Phys2010 Fall 2015 Lab 3: Projectile Motion Sep 29-Oct 2 Name Section Tues Wed Thu Fri 8am 10am 12pm 2pm 4pm THERE IS A PRELAB THIS WEEK! IT IS A SEPARATE DOCUMENT, DOWNLOAD AND PRINT IT FROM THE WEBSITE, AND DO IT BEFORE YOU COME TO LAB! Bring it with you, thanks. Intro You will be investigating the trajectory of a projectile. Part of the lab is a virtual exploration, and part involves shooting a projectile from a launcher that allows both the initial velocity and the angle of launch to be accurately set. The goal is to predict, not to do this by trial and error. So, for most parts below, you will have ONE TRY. Please wait until your TA, or another group, is watching for your tries! Activity 1: Calculating Initial Velocity These launchers should give very repeatable results. Set your launcher horizontal (there is a small protractor on the side to help you set the angle). To load the ball, it s like an oldfashioned musket. Place the ball in the end, then use the plastic plunger to gently but firmly push the ball into the launch tube. These launchers can be broken, and they are expensive, please pay attention and use care. You should feel it click, there are 3 launch starting positions, go for the middle one: we will use the middle Medium range setting (look for the yellow tab) EVERY TIME this week. - Get a box to stop/catch the ball, and launch horizontally. (Gently but firmly pull the release cord) There are two photogates in front of the launcher. Set them up (pulse mode) to measure the time taken as the ball passes, and use this to calculate the initial speed. Do this a few times and estimate an uncertainty. What value (and uncertainty) do you measure? (Show your work!) v 0 ± v =
Activity 2: Predicting and testing the range Name Given a launch speed, the range formula (derived in the prelab) tells you how far you expect the ball to land horizontally: RRRRRRRRRR = 2vv 0 2 cos θθ sin θθ gg Assuming we set the launcher at 45 degrees (for maximum range!), use the formula above to predict not just the range, but the uncertainty in that range, and record them below: Range ± (range) = PLEASE DO NOT LAUNCH YET! You have one and only one try, so read on before you use it up! Given your predicted uncertainty, set up a catching box located so that you think the ball will land squarely in the middle of the box. (Think about both the horizontal position and also the height. The range formula only works if the ball ends at the same height it started from) There should be a variety of boxes available, choose one with the smallest opening that you are still confident you will land in (that s where the uncertainty above factors in) Please wait, and only launch when your TA is with you (or, if they are busy, work with a second group so you have an outside observer to verify whether or not you succeeded in landing in the box) This is not Angry Birds : think about NASA launching a rocket, there are no do-overs! Note: Landing in and then bouncing out of the box is a success (if confirmed by the outside observer!), but hitting an edge of the box (not the bottom), or missing entirely, is a fail. The goal here is to be the group with the SMALLEST box that catches the ball! To discuss briefly below: How did you do? How about other groups around you? Do you consider what happened luck or physics? If you did not succeed, briefly discuss what factors contributed to the miss, and how you might succeed if you were allowed a second launch (perhaps with a different angle) Was the main problem with your calculation of range, or the uncertainty in that calculation (or something else?) - If you did hit the landing, briefly discuss what you might do to succeed with a smaller box (What if we gave you a different angle?) p. 2
Activity 3: Trajectory Name 1. Have one member of your group drop a ball through the Plinko board to decide on the angle of launch you will be investigating. yyyyyyyy aaaaaaaaaa = 2. Calculate the expected range for your launch angle, (using # s from Activities 1 and 2) Range ± (range) = 3. Your goal this time will be to hang 3 hoops along the trajectory of the projectile such that the ball goes approximately through the center of each hoop! The rules are: Hoop 1 should be hung at the apex of the motion Hoops 2 and 3 should be hung between 20 cm and 70 cm from the apex hoop There must be at least a 20 cm difference in the distance from Hoops 1 and 2 and Hoops 1 and 3. For example, if Hoop 2 is hung 20 cm from Hoop 1, Hoop 3 needs to be hung at least 40 cm from Hoop 1. 4. Discuss your strategy with your group. Describe your strategy and final calculations for deciding on your hoop placements below. Take your time. You again have only ONE TRY, so first complete all calculations, set it up, and double check your measurements! Sight the launcher very carefully, but don t launch yet see next page! p. 3
Name 5. Once you are confident in the placement of your hoops, wait for a TA to watch your launch. (If they are too busy, as before, you can work with another group so you have an outside observer to verify your one launch! ) How well did you do? Any thoughts for improving your trajectory? 6. Repeat steps 1-5 for an angle which is 90 degrees minus your initial angle. For example if the initial angle you selected was 30 degrees, repeat for an angle of 60 degrees. How do the ranges compare in this case? What was different about the trajectory? 7. How would your hoop placement change if just the vertical position of the launcher was modified? (E.g. if the launcher were lifted 10 cm higher or lower than it was) p. 4
Name Activity 3b (optional, if time. But be sure to look at activity 4 as well!) Trajectory Challenge: 1. If you accurately hung the hoops in Activity 3, go back to the Plinko board for a new angle. Follow the same procedure as steps 1-5, but this time use the challenge hoops (which are a much smaller diameter). 2. Set up, sight the launcher, and once again, you are allowed only one launch of the projectile find an observer. Were you able to get through the center of the smaller hoops? What effects made this challenge more difficult? Can you think of ways to improve the likelihood of success? p. 5
Name Activity 4: Projectile Motion PhET Sim For this activity, work in groups of two. 1. On the provided laptops (or your own) open the PhET sim called Projectile Motion. Take about 5 minutes to play with the sim and explore all the features. What are all the ways you can change the range of the projectile? 2. Choose an object and an initial velocity. Try only varying the angle of the shot. Which angle appears to give you the maximum range? Try to reproduce your numbers from Activity 3 (Use user choice to set the parameters to match your projectile) How does the sim compare to your observed trajectory? Be sure to investigate the difference between using your initial angle, and 90 minus that angle. 3. With the user choice option for the object and air resistance turned on, investigate the various factors that affect air resistance. How does the altitude setting change the trajectory? How would you explain the change? In your own words, how would you describe the effect of the drag coefficient? For a given drag coefficient, how does the diameter of the object change the trajectory? Give an example from your experience in which the size of an object changes the amount of air resistance. p. 6