thermal shock resistance. Glass: α=9∗10−6C−1 Pyrex: α=4∗10−6C−...
*September 28, 2014*

**Material Sciences**

A steel bar of length L and cross-sectional area A is fitted between rigid supports. Write an expression for the stress in the bar when the temperature is raised by an amount ΔT. Write your answer in terms of α,E and ΔT. Is the stress compressive or tensile? ...
*September 27, 2014*

**Material Sciences**

Given an aluminum shell perfectly bonded to a steel core, what is the thermal stress in the aluminum shell when the system is heated from the unstressed state (20∘C) to 180∘C? Consider only deformations in the axial directions. Give your answer in MPa αAl=23.6...
*September 27, 2014*

**Material Sciences**

The energy-separation curve for two atoms, a distance, r, apart is: U(r)=−A/r^m+B/r^n Derive and expression for the stiffness of the bond at the equilibrium spacing, in terms of A, B, m, n, and r0. S=dF/dr at r=r0:
*September 27, 2014*

**Material Sciences**

The energy-separation curve for two atoms, a distance, r, apart is: U(r)=−A/r^m+B/r^n Derive an expression for the equilibrium spacing, r0, as a function of A, B, m, and n.
*September 27, 2014*

**Material Sciences**

Rubber particles are sometimes dispersed in an epoxy matrix to increase its resistance to the propagartion of cracks. Rubber has a Young's modulus of Erubber=0.001GPa. For a composite with 15 volume percent rubber particles uniformly dispersed in an epoxy matrix, give ...
*September 20, 2014*

**Material Sciences**

A unidriectional, continuously reinforced glass fiber composite has Vf=60 volume percent fibers in a matrix of epoxy. Glass has a Young's Modulus of Eglass=70GPa; epoxy has a Young's Modulus of Eepoxy=2GPa (a) Give numerical values for (i) the Young's Modulus for ...
*September 20, 2014*

**Material Sciences**

The boron fibers are produced by chemical vapor deposition of boron onto fine tungsten wires of dW=10μm. The final wire diameter is dB=75μm. The boron coated wires are then added to an aluminum matrix with VW+B=0.35 fiber volume fraction. The Youngs Moduli of the ...
*September 20, 2014*

**Acoustic**

Consider a room that is 6m long, 5m wide and 3m high and whose walls, floor and ceiling have the same sound absorption coefficient (α=0.01). When all machines are in use, the sound pressure level (SPL) within the room amounts to 90 dB. A carpet is to be placed on the ...
*September 11, 2014*

**Acoustic**

How would you expect the directivity of the piston to vary as the temperature of air decreases? (Hint: Low temperature -> decrease of sound speed) 1.Directivity of the piston is independent of the temperature. 2.Directivity of the piston becomes smooth (low directivity) ...
*September 11, 2014*

**Acoustic**

A breathing sphere of radius 0.1m radiates spherical waves into air at 100Hz with an intensity of 200mW/m2 at the distance 0.5m from the center of the sphere. In this case, 1) Compute the radiated acoustic power. 2) Compute the intensity at the surface of the sphere. 3) ...
*September 11, 2014*

**Music**

List one of the vocal technology topics and describe how it has changed and/or affected popular music. 1. Time Based Effects Processing 2. Autotune or Melodyne 3. Talkbox or Vocoder 4. Vocal Sampling 5. Elastic Audio and Flex
*July 14, 2014*

**Astronomy**

The current peak wavelength of the CMB corresponds to a blackbody of temperature 2.74K. What was the peak wavelength of the CMB when it was emitted? The current redshift of the CMB is z=1100. Give your answer in microns. Which waveband regime is this?
*June 24, 2014*

**Astronomy**

What about a for a supernova in an elliptical galaxy of 10^12 solar masses and radius of 50 kpc? Yes or No
*June 24, 2014*

**Astronomy**

Supernovae can eject material at velocities of 0.001c. Will gas ejected by a supernova which occurs inside a dwarf galaxy with a mass of 10^8 Msolar and a radius of a few kpc be able to escape the galaxy? Yes or No
*June 24, 2014*

**Astronomy**

The star Polaris has a surface temperature of roughly 6500 K and a radius of approximately 50 solar radii. Find the luminosity of Polaris in terms of the Sun's luminosity.
*June 24, 2014*

**Astronomy**

A galaxy is receding away from us with a recession velocity of 100,000 km/s. What is the distance to this galaxy? What is the redshift z of this galaxy?
*June 24, 2014*

**Astronomy**

A quasar is observed at a redshift of z=1, which corresponds to a comoving radial distance of 3 Gpc. Its observed bolometric flux is 10^{-12} erg/cm^2/s. An interacting companion galaxy is seen 4 arcsec apart in projection. What is the bolometric luminosity of the quasar, in ...
*June 19, 2014*

**Maths Probability**

3) 0.0625
*May 20, 2014*

**Probability**

A dedicated professor has been holding infinitely long office hours. Undergraduate students arrive according to a Poisson process at a rate of per hour, while graduate students arrive according to a second, independent Poisson process at a rate of per hour. An arriving ...
*May 13, 2014*

**Probability**

Uncle Henry has been having trouble keeping his weight constant. In fact, during each week, his weight changes from the beginning of the week to the end of the week by a random amount, uniformly distributed between -0.5 and 0.5 pounds. Assuming that his weight change during ...
*May 13, 2014*

**Probability**

Suppose that you move to a new house and you are 10% sure that your new house's phone number is 561290. To verify this, you use the house's phone to dial 561290, obtain a busy signal, and conclude that this is indeed your phone number. (Suppose that you were incorrect...
*May 13, 2014*

**Astronomy**

Imagine that you have found an object out in space. When you use the inverse square law to compute its luminosity, you get a ridiculously large value. You therefore suspect that relativistic beaming is at work - the radiation is coming from a jet traveling at speed v directly...
*May 11, 2014*

**Astronomy**

Imagine that you are an astronomer, and you are planning to do a survey to find new brown-dwarf stars. You need to calculate how sensitive your search needs to be (and hence how big a telescope you will need and how long the survey will take). You want to find ten times more ...
*May 11, 2014*

**Astronomy**

Imagine that you are a particle physicist, and you have come up with a theory that some gamma-ray bursts come from the decay of a new and exotic sub-atomic particle that you think might exist. You calculated that if this particle really does exist and decay as you predict, ...
*May 11, 2014*

**Astronomy**

The luminosity of the Sun is 4*10^33 erg/s, and its radius is 7*10^10 cm. What is the Sun's effective temperature? Please enter your answer in units of Kelvin.
*May 6, 2014*

**Astronomy**

black hole of mass 2 * 10 ^ 30 kg
*May 4, 2014*

**Astronomy**

Imagine you are in a spaceship one million km from a black hole of mass kg. Initially you are flying directly away from it at a velocity of 16.9 km/s. Using the n-body approximation, what is your velocity one second later? Type your answer in metres per second.
*May 4, 2014*

**Astronomy**

The luminosity of the Sun is 4*10^33 erg/s, and its radius is 7*10^10 cm. What is the Sun's effective temperature? Please enter your answer in units of Kelvin. What is the Sun's effective temperature? Please enter your answer in units of Kelvin. - unanswered What is ...
*April 29, 2014*

**Astronomy**

Choose one answer for each question How old is the universe? 6,000 years old 380,000 years old 13.8 billion years old 13.8 million years old A parsec is: Longer than a light year The distance between the Earth and the Sun Approximately 3x10^24 cm None of the above The distance...
*April 29, 2014*

**Probability**

We are given the following statistics about the number of children in the families of a small village. There are 100 families: 10 families have no children, 40 families have 1 child each, 30 families have 2 children each, 10 families have 3 each, and 10 families have 4 each. ...
*April 29, 2014*

**Probability**

All ships travel at the same speed through a wide canal. Each ship takes days to traverse the length of the canal. Eastbound ships (i.e., ships traveling east) arrive as a Poisson process with an arrival rate of ships per day. Westbound ships (i.e., ships traveling west) ...
*April 29, 2014*

**Probability**

In parts 1, 3, 4, and 5 below, your answers will be algebraic expressions. Enter 'lambda' for and 'mu' for . Follow standard notation. 1. Shuttles bound for Boston depart from New York every hour on the hour (e.g., at exactly one o'clock, two o'clock, ...
*April 29, 2014*

**Probability**

Fred is giving away samples of dog food. He makes visits door to door, but he gives a sample away (one can of dog food) only on those visits for which the door is answered and a dog is in residence. On any visit, the probability of the door being answered is 3/4 , and the ...
*April 29, 2014*

**Astronomy**

Another possible energy source would be gravity. Imagine that the strange object contains a black hole of mass mbh. You drop a star of mass m into it. If the star of mass m starts at rest at infinity, what will the kinetic energy of the star be when it crosses the event ...
*April 28, 2014*

**Astronomy**

One possible energy source for the mysterious object you've just discovered is the fusion of hydrogen to form carbon-12 (the most common and stable isotope of Carbon). Hydrogen has an atomic mass of 1.00794 u, while carbon has an atomic mass of 12.010 u. If you fuse ...
*April 28, 2014*

**Astronomy**

Imagine that a rival astronomer proposes that your mysterious object is actually the mouth of a wormhole to another dimension. The rival astronomer calculates that the wormhole mouth would need to be 10^13 m in diameter and would last for around a thousand years. Which one or ...
*April 28, 2014*

**Astronomy**

Imagine that you have just discovered a strange object. This object is a distance D away, has a redshift z, and you measure a flux for it f. Derive an equation for the luminosity L of this object.
*April 28, 2014*

**Probability**

For any given flight, an airline tries to sell as many tickets as possible. Suppose that on average, 10% of ticket holders fail to show up, all independent of one another. Knowing this, an airline will sell more tickets than there are seats available (i.e., overbook the flight...
*April 27, 2014*

**Astronomy**

A roughly spherical asteroid with a diameter of 1 km, and a mean density of 4 g/cm^3, hits the Earth with a relative speed 1.5 times the orbital velocity of the Earth. Compute the amount of energy released upon the impact. Compare that with the energy released by a 10-Megaton ...
*April 25, 2014*

**Probability**

Let X be a continuous random variable. We know that it takes values between 0 and 3 , but we do not know its distribution or its mean and variance. We are interested in estimating the mean of X, which we denote by h . We will use as a conservative value (upper bound) for the ...
*April 21, 2014*

**Probability**

The random variables X1,..,Xn are independent Poisson random variables with a common parameter Lambda . Find the maximum likelihood estimate of Lambda based on observed values x1,...,xn.
*April 21, 2014*

**Probability**

For any given flight, an airline tries to sell as many tickets as possible. Suppose that on average, 10% of ticket holders fail to show up, all independent of one another. Knowing this, an airline will sell more tickets than there are seats available (i.e., overbook the flight...
*April 21, 2014*

**Probability**

Let Sn be the number of successes in n independent Bernoulli trials, where the probability of success for each trial is 1/2 . Provide a numerical value, to a precision of 3 decimal places, for each of the following limits. 1. lim P (n/2 - 20 < Sn < n/2 + 20) 2. lim P (n/...
*April 21, 2014*

**Astronomy**

In the past few million years of Mars the main source of climate variation has been the slow escape of the atmosphere. Probably true Maybe Probably false
*April 21, 2014*

**Astronomy**

Sometimes a crater from the Noachian period can be found inside of a crater from the Amazonian period. Probably true Maybe Probably false.
*April 21, 2014*

**Astronomy**

The northern plains of Mars have many characteristics that you would expect from their being an ancient ocean floor. Probably true Maybe Probably false
*April 21, 2014*

**Astronomy**

Rain was once common on Mars. Probably true Maybe/unclear Probably false
*April 21, 2014*

**Astronomy**

Rank the following hypothetical planets -- all of which have the same total mass and same radius -- from lowest moment of inertia to highest moment of inertia: 1) A uniform sphere of mixed up rock and iron 2) A hollow sphere 3) A rocky planet with an iron core 4) A rocky ...
*April 21, 2014*

**Astronomy**

Which of the following statements is correct? (check all that apply) If liquid water is not stable on the surface of Mars, liquid CO2 is definitely not stable. If liquid CO2 is not stable on the surface of Mars, liquid water is definitely not stable. If liquid water is stable ...
*April 21, 2014*

**Probability**

a=? b=?
*April 20, 2014*

**Probability**

θˆLMS= ?
*April 20, 2014*

**Probability**

Random variables X and Y are distributed according to the joint PDF fX,Y(x,y) = {ax,0,if 1≤x≤2 and 0≤y≤x,otherwise.} 1. Find the constant a. 2. Determine the marginal PDF fY(y). (Your answer can be either numerical or algebraic functions of y). For 0&#...
*March 18, 2014*

**Probability**

Let X and Y be normal random variables with means 0 and 2, respectively, and variances 1 and 9, respectively. Find the following, using the standard normal table. Express your answers to an accuracy of 4 decimal places. 1. P(X>0.75)= 2. P(X≤−1.25)= 3. Let Z=(Y&#...
*March 18, 2014*

**Probability**

Consider the following joint PMF of the random variables X and Y: pX,Y(x,y)={1/72⋅(x2+y2),if x∈{1,2,4} and y∈{1,3}, 0, otherwise}. 1. P(Y<X)= 2. P(Y=X)= 3. Find the marginal PMF pX(x). pX(1)= pX(2)= pX(3)= pX(4)= 4. Find E[X] and E[XY]. E[X]= E[XY]= 5. var...
*March 7, 2014*

**Probability**

We start with a collection of n (distinguishable) individuals whom we want to split into three possible teams: the blue team, the red team, and the white team. Each individual is randomly assigned to exactly one of the teams according to the following procedure. An individual ...
*March 7, 2014*

**Probability**

To get a complete collection of stamps, Ann needs to have 5 specific (and different) kinds of stamps, denoted by k1,…,k5. She starts out with no stamps. She then receives 5 stamps randomly from a lottery. Each of these 5 stamps has probability 1/5 of being of any ...
*March 7, 2014*

**Probability**

Alice has five coins in a bag: two coins are normal (i.e., fair with one face Heads and the other face Tails), two are double-headed (i.e., both sides are Heads), and the last one is double-tailed (i.e., both sides are Tails). She reaches into the bag and randomly pulls out a ...
*March 7, 2014*

**Probability**

For each of the following statements, determine whether it is true (meaning, always true) or false (meaning, not always true). Here, we assume all random variables are discrete, and that all expectations are well-defined and finite. Let X and Y be two binomial random variables...
*March 4, 2014*

**Astronomy**

What is the estimated total lifetime for a star with mass 0.7 Msun ? (in billions of year)
*March 4, 2014*

**Astronomy**

The coolest star whose peak blackbody wavelength can be observed with the human eye (the exact range of visible light varies from publication to publication, so use 380nm to 750 nm).
*March 4, 2014*

**Astronomy**

How close to the sun Ganymede would have to be in order to hold water in its atmosphere. (in AU)
*March 4, 2014*

**Astronomy**

A newly discovered planet has a surface temperature of 330 K and a density of 6 g/cm3. What minimum radius does the planet need to have in order to hold water in its atmosphere indefinitely? (in km)
*March 4, 2014*