ROTATIONAL MECHANICS Name: Period: Date: preap Physics Exploratory Lab Weight = 1 DIRECTIONS: Do the activities below and fill the blanks/spaces provided. 50 points available. STATION 1: ROTATIONAL INERTIA Choose two objects from the available ones that roll down the hill at roughly the same rate 1. PREDICTION: Which do you think will reach the bottom of an inclined plane first, if released from the same height at the same time: solid cylinder? hollow cylinder? 2. Roll the cylinders down the inclined plane and see if your prediction was right. YES NO 3. Find the common mass of the cylinders(avg.): kg 4. Find the outer radius of the solid cylinder and hollow cylinder: m 5. Define inertia (we talked about this first semester). 6. Use the following chart to calculate the inertia of the two shapes you are experimenting with. I, Solid Cylinder: kg m 2 I, Hollow Cylinder: kg m 2 7. Does having more rotational inertia make the shape go down the inclined plane slower or faster? slower faster 8. What affects the rotational inertia more, the mass or the distance the mass is from the axis of rotation? mass (m) distance of mass (r) 9. How does the formula for rotational inertia reveal which variable has more effect the mass or the radius? 10. When does a top loading washing machine have more rotational inertia? wash cycle (full of water) spin cycle (at the end when it is spinning real fast to dry and clothes are pressed at the sides) hard to tell Why? Give a reason or two.
STATION 2: ANGULAR MOMENTUM d 1 1. PREDICTION: How will you spin faster: with your arms extended from your body or with your arms closer to your body: arms extended arms closer 2. Stand on the rotating platform with your arms extended. 3. Have your partner start you spinning with your arms extended. NOTE: It is easy to lose your balance here so have him/her spin you at a reasonable speed. Have him/her count how many times you spin in a period of time, say 5 to 10 s. 4. Stand again on the rotating platform with your arms extended. Have your partner start you spinning but as soon as s/he starts, bring your arms tight around your body. Have him/her count how many times you spin in a period of time, say 5 to 10 seconds, with d 2 your arms close to your body. 5. What was your speed with your arms extended (in number of rotations per second)? 6. What was your speed with your arms close to your body (in number of rotations per second)? 7. Was your prediction right from Number 1? YES NO 8. Extend your arms. Measure the distance d 1 from fingertip to fingertip (see Figure at above right) and divide by 2: m. This is your radius of rotation with your arms extended. 9. Now stand with your arms down by your sides. Measure the distance d 2 from shoulder to shoulder (see Figure at above right) and divide by 2: m. This is your radius of rotation with your arms by your side. 10. From the inertia chart on the previous page, find the shape that most looks like you in both positions arms extended and arms by your sides and the corresponding formula for the moment of inertia: FORMULAS: arms-extended position: arms-retracted position.: 11. How much do you weigh. In other words, what is your mass? kg. 12. Find your inertia in each position using your radii and mass. a. inertia in the arms-extended position: 13. b. inertia in the arms-retracted position: Ideally, your inertias should be in the same proportion as the rotational speeds, so that if you spun three times faster (compare your answers for 5 & 6), for example, when you had your arms retracted, then your inertia was three times smaller with your arms retracted (compare your answers for 12.a. and 12. b). Do your numbers more or less pan out that way? YES NO FOR BONUS POINTS: Sit down on the rotating platform. How many spins can you do without falling off? Call me to compete for the record and get checked off
STATION 3: ROTATIONAL KINETIC ENERGY VS. TRANSLATIONAL KINETIC ENERGY PREDICT: If you dropped two identical rolls of toilet paper (assuming there is no friction to speak of) from the same height, allowing one of them to fall freely while holding one end of the other one and letting it unroll, which would hit the ground first? The free falling one The one unrolling They would hit the ground at the same time 1. Try the experiment. Was your prediction right? YES NO 2. Find out from what height you need to drop them both so that they will hit the ground at the same time. Record your values here (also find the inner & outer radii of the toilet paper roll and record): Free-falling roll Unrolling roll Release Height (in m) Inner radius (in m) Outer radius (in m) 3. What is the initial form of energy of both rolls of toilet paper while suspended above the ground? What is the final form(s) of energy for the freely falling toilet paper right before hitting the ground? What is the final form(s) of energy for the unrolling toilet paper? Check all that may apply. 4. Will mass affect the final velocity of either roll before hitting the ground? YES NO Explain. 5. The height ratios of the free falling vs. unrolled toilet papers in theory is given by: Where H is the drop height of the free-falling roll, h is the drop height of the unrolling roll, r and R are the inner and outer radii of the toilet paper rolls. What is the theoretical ratio of the heights H (using the formula): h What is the experimental ratio of the heights H (find from your experiment/chart above): h 6. Calculate the experimental error between the ratio of the heights (you found through experimentation) and the theoretical heights (you found from the formula) % error = Theoretical value Experimental Value Theoretical Value x 100% =
STATION 4: ROTATIONAL INERTIA Spin the two suspended PVC pipes given with the same amount of force. They both have the same mass and to outward-seeming are identical. 1. What is different about the way they spin? 2. Explain why this happens. 3. Try balancing the baseball bat on one finger from one end and then flip it upside down and try to balance it on one finger from the other end. Which end is it harder to balance from? Heavier end down Lighter end down 4. Is the center of gravity of the bat over the base (your finger) in both cases? YES NO So if not a center of gravity difference, why is it harder to balance the baseball bat in one orientation? STATION 5: PRECESSION 1. Hold the bicycle wheel from its axles in a vertical position and stand on the rotating platform. Have your partner spin the bicycle wheel very fast. Now try turning the bicycle wheel 90. What happens to you? Why does this happen? 2. You are mountain biking off road. You are about to encounter a pretty rough patch of terrain but the greatest danger would be falling sideways into the ravines. You are challenged to get through without falling over to the sides. If you trust in the laws of physics and believe you have decent enough mountainbiking skills, should you go over the rough terrain faster or slower to help ensure that you don t fall off to either side and maintain balance? faster slower Why do amateur mountain bikers often opt to do the opposite of what the physics laws say then?
STATION 6: GYROSCOPIC MOTION 1. Slide the string into the gyroscope hole and wind it up. Spin the gyroscope and release it on the table. How long can you keep it spinning? s 2. Compete for the record (get my initials): (5 bonus points) 3. Why doesn t the gyroscope fall over when spinning? How is it able to stay balanced on such a small base?. 4. What does the balance attained by a spinning gyroscope have to do with the rifling of a barrel of a gun or the spinning of a football by a quarterback as s/he throws it? STATION 7: CENTRIPETAL VS. CENTRIFUGAL FORCE GET AWAY FROM ALL ELECTRONIC DEVICES AND OUTLETS FOR THIS ACTIVITY: 1. Can you fill the bucket with water halfway full and spin it in a vertical circle four times without the water spilling out? Call me to show me (or film it) and get checked off: INITIALS 2. How does this simple demo show how satellites stay up in orbit around the earth? In other words, will a satellite remain in orbit at just any speed? STATION 8: MAGIC SPINNER In a vertical position, spin the wheel suspended from one axle by a vertical spring and then release it. Spin it really fast. What cool and unexpected thing does the wheel do?