AP · AP 1 · free response
AP Physics 1: Algebra-Based · Question 9
AP Physics 1: Algebra-Based · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 12
- Topics
- 1
- Answer
- Complete
A student is given a wooden block, a horizontal board of the same material, a set of known masses, a spring scale, a meter stick, and a stopwatch. The student wants to determine the coefficient of kinetic friction μ between the block and the board.
- (a)
Describe Describe an experimental procedure the student could use to collect the data needed. Include the quantity that is deliberately varied, the quantity measured in response, and at least two quantities held constant.
4 marks - (b)
Describe Describe how the student should graph the data so that the coefficient of friction can be determined from a straight line, and state what quantity the slope of that line represents.
3 marks - (c)
Determine The student's best-fit line has a slope of 0.32 when friction force is plotted against normal force, and a vertical intercept of +0.15 N rather than zero. Determine the coefficient of kinetic friction, and explain what the non-zero intercept suggests about the experiment.
3 marks - (d)
Explain Explain why taking the slope of the best-fit line gives a more reliable value for μ than calculating f/N from a single pair of readings.
2 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Describe Describe an experimental procedure the student could use to collect the data needed. Include the quantity that is deliberately varied, the quantity measured in response, and at least two quantities held constant.
Answer the command word directly and use precise physical vocabulary. Include only the distinct features or facts that earn marks, without burying them in unrelated background information.
- 1
1 point: pull the block along the board with the spring scale at constant velocity, so that the applied force equals the friction force
- 2
1 point: vary the total mass by adding known masses on top of the block — this is the independent variable
- 3
1 point: record the spring-scale reading for each total mass — this is the dependent variable
- 4
1 point: hold constant the surfaces in contact and the surface area, and keep the pull horizontal for every trial
(b)
Describe Describe how the student should graph the data so that the coefficient of friction can be determined from a straight line, and state what quantity the slope of that line represents.
List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.
- 1
1 point: plot the friction force (spring-scale reading) on the vertical axis against the normal force, or against the total mass, on the horizontal axis
- 2
1 point: the relationship f = μN is linear through the origin
- 3
1 point: the slope equals μ if the horizontal axis is the normal force, or μg if the horizontal axis is the mass
(c)
Determine The student's best-fit line has a slope of 0.32 when friction force is plotted against normal force, and a vertical intercept of +0.15 N rather than zero. Determine the coefficient of kinetic friction, and explain what the non-zero intercept suggests about the experiment.
List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.
- 1
1 point: μ = 0.32, the slope, which is dimensionless
- 2
1 point: the model predicts an intercept of zero, since zero normal force means zero friction
- 3
1 point: a positive intercept indicates a systematic error — for example the spring scale reads high at zero, or the pull was not horizontal, adding a constant component to every reading
(d)
Explain Explain why taking the slope of the best-fit line gives a more reliable value for μ than calculating f/N from a single pair of readings.
State the outcome first, then link cause to effect with the relevant physical principle. Each link in the reasoning should be explicit enough to earn its own marking point.
- 1
1 point: the slope uses every data point, so random errors in individual readings tend to average out
- 2
1 point: a systematic offset such as the intercept above shifts a single-point ratio but does not change the slope, so the slope is insensitive to it
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