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Joyce FielFebruary 18, 2008AP Physics1?LAB #___: Rotational Mechanics PUPOSES:To investigate all the lab stations To explore torque To explore the rotation objects To differentiate between torque and force, the Center of Gravity, and the Moment of Inertia To differentiate and relate aspects of linear motion and angular motion Station 1: Coin Pick-UpAPPARATI:MaterialQty.MaterialQty.Flat wall 1Partner1Quarter1Camera 1THEORY:1] Center of GravityPICTURES:METHOD:Stood up straight against flat door surface (heels and butt remained touching door surface)Coin was dropped between my feet but not past my toesTried to pick up coin without taking my heels or butt of door surface (did not bend my knees)Fell over and used hand to right hand to catch myself (made some kind of rectangle with the floor)Used left hand to grab the coinCould not get back up unless I butt and heels were removed from the surface Repeated process but found another way to try to get coinBent my knees to lower myself closer to the coin and put my arm in between my legs to reach and get coinRepeat process again: bent my knees and reach over around the side to get coinOther two processes worked, however, I think my butt left the door surface for short intervals when trying to pick up the coinSwitch roles with partnerDATA TABLE: -No data tableANALYSIS:Calculations-no calculationsFree Body Diagrams Questions Why is it so hard to pick up the coin in the first position?When the person is standing straight up against the wall, the center of gravity is somewhere inside the person between their stomach and their back . When the person bends over to try to pick up the coin, a pivot point is created at her hips and the center of gravity and/or center of mass shifts away from the wall. It is harder for the her legs to support her upper body and therefore, she falls. However, if she bends her knees, the pivot point is at her knees instead of her hips and the center of mass/gravity does not move as farther away. The downward gravitational force is not as strong as it was in the first position so she does not fall over and is successful in picking up the coin. How would this experiment work with a pregnant lady? This experiment would be very difficult with a pregnant lady. She probably wouldn’t even be able to stand up straight against a wall, leaving her heels and butt against the wall because the center of gravity/mass is already away from her body. Pregnant women tend to stand a bit bent backwards to get their center of gravity in line with their body. When a pregnant woman picks up the coin, she would need to bend her back backwards a little bit, bend her knees to lower herself to the ground, use a hand to support her weight, while she uses her other hand to pick up the coin. (See FBD and diagrams above)Station 2 – Bicycle WheelAPPARATI:MaterialQty.MaterialQty.wheel of bike1camera1THEORY:Rotational speed and force right hand rule Rotational InertiaPICTURES:METHOD:Held handles of bicycle wheel Asked someone to spin the wheel for meTried to move the wheel around in different directions but it forced me to move it in a circular pathThe faster the wheel was going, the harder it was to push it the way I wanted it to goThe slower, it went, the easier it wasDATA TABLE: no data table ANALYSIS:Calculations-no calculationsDiagrams Questions How is rotational inertia different from linear inertia?Linear inertia depends solely on mass. Angular inertia, however, depends on mass AND how it is distributed with respect to the axis it is revolving around. In linear motion, mass is assumed to be concentrated at the center of mass. The diameter of the body that is rotating will determine the angular inertia which is also called the moment of inertia. Because the bike wheel has a relatively large diameter, it’s moment of inertia will too be relatively large compared to a wheel of a smaller tricycle for example. What can you say about the rotational speed and rotational acceleration and the forces acting on the spinning wheel? According to the right hand rule: If the fingers are around the axis of rotation and pointing to the direction of rotation, then the thumb is pointing to the angular velocity points along the axis of rotation. Angular acceleration also points along the axis of rotation. How do you explain the motion of the wheel when you try to force it in a certain direction?When the wheel is spinning, linear acceleration and velocity points toward the direction the wheel is spinning. However, since the angular velocity and acceleration points in the direction of the axis, any force applied to the axis will be forced to go the way the angular velocity and angular acceleration are going. That is why the wheel will only move seemingly sideways while it is in motion. (see diagram above)Station 3- FlywheelAPPARATI:MaterialQty.MaterialQty.Flywheel1Masses4Pins2Camera 1THEORY:I=mr2v=rwPICTURES:METHOD:Set pins at point C on each armSet masses in the centerTurned flywheel on setting 3This setup spun the fastestTurned off flywheel, put the pins at point B on each arm so the masses would be stuck in the center of the armsTurned flywheel setting on 3This setup spun a little slowerRepeat step 5 except pins were put at point A on each armThis setup spun the slowest, however all 3 setups seemed to have constant angular velocityNoticed all three setups were also loud and didn’t vibrate muchExperimented with two masses on point A on one arm and two masses on point C on the other armNoticed that motor vibrated, less sound was made, the shape was more like an oval instead of a circle, and acceleration wasn’t constantAlso experimented with one mass at point A on one arm and 3 masses on point A on the other armResults were similar to step 12. DATA TABLE: no data table ANALYSIS:Calculations-no calculationsFree Body Diagrams Questions How did motion change as the weights moved from outside to inside?The flywheel moved slower as the weights were moved from the outside to inside. This is because of the relationship v=rw. Assuming that the rotational velocity is constant, if the radius between the rotator and the mass is smaller, then the linear velocity will be a smaller number (meaning it is slower). The greater the radius, the greater the linear velocity, which is what we see when we look at the spinning flywheel.What position is rotational inertia the greatest?Rotational inertia is greatest at point C. Rotational inertia is equal to mass times radius squared. For our first three trials, we kept the masses constant so the radius was the determining factor. The greater the radius, the greater the rotational inertia will be. Therefore, the point with the greatest rotational inertia is Point C.Why caused the change in the level of noise the flywheel made?When the arms have even masses and even distances, the flywheel made a lot of noise. However, when the arms weren’t equally loaded, there was significantly less noise. This is because when the arms weren’t equally loaded, the acceleration and velocity weren’t constant so the system began to vibrate. The motor absorbed the vibrations so there was so sound (vibration is a form of sound).What effect did putting different masses on different arms or putting the masses as different lengths from the rotator have on the motion of the flywheel?The flywheel spun in different speeds and the shape it made was more oval instead of circular. Station 4- Rolling, Sliding, SlippingAPPARATI:MaterialQty.MaterialQty.Wooden crates/ boxes 3+Thick sheet of plastic1Thick sheet of wood1Thick sheet of glass1Ruler1Compass1Large steel ball1Small steel ball1Pickle ball1Ping-pong ball1Cans with different contents1Stopwatch1Camera1THEORY:Moment of InertiaRolling, sliding, slippingPICTURES:METHOD:Used boxes as a base of the slope Used wood as first surface for 30? slopeRaced two sizes of steel balls to see which would reach bottom of the slope firstTwo balls appeared to reach bottom at the same timeRaced hollow ball vs. steel ball to see which would reach bottom firstSteel ball reached the bottom firstTimed how long it would take for each type of can and ball to go down the rampConcluded that the denser the can, the faster it would go downUsed a different surface for slope: window plastic thingRepeated Step 3Big ball seemed to reach bottom fasterRaced pickle ball and ping pong ball –pickle ball wonTimed the pumpkin can again –noticed that time interval decreasedChanged surface to glassTimed some small steel ball, small hollow ball, and oriental can. Times decreased DATA TABLE:Time it took for rolling object to reach bottom of slope (seconds)SurfaceSmall hollow Large hollowSmall steelLarge steelCorn bread canPumpkin canOriental canWood.66.66.51.44.54Plastic.36Glass .42.35.42ANALYSIS:Calculations-no calculationsFree Body Diagrams Questions Which object had the greatest rotational inertia?I think the large hollow ball (pickle ball) had the greatest inertia. I didn’t record the time it took for the pickle ball to go down the incline, but I know that it took longer than the steel ball so it must have the greatest rotational inertia because it was harder for it go down the slope.Which object took the least time to roll down the incline?The can of corn bread mix took the least time to roll down the incline on the wooden surface. This was the densest object.What evidence do you have for instances of objects rolling, sliding, and or slipping?Objects on the glass went down faster than objects on the wood. This is probably because the object was rolling AND slipping/and or sliding (due to less friction).What are the moments of inertia involved with the objects used?The moment of inertia for the balls (which are uniform spheres) is (2MR2)/5. The moment of inertia for the cans (which are solid cylinders) is 0.5MR2. This is assuming that the objects revolve around a central axis.What would you do differently to improve this experiment? I would make sure to time all the objects for each slope at least once. It’s hard to make conclusions when my data table is so empty. CONCLUSIONSResults:See individual labsPurposes: My partner Leslie and I were able to accomplish all of our objects. The first thing that contributed to this was that we came early enough to do at least one thing in each station. We investigated most of the stations pretty thoroughly. There was only a couple that we didn’t get to do all of. We also accomplished our objectives with the help of the pre-lab that Mr.Tillay gave us. There were guide questions for each section and we made sure we answered those during and after each lab to ensure that we comprehended the point of the lab.Knowledge: With the bike experiment, the right hand rule for angular motion finally clicked to me. I now understand that the bike moved sideways because the angular velocity points that way. It is somewhat perpendicular to the linear velocity. I also understood center of mass better and that the farther away it is from the pivot point, the harder it is to balance and to move. I saw that in the pick-up-the-coin station and in the ruler and mass station. I also learned how mass and moment of inertia are different because moment of inertia depends on the distribution of mass as well. I also learned that torque includes force. MOTION rotation and MOTION linear And Rotational Mechanics LABLeslie Terry and Joyce FielFebruary 18, 20081? AP Physics ................
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