X WCLN.co How do CS investigators use skid marks to analyze an accident scene? Your friend was touring Northern British Columbia in a car. It was getting late and your friend noticed a large shadow leap out onto the road ahead. The brakes were slammed on and the car attempted to skid to a stop. Your friend claims it was a moose frozen in the headlights (as they tend to do). Just before the car stopped it struck the moose taking its legs out. It collapsed onto the car. The car was totalled. Fortunately, your friend was insured. Pictures were taken of the damage, the moose, and the location. Police were notified and reports were filed. Unfortunately the insurance company was not so willing to replace the damaged vehicle. It was stated on their report that the accident could have been avoided had there not been excessive speeding. Insurance is said to have based this decision on the length of the skid marks at the sight. Your friend needs your help. She assures you that she was not speeding and feels that this is simply a case of \"guilty by age\" (she just recently got her license). You are to help her construct a solid case against the insurance company that proves that she was not speeding (and not driving recklessly). 2022-06-10 Page 10f12 X WCLN.C: Instructions Using a pencil, answer the following questions. The lab is marked based on clarity of responses, completeness, neatness, and accuracy. Do your best! Please ensure that any data measured (or recorded) includes the appropriate number of significant digits (only one uncertain digit). This activity is divided into three sections: Core this first section explores only the basic \"core\" ideas involved in understanding. @ wclin.ca 2 Of 12 Page 1 0f 12 X WCLN.c? ' Instructions Using a pencil, answer the following questions. The lab is marked based on clarity of responses, completeness, neatness, and accuracy. Do your best! Please ensure that any data measured (or recorded) includes the appropriate number of significant digits (only one uncertain digit). This activity is divided into three sections: + Core - this first section explores only the basic \"core\" ideas involved in understanding. Students must demonstrate a sound understand with all of their answers in this section BEFORE attempting the next section. e Mastery Your instructor will NOT review this section if the Core section above shows any misconceptions. In this section students will make predictions and apply the concepts and ideas learned above. For complete mastery it is expected that data collection and scientific procedures will be as accurate as possible. All work shown should be clear with any units included. Answers should be rounded off to the correct number of significant figures based on the data collected. e Ace - Once again, your instructor will only look at this section provided he/she is confident that the above Mastery criteria has been met. In this section students will demonstrate a deeper understanding of the concepts through error analysis, experimental design etc. Physics concepts from other units already covered will often be required here. Part 1: Core Being the solid physicist that you are, you proceed to photograph all the necessary information that you need to present the case. You pull a tape measure from the trunk of your car and record the following data: length of the skid marks = 24.2 m * speed limitin this area is 70 kph Now you can build your case... You are to present an easy to follow report that shows the insurance company (who likely has limited Physics experience) that you were not speeding based on the laws of Physics. 1. Assuming that the brakes were \"locked\" and the car skidded for the entire 24.2 m before striking the moose, construct a free-body diagram that illustrates the forces which act on the car. Free-body diagram of skidding car: Be sure to |abel all forces with appropriate symbols. Define each force below: Where: (define each force first one done for you) 2022-06-10 Page 2 of 12 X WCLN.C? - F4 = force of gravity & wclin.ca 30f12 2022-06-10 Page 2 of 12 fine each force first one done for you) X WCLN.c? F, = force of gravity 2. Based on your free-body diagram above, determine the equation of motion for this skidding car. Equation of motion: Fhret = 3. Substitute an appropriate equation for each force represented in your equation of motion. For example, you will substitute ma for Fretsince Fnet = ma. You final equation should contain only physical constants (g, and y) along with any necessary masses, m, and accelerations, a. Be sure to solve your equation for the acceleration, a. Equation of motion simplified: 4. If done correctly, your equation above will suggest to you a great deal of physics for a sliding vehicle. Let's examine the mass, m, of the sliding vehicle. What happens to m once the above equation is fully simplified? 5. Mass is said to be a measure of an object's inertia. Define inertia in the context of this skidding car. 2022-06-10 Page 3 of 12 X WCLN.C? & wclin.ca Page 3 0f 12 X WCLN.C;\\ 6. Now you can readdress question (4). An object with more mass, m, should be more difficult to slow down. However, if you examine your free-body diagram you will notice that many of the forces present also depend directly on mass, m. Explain why the mass of the vehicle does not matter by discussing how the forces present can offset or counter the increase in inertia (mass). . Solve for the acceleration of the skidding car using your equation found in question (3). You will have to research any constants that appear in the formula. Be sure to state your source. Constants and given information (quote any sources): Solve for acceleration, a a= m/s? With respect to the car, will this acceleration be positive or negative? Explain? 2022-06-10 Page 4 of 12 X WCLN.ceT & wclin.ca 50f12 2022-06-10 Page 4 of 12 X WCLN.C? 8. Now, using kinematics combined with your answer above, determine the initial velocity of the car based on the length of the skid marks. State any assumptions made as clearly as possible. Calculate initial velocity, ve, of car: Given information and assumptions: Calculations: (show all work) Vo= m/s = km/h 9. Conclusion: Was the car speeding? What was the major assumption(s) that you made to make this conclusion? Remember, the \"best case scenario\" for the tire and road conditions will be your best option since these choices would predict your maximum possible speed assuming everything was ideal. 2022-06-10 Page 5 of 12 e xx & wclin.ca 6 of 12 2022-06-10 Page 5 0f 12 X WCLN.C: Part 2: Mastery Approximate Coefficients of Friction Kinetic Static Rubber on concrete (dry 068 0.90 Rubber on concrete (wet) 058 Rubber on asphalt (dry 067 085 Rubber on asphalt (wet) 053 Rubber on ice 0.15 Waxed ski on snow 0.05 0.14 Wood on wood 030 0.42 Steel on steel 0.57 0.74 Copper on steel 0.36 053 Teflon on Teflon 0.04 1. Your friend tells you that it was raining on the day of the collision. Discuss how this information would affect your overall answer for the initial velocity that you previously calculated. Be sure to support any claims with equations used above. 2. Recalculate your acceleration based on the wet conditions (use above table): Solve for acceleration, a 2022-06-10 Page 6 of 12 & wclin.ca 7 of 12 2022-06-10 Page 6 of 12 X WCLN.C? You notice on the report that the road was recorded as \"a slight decline\". Discuss, in detail, how this decline (think of it as a ramp) would affect the overall acceleration by answering the questions below. 3. Reconstruct your free body diagram by considering your car as if on a ramp. Free-body diagram for car skidding down a ramp (exaggerated ramp): 4. From your previous studies on simple machines you are aware that the function of ramp is to dilute the acceleration due to gravity. Explain, in your own words what this means. 2022-06-10 Page 7 of 12 & wclin.ca 8 of 12 2022-06-10 Page 7 of 12 X WCLN.C? 5. Your friend is in Physics 12 and has just completed a lab where a low friction cart is rolled down a ramp at various angles and the acceleration is determined. Your friend offers you their results in the form of the graph below. Acceleration of a Low Friction Cart Down a Ramp versus the Sine of the Angle of the Ramp 10 i t 11 b 8 & m - g B B equation of best-fit line: & & 4 a= 9.94 m/s?sin 6 v