AP PHYSICS 2011 SCORING GUIDELINES General Notes About 2011 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 earned. 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 earn 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 earned. However, when students are asked to derive an 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. 2 4. The scoring guidelines typically show numerical results using the value g = 9.8 m s, but use of 2 10 m s is 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 earn 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.
AP PHYSICS B 2011 SCORING GUIDELINES Question 4 10 points total Distribution of points (a) 2 points For any statement indicating that the total weight is the sum of three terms, that of the beaker, ball, and water W = W + W + W tot beaker ball water For calculating W water r = mv, so m = rv W = m g = r V g water water water water 3-3 3 2 ( )( )( ) W water = 1000 kg m 5.0 10 m 9.8 m s = 49 N W = 49 N water W tot = 2.0 N + 3.0 N + 49 N W = 54 N (or 55 N using tot 2 g = 10 m s ) (b) 3 points One point for each of the three correctly drawn and appropriately labeled forces One point deducted for any extraneous forces or forces not drawn near the object 3 points (c) S F = ma y y The ball is in equilibrium, so a y = 0. S Fy = FB - T - mbg = 0 F = T + m g B F = 4.0 N + 3.0 N B For the correct answer F = 7.0 N B b
AP PHYSICS B 2011 SCORING GUIDELINES (d) 2 points Question 4 (continued) Distribution of points P = rgh For correct substitutions P = ( 1000 kg m 3 2 )( 9.8 m s )( 0.20 m ) For the correct answer with units P = 1960 Pa (or 2000 Pa using 2 g = 10 m s ) (e) 2 points For selecting Lower For an appropriate justification Example: Less fluid is displaced by the rubber ball because less of the ball s volume is submerged in the water.
AP PHYSICS B 2011 SCORING COMMENTARY Question 4 Overview This question assessed students understanding of density, buoyancy and pressure under static conditions. Sample: B4A Score: 10 The work shown on this problem is clean, orderly and easy to follow. Note that the explanation in part (e) would be complete even without the calculation shown. Sample: B4B Score: 7 Part (a) earned one point for calculating the weight of the water. The summation shown has an incorrect term (which may be from subtracting the weight of the ball from the tension). Part (b) earned only 2 points because weight vector is missing. Part (c) calculates the weight of the water instead of the buoyant force and earned no points. Parts (d) and (e) earned full points. Sample: B4C Score: 4 Part (a) earned no points because the student does not correctly calculate the weight of the water and does not show a clear summing of forces. In part (b) the word gravity or g was taken by the graders to mean an acceleration, not a force. Use of F g, mg, or weight of ball were all acceptable. The student also omits the tension of the string; therefore part (b) earned only. Part (c) calculated the weight of the water instead of the buoyant force and earned no credit. Part (d) earned for the correct substitutions but there are no units. Part (e) earned full credit.