Originally posted by Ramned 10. In astronomy, the temperature at the core of a star is given to be 1.5 X 10^7 degrees. How would you respond if an astronomer studying this star asked you if the temperature is Celsius of Kelvins? Explain.[/b]
I'd tell him it was either Rankine or Fahrenheit and that he'd have to guess which one.
Solve. Showing your work is not necessary, but I'd like to see it in case you have another route to solving it than I did. Use precise and necessary significant figures.
A van accelerates down a hill, going from rest to 30.0 m/s in 6.00 s. During the acceleration, an object (m = 0.100 kg) hangs by a string from the van's ceiling. The acceleration is such that the string remains perpendicular to the ceiling. Determine the tension in the string.
2 forces are acting on the mass, the tension T and its weight W = mg = 0.100 kg * 9.81 m/s^2 = 0.981 Newtons. The resultant force of these two forces must be accelerating the weight downhill along with the van at acceleration a = (30.0 m/s)/6.00 s = 5.00 m/s^2. So net force F = ma = 0.500 N.
Construct a right triangle. The lower left side of the triangle is the vector F, which = ma. The top left side is the tension vector T. The hypotenuse is the weight vector, pointing straight down. The vectors sum T + W = F. The geometry tells us that the magnitudes of the vectors are such that T^2 + F^2 = W^2, so T = the square root of (W^2 – F^2).
Putting the numbers in, T = square root of (0.981^2 – 0.500^2) = 0.844 N.
(The acceleration is constant since the problem states that the string REMAINS perpendicular to the ceiling. If the ceiling is parallel to the direction of motion, then the string is perpendicular to the ceiling, and the tension force in the string is perpendicular to the net force, which is in the direction of acceleration and motion.)
Originally posted by HolyT 2 forces are acting on the mass, the tension T and its weight W = mg = 0.100 kg * 9.81 m/s^2 = 0.981 Newtons. The resultant force of these two forces must be accelerating the weight downhill along with the van at acceleration a = (30.0 m/s)/6.00 s = 5.00 m/s^2. So net force F = ma = 0.500 N.
Construct a right triangle. The lower left side of the triangle ...[text shortened]... string is perpendicular to the net force, which is in the direction of acceleration and motion.)
Well - done. You were off a touch according to the answer I have - 0.843. Not too hard, was it?
That wraps up the first 10 - the easy bit.
Now, the intermediates, if you will.
11. Concrete has a higher specific heat than soil. Use this fact to (partially) explain why a city has a higher average temperature than the countryside around it. Would you expect the evening breezes to blow from city to country, or country to city? Explain.
Originally posted by Ramned Well - done. You were off a touch according to the answer I have - 0.843. Not too hard, was it?
Thanks! Using g = 9.80665 m/s/s, I get 0.843625 Newtons. Rounding to significant digits gives 0.844 N. Actually, some of those questions were a bit tough, including the last one. You must have some doozies in the next 5 sets of 10.
[b]11. Concrete has a higher specific heat than soil. Use this fact to (partially) explain why a city has a higher average temperature than the countryside around it. Would you expect the evening breezes to blow from city to country, or country to city? Explain.[/b]
I don't think specific heat is the reason. the plants of the countryside are evaporating water - that provides a huge cooling effect.
Since the city is warmer than the countryside, the breezed would, on average, vector into the city and upward drawn by the rising air from the concrete that is warming the air above it
Originally posted by coquette I don't think specific heat is the reason. the plants of the countryside are evaporating water - that provides a huge cooling effect.
Since the city is warmer than the countryside, the breezed would, on average, vector into the city and upward drawn by the rising air from the concrete that is warming the air above it
[b]11. Concrete has a higher specific heat than soil. Use this fact to (partially) explain why a city has a higher average temperature than the countryside around it. Would you expect the evening breezes to blow from city to country, or country to city? Explain.[/b]
THe countryside takes in CO2 and lets off oxygen. However, the city has pollutants and CO2 out the wazoo (leading to a greenhouse effect).
Heat is allowed to come and go more freely in the city from the reduction in albedo (thermal reflection).
Urban canyons also help to trap more radiation than an open countryside, so heat is also added from this part as well.
(This is just the first part. The second part is left to whoever wants it if I don't find out first.)
Well, the question is really about how the "specific heat" of concrete, being higher than that of soil, affects the average temperature. Specific heat is how much heat energy is needed to raise the temperature of the mass.
Since the specific heat of concrete is higher than soil, the average temperature of the city is higher because solar radiation has heated both the sol and the concrete, but the concrete has much more energy stored and radiates it back longer as the evening air cools.
i think that might be the answer that he's looking for, but i still don't think it's the right concept. I still go with plants and cooling from evaporation being the real reason that the country is cooler.
Originally posted by coquette Well, the question is really about how the "specific heat" of concrete, being higher than that of soil, affects the average temperature. Specific heat is how much heat energy is needed to raise the temperature of the mass.
Since the specific heat of concrete is higher than soil, the average temperature of the city is higher because solar radiation has he ...[text shortened]... with plants and cooling from evaporation being the real reason that the country is cooler.
Pardon me for not answering in exactly the right manner.
However these are all effects that contribute towards the city being warmer than the countryside.
Originally posted by coquette think that might be the answer that he's looking for, but i still don't think it's the right concept. I still go with plants and cooling from evaporation being the real reason that the country is cooler.
Plus the greater number of heat sources in the city.
You can use the specific heat thing to explain evning sea-breezes though (water having a higher specific heat than soil).
21 Sep '07 23:36
Anyone a Master of Physics?
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