【AP物理C】【真题】2004年力学解答题答案

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2. Generally, double penalty for errors is avoided. For example, if an incorrect answer to part (a) is correctly substituted into an otherwise correct solution to part (b), full credit will usually be awarded. One exception to this may be cases when the numerical answer to a later part should be easily recognized as wrong, e.g. a speed faster than the speed of light in vacuum. 3. Implicit statements of concepts normally receive credit. For example, if use of the equation expressing a particular concept is worth one point, and a student’s solution contains the application of that equation to the problem but the student does not write the basic equation, the point is still awarded. 4. The scoring guidelines typically show numerical results using the value g = 9.8 m s 2 , but use of
AP® Physics C: Mechanics 2004 Scoring Guidelines
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AP PHYSICS C MECHANICS 2004 SCORING GUIDELINES
Question 1 15 points total (a) 2 points For any statement of conservation of energy Taking the zero of potential to be at the height of point B, and setting the kinetic energy at point B equal to the potential energy at point A: 1 m u 2 = m1 gL 2 1 B For the correct answer uB = 2 gL 1 point Distribution of points
1 point
(b)
4 points For any indication that there are two forces acting on the person For an indication that the acceleration of the person is centripetal, i.e. equal to u 2 r or u 2 L For a correct application of Newton’s second law that includes the two forces (tension T and weight) and a non-zero acceleration m u2 T - m1 g = 1 B r 2 mu T = 1 B + m1 g r For substitution of the expression for uB from part (a) and L for the radius 1 point 1 point 1 point
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AP PHYSICS C MECHANICS 2004 SCORING GUIDELINES General Notes about 2004 AP Physics Scoring Guidelines
1.
The solutions contain the most common method(s) of solving the free-response questions, and the allocation of points for these solutions. Other methods of solution also receive appropriate credit for correct work.
1 point
T =
m1 (2 gL ) + m1 g = 2 m1 g + m1 g L T = 3m1 g
3 points
(c)
For any statement of conservation of momentum For a correct application of conservation of momentum, with a clear indication that both masses have a common final velocity m1uB = (m1 + m2 ) uafter m1 uafter = u (m1 + m2 ) B For an answer in terms of the required quantities and of the form: m1 (answer from part(a)) (m1 + m2 ) m1 uafter = 2 gL m ( 1 + m2 )
10 m s 2 is of course also acceptable.
5.
Numerical answers that differ from the published answer due to differences in rounding throughout the question typically receive full credit. The exception is usually when rounding makes a difference in obtaining a reasonable answer. For example, suppose a solution requires subtracting two numbers that should have five significant figures and that differ starting with the fourth digit (e.g. 20.295 and 20.278). Rounding to three digits will lose the accuracy required to determine the difference in the numbers, and some credit may be lost.