Concept Questions submitted during the conference (and subsequently by July 4th)
Comments
Some of these questions might be considered to be "brainteasers" rather than true concept questions. However all of them require thought and for the moment none have been excluded. In the first 80 questions there were only two which were submitted twice!
Many of the questions need answers because I am not sure that I can do them correctly! (I certainly did not know the answer to Q69)..
There is at this stage a strong preponderance of questions relating to probability, logic, and to simple physical concepts such as mass, density, force and g.
Peter Goodhew (23rd June 2011)
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A: A single force vertically downward
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A: Remain constant (does the question need to specify the phase of the H2O as gaseous?)
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A: b, concentration stays constant (assuming solution is saturated)
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- A system consisting of a quantity of ideal gas is in equilibrium state "A". It is slowly heated and as it expands its pressure varies. It ends up in equilibrium state "B". Now suppose that the same quantity of ideal gas again starts in state "A" but undergoes a different thermodynamic process (i.e. follows a different path on a P-V diagram) only to end up again in the same state "B" as before. Consider the net work done by the system and the net heat absorbed by the system during these two different processes. Which of these statements is true?
- The work done may be different in the two processes but the heat absorbed must be the same;
- The work done must be the same in the two processes, but the heat absorbed may be different;
- The work done may be different in the two processes, and the heat absorbed may be different in the two processes;
- Both the work done and the heat absorbed must be the same in the two processes, but are not equal to zero;
- Both the work done and the heat absorbed by the system must be equal to zero in both processes.
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A: I'm not sure about this one. Is it (a)?
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- If atomic bonding in metal A is weaker than metal B, then metal A has:
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- What do these three processes have in common?
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Rust forming on iron nail
Water evaporating from a dish
A piece of candy dissolving in your mouth
a) The rate of change depends on the mass of the substance.
b) All three processes involve a change in phase.
c) All three processes are chemical reactions.
d) All three processes occur at the surface of the substance.
e) All three processes depend on the solubility of the substance. \ [Light et al\]
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A: All are surface phenomena (d)
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- You are in an elevator travelling upwards at constant velocity. Suddenly you drop your keys: It so happens that when they strike the floor they are at the same height above ground level as when they left your hand. The keys fall dead on the floor without bouncing. Make a single graph showing qualitatively the height above ground of both the keys and the elevator as a function of time, starting before the keys are released until after they strike the floor.
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A: the station is in free fall and the only force on it is directed towards the centre of the earth
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- A cylinder of water in a body of water has weight = height X pressure at area of bottom (W = ρAgh). A cone of water with the same bottom area and height should surely therefore weigh the same? Why not? \ [diagram needed\]
A: There are forces perpendicular to the "vertical" walls. In the case of the cylinder these cancel exactly. For the tapered shape there is a non-zero resultant in the vertical direction which cancels part of the bottom force.
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Windy, and not windy. Model planes are flown from post A to B, and back. Constant wind – if some – from B to A. Will the planes take more, less, or the same time to do the round trip with or without wind?
A:
- ...But can you float?
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- Red chair
On the ski slope, there is a little flat ¾ of the way up, where you like to stop for the sight. Alongside is the chair lift, with all blue chairs but one, which is red. Playful expectation, you stop on your little flat until the red chair comes by. But, but... Your statistics show you see the red chair going up much more often than it goes down. How can this be?
A: ??
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A ferry boat runs along a cable because the cross current is too strong. The motor breaks down. What can you do to go on ferrying along without a motor, and not touching or pulling on the cable?
A: ??
- So hot
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- Three switches individually connected to three matching floodlight projectors hundreds of meters away. You want to know what switches what, but cannot see the lights unless you walk to them. How can you do it in one and only one trip to the floodlights?
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