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1. A boy throws a ball straight up in the air and then catches it again. Which of the above graphs best represents the ball’s position with respect to time?
You can usually answer classification questions a bit more quickly than the standard five-choice completion questions, since you only need to review one set of answer choices to answer a series of questions.
The answer to the question is B. The ball’s position with respect to time can be expressed by the equation y = –1/2 gt2, where g is the downward, acceleration due to gravity. As we can see, the graph of y against t is an upside-down parabola. In more intuitive terms, we know that, over time, a ball thrown in the air will rise, slow down, stop, and then descend.
2. A gas in a closed container is steadily heated over a period of time. Which of the following statements is true of this process?
(A) The average kinetic energy of the gas molecules decreases
(B) The mass of the container increases
(C) The pressure exerted by the gas on the walls of the container increases
(D) The gas changes phase into a liquid
(E) The specific heat of the gas decreases
The answer to this question is C. The key lies in remembering the ideal gas law: PV = nRT.
According to this formula, an increase in temperature is accompanied by an increase in pressure. A is wrong, since the average kinetic energy of gas molecules corresponds to their temperature: if the temperature increases, so does the average kinetic energy of the molecules. B is wrong because we’re dealing with a closed container: the mass cannot either increase or decrease. D is wrong because a gas must be cooled, not heated, to change phase into a liquid. Finally, E is wrong because the specific heat of any substance is a coonstant, and not subject to change. We’ll touch on all this and more in Chapter 9: Thermal Physics.