Ap07 physics b form b sg - College Board
AP? Physics B
2007 Scoring Guidelines
Form B
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AP? PHYSICS B (Form B)
2007 SCORING GUIDELINES
General Notes About 2007 AP Physics Scoring Guidelines
1.
The solutions contain the most common method of solving the free-response questions and the
allocation of points for this solution. Some also contain a common alternate solution. Other methods of
solution also receive appropriate credit for correct work.
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. However, when
students are asked to derive and expression it is normally expected that they will begin by writing one or
more fundamental equations, such as those given on the AP Physics exam equation sheet. For a
description of the use of such terms as ¡°derive¡± and ¡°calculate¡± on the exams, and what is expected for
each, see ¡°The Free-Response Sections?Student Presentation¡± in the AP Physics Course Description .
4. The scoring guidelines typically show numerical results using the value g = 9.8 m s2 , but use of
10 m s 2 is of course also acceptable. Solutions usually show numerical answers using both values
when they are significantly different.
5.
Strict rules regarding significant digits are usually not applied to numerical answers. However, in some
cases answers containing too many digits may be penalized. In general, two to four significant digits are
acceptable. Numerical answers that differ from the published answer due to differences in rounding
throughout the question typically receive full credit. Exceptions to these guidelines usually occur 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.
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AP? PHYSICS B
2007 SCORING GUIDELINES (Form B)
Question 1
15 points total
(a)
Distribution
of points
4 points
For each of the forces above with arrow correctly drawn and labeled, 1 point was awarded
Notes:
The force vector T could be shown in component form or as one force, but not both.
For extraneous forces, 1 point was deducted.
(b)
4 points
4 points
? Fy = 0
N + T sin q - mg = 0
For a correct force equation
N = mg - T sin q
For calculating the mg term correctly
For calculating the T sin q term correctly
(
1 point
1 point
1 point
)
N = (15 kg + 5 kg) 9.8 m s2 - ( 55 N ) sin 20¡Ş
For the correct numerical answer with units
1 point
N = 177 N (181 N if g = 10 m s is used)
2
(c)
3 points
? Fx = ma
For a correct Newton¡¯s second law equation involving horizontal forces
T cos q - f = ma
T cos q - f
m
For the correct relationship between the frictional and normal forces
f = mN
For correct calculation of the frictional force using the value of N from part (b)
1 point
a=
1 point
1 point
f = ( 0.22)(177 N) = 38.9 N (39.8 N if g = 10 m s is used)
(55 N) cos 20¡Ş - 38.9 N
a=
20 kg
2
a = 0.64 m s2 ( 0.59 m s2 if g = 10 m s2 is used)
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AP? PHYSICS B
2007 SCORING GUIDELINES (Form B)
Question 1 (continued)
Distribution
of points
(d)
2 points
For the correct work equation
W = (T cos q ) d
For substituting the correct displacement
W = ( 55 N ) cos 20¡Ş ( 7.0 m )
W = 360 J
(e)
1 point
1 point
2 points
For each section of the graph correctly drawn as above, 1 point was awarded.
Note: The second point was awarded only if the change at tr was abrupt.
2 points
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AP? PHYSICS B
2007 SCORING GUIDELINES (Form B)
Question 2
10 points total
Distribution
of points
(a)
(i)
2 points
For at least two correct electric field lines in the right directions
For the lines going through the entire Region I from top to bottom
Note: A single line going from top to bottom could earn a maximum of 1 point.
(ii)
2 points
For either of the first two equations below
qu B = qE
u = E B
u = ( 4800 N C) ( 0.12 T )
u = 4.0 ? 10 4 m s
For the correct answer with units
(b)
1 point
1 point
1 point
1 point
2 points
mu 2
r
For the correct equation for the radius
mu
r =
qB
For correct substitutions consistent with the answer to (a)(ii)
6.68 ? 10 -26 kg 4.0 ? 10 4 m s
r =
3.2 ? 10 -19 C ( 0.12 T )
qu B =
(
(
r = 0.070 m
)(
)
)
1 point
1 point
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