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AP Physics C: Mechanics How to Approach the Multiple-Choice Section General Preparation Information Allowed / Not allowed • • No Calculator – you don’t need to use a calculator to answer the multiple choice questions No Formula Sheet – you will be tested over your ability to use the formulas, thus you should know the formulas. Easy Calculations • What kinds of questions will be asked on the multiple choice section? Designed to test your knowledge of formulas Easy Calculations Example A ball is dropped from a 45 m high platform. Neglecting air resistance, how much time will it take for this ball to hit the ground? a) b) c) d) e) 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 1 Easy Calculations Example A ball is dropped from a 45 m high platform. Neglecting air resistance, how much time will it take for this ball to hit the ground? a) b) c) d) e) Planet X is twice as massive as Earth, but its radius is only half of Earth’s radius. What is the acceleration due to gravity on Planet X in terms of g, the acceleration due to gravity on Earth? a) b) c) d) e) ¼g ½g g 4g 8g Designed to test your understanding of the size of things, measurements, or just numbers. Proportional Reasoning Example • Designed to test your knowledge of how to use equations, except that you don’t have to plug in numerical values to solve them. Proportional Reasoning Example • • 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s Proportional Reasoning • Order of Magnitude Estimates Planet X is twice as massive as Earth, but its radius is only half of Earth’s radius. What is the acceleration due to gravity on Planet X in terms of g, the acceleration due to gravity on Earth? a) b) c) d) e) ¼g ½g g 4g 8g Concept Questions: “WHY?” • These are designed to test your understanding of vocabulary and explanations for physical phenomena. 2 Concept Questions: “WHY?” Example • At a smooth, sandy beach, it is observed that the water wave fronts are traveling almost perpendicular to the shoreline. However, a helicopter pilot notices that several hundred meters away from the shore, the wave fronts are traveling at about a 45° angle to the shore. What is a possible explanation for the change in the direction of the water wave fronts? a) b) c) d) e) These ask you a simple question based (obviously) on a diagram. Direct Solution with Variables Example • A pendulum of length L is drawn back to position P, as shown in the above diagram, and released from rest. The linear distance from P to the lowest point in the pendulum’s swing is d; the vertical distance from P to the lowest point in the swing is h. What is the maximum speed of this pendulum in terms of the above variables and fundamental constants? a) d) 2 gd 2 gL L b) e) 2 gd c) • At a smooth, sandy beach, it is observed that the water wave fronts are traveling almost perpendicular to the shoreline. However, a helicopter pilot notices that several hundred meters away from the shore, the wave fronts are traveling at about a 45° angle to the shore. What is a possible explanation for the change in the direction of the water wave fronts? The water waves have a greater speed near the shore. The water waves have a smaller speed near the shore. The water waves have reflected off of the shore. The acceleration due to gravity is greater near the shore. The acceleration due to gravity is smaller near the shore. Concept Questions: Diagrams • Concept Questions: “WHY?” Example 2 gh 2 gh L a) b) c) d) e) The water waves have a greater speed near the shore. The water waves have a smaller speed near the shore. The water waves have reflected off of the shore. The acceleration due to gravity is greater near the shore. The acceleration due to gravity gravity is is smaller smaller near near the the shore. shore. Direct Solution with Variables • Because the AP writers can’t ask you to do any kind of difficult number crunching on the multiple choice section, often they will ask you to do your problem solving using variables only. Direct Solution with Variables Example • A pendulum of length L is drawn back to position P, as shown in the above diagram, and released from rest. The linear distance from P to the lowest point in the pendulum’s swing is d; the vertical distance from P to the lowest point in the swing is h. What is the maximum speed of this pendulum in terms of the above variables and fundamental constants? a) d) 2 gd 2 gL L b) e) 2 gd c) 2 gh 2 gh L 3 Skipping Questions? • • • • • You earn 1 point for each correct answer You lose ¼ point for each incorrect answer You neither gain or lose for blank answers Thus, if you have no clue, it might be best to leave the question blank. If you are able to reduce the number of answer choices (check for correct units) then guessing might help you. How to Approach the Free-Response Section Multiple Choice Final Advice • • • What do the graders look for? • Free-Response Tools You can use most calculators on the free-response section of the exam. – Check and make certain that your calculator is an allowed type. You don’t want any surprises on the day of the exam. Before looking at any student papers, the graders set a Rubric (a grading guide) for each question. This includes what is expected for partial credit and full credit to be awarded. Free-Response Tools • • Know your pace. Know your strengths and weaknesses. Multiple-choice questions do not necessarily start easy and get harder. Thus, if you are stuck, skip it and come back. There may be easier ones further in the test. The AP authors know the time limit. They will not write a problem that really takes 3-4 minutes to solve. Thus, look for the easy problem approaches. You will also be allowed to have the formula sheet for this section of the exam. • • • You should have most of these formulas memorized, but you may use the formula sheet to make certain you remember the equation correctly. You may also use the formula sheet to jog your memory on a particular equation. Beware: The formula sheet sheet can can be be dangerous. dangerous. Don’t Don’t randomly start searching the formula sheet for formulas that “look “look like like they they might might work”. work”. 4 Free-Response Advice • • • • Always show your work (neat is a plus)! If you are unsure exactly how to solve a problem, explain your thinking process to the grader. You might be able to get several points of partial credit for your physics thought process. Keep your answers succinct (don’t write a book). ALWAYS remember units if they are appropriate. Free-Response Advice (cont) • • If you make a mistake, cross it out. If your work is messy, circle your answer. Make sure the AP readers know what to grade and what to ignore. If you get stuck on one question, try another. Question 3 might be easier than question 2. Get the easy points first, then try to get the harder points if you have time. Free-Response Advice (cont) • If you don’t know how to solve part a, but know how to solve part b and it involves part a’s answer, you can: – – Make up a reasonable answer for part a then use it to calculate part b. Set some variable equal to the answer from part a, write a short note (“let v be the velocity found in part a”). Then solve in terms of that variable. Free-Response Advice (cont) • • Lab Questions It may be helpful to include a drawing or a graph in your answer to a question, but make sure to label your drawings or graphs so that they’re easy to understand. No free-response question should take you more than 15 minutes to solve. They’re not designed to be outrageously difficult, so if your answer to a freeresponse problem is outrageously complicated, you should look for a new way to solve the problem, or just skip it and move on. Lab Questions • Each free response section will contain at least one question that involves experiment design and analysis. 5 Steps to Answering Lab Questions • Follow the directions – – – – There is no single correct answer. – – – – Most of the lab questions are open-ended. There might be four or more different correct approaches. Don’t try to “give them the answer they’re looking for.” Give them an answer that makes sense – you might be right. Steps to Answering Lab Questions • • This sounds simple. When the test says “Draw a diagram,” it means they want you to draw a diagram. When the test says, “Label your diagram,” it means that they want you to label your diagram. Yes, you will earn points for these easy steps. Steps to Answering Lab Questions • Steps to Answering Lab Questions Don’t over-think the question – They’re normally not too complicated. – Remember, you’re supposed to only take 10-15 minutes to write your answer. – You’re not exactly designing a subatomic particle accelerator. Use as few words as possible. – Answer the question, then stop. – You can lose credit for an incorrect statement even if the other 15 statements in your answer are correct. – KEEP IT SIMPLE! Steps to Answering Lab Questions • Don’t assume you have to use all the stuff they give you. – It might sound fun to use a force probe while determining the index of refraction of a glass block, but, really! A force probe!?! Steps to Answering Lab Questions • Don’t state the obvious – You may assume that all basic lab protocols will be followed. – There’s no need to tell the reader that you recorded your data carefully, nor do you need to remind the reader to wear safety goggles. 6