Monday, October 11, 2010

A Basic Physics Test

One of the more annoying aspects of blogging on current topics that relate to physics, is seeing another blogger criticize a blog on some physics issue (whether quantum mechanics in general, or Stephen Hawking’s brane-based multiverse, or dark energy or the spontaneous inception of the cosmos) while never having been exposed to even basic physics themselves – which means their opinions aren’t worth a gallon of doggie lickspittle. Therefore, this basic test in physics is offered as means to test their mettle in terms of being qualified to comment on anything physics! Score less than 70% and be ruled out for credible commentary – or maybe stick to the Bible)

Time: 1 hr 30 mins. Maximum - NO Googling for answers!

1.A 1.8 m (5’ 11”) tall man tosses a ball from his height position (e.g. ball leaves his hand at level of the top of his head) to a hard board 2m high and 3.0 m distant. If the ball strikes the board at a height of 1.5 m, find: a) the initial speed of the ball; (b) the time of flight, and c) the speed when it hits the board. (Assume g = 10 m/s^2)

2. A rocket is fired at a launch angle of 30 degrees, and at an initial velocity of u = 500 m/s. Find: a) the horizontal range, b) the greatest altitude (h) reached, and c) the time taken to reach h. (Assume g = 10 m/s^2)

3. Find the acceleration with which a 2 kg mass accelerates down a frictionless inclined plane (angle of incline = 30 degrees) (Assume g = 10 m/s^2)

4. An air conditioner has a power rating of 1.0 kilowatt. A solar panel on the roof of a house has a collective efficiency of 12% and is 5 meters by 5 meters. The solar power (insolation) constant is 1360 watts per square meter. Assuming the ac will be run from 11 a.m. to 4 p.m. , will the panel be able to achieve the objective? Assume no cloud cover, and no significant variation in insolation. Show all working.

5. Two identical copper calorimeters of mass 0.1 kg contain 1 kg of water and 2 kg of alcohol, respectively. It’s found that they both take 60 minutes to cool from 350 K to 300 K under similar conditions. Find the specific heat of the alcohol (in J/ kg K) ) based on this information. (Assume c(H2O) = 4,200 J/kg K)

6. The diagram shows A and B in thermal contact.

a) Find the temperature of the system of two bodies in thermal equilibrium if T(A) is the initial temperature of A, and T(B) is the initial temperature of body B. Is this the same as the reading of A’s temperature? (Show work, explain)
b) Which body undergoes a positive change in entropy?
c) Which body undergoes a negative change in entropy?
d) What is the total entropy change for the system, A + B?

7. A particle of charge q = 3 x 10^-9 C moves as shown:

(a)x --------------------(d)----------------o q-------->x(b)

Where d = 0.5m

Assume the electric field E is uniform along the line ab and has a magnitude of 200 Volts/m. Find: a) the electric force on charge q, b) the work done on it by the E-field, and c) the potential difference: V(a) – V(b)

8. Estimate the effect of the Earth’s magnetic field on an electron beam in a conventional (cathode ray-based) TV tube. Suppose that the accelerating voltage = 20,000 V. Then calculate the approximate deflection of the beam over a distance d = 0.4m (e.g. from the electron gun to the screen) under the action of a transverse magnetic field of 0.00005 T.

9. In the principle of the current balance, a rectangular wire frame is connected to a pan balance holding a weight (mg) such that the wire frame (with current running through two parallel wires facing each other) causes a force that balances the mechanical weight. If the currents running through parallel 1m (= L) wires of the frame are I1, I2 respectively, and the separation between wires is r then we have for force balance:

F = mg = u(o) I1 (I2) L/ 2 pi r

Where u(o) is the magnetic permeability = 4 pi x 10^-7 H/m

Find the current needed to support a weight mg = 10N if the separation between the wires carrying the currents I1 and I2 is r = 0.10 m.

10. In an earlier blog:

We examined the basics of radioactive decay. Now, for the radioactive decay curve shown in the blog- sketch what will the given decay curve look like if the half life T(1/2) was 250,000 years?How much of an original sample of N(o) = 1 gram would remain after 3 million years? Work this out without using the graph!

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