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Centripetal Force Lab Dec 7, 2005

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The purpose of this lab is to attempt to measure the centripetal force acting on a small object that is moving in uniform circular motion. If you follow the directions, you will be able to determine the centripetal force and the centrifugal force in our system. In the conclusions you will compare the two.

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Materials You will need a rubber stopper, a tube, some string, two paper clips, and an object of known mass - preferably 0.100 kg to 0.500 kg.

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Materials Tie the stopper to one end of the string. Thread the string through the tube. Tie one paper clip on to the string, on the opposite side of the tube from the stopper. Then tie the other paper clip on to the end of the string farthest from the stopper. It should look something like this:

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Pre-lab calculation For this lab, we will have to know how much distance the stopper travels every time it makes one revolution. This will be the circumference of the circular path that the stopper is traveling, which you must calculate with the equation C = 2 π r. So, measure the radius of the circle with a meter stick. (the distance from the middle of the stopper to the end of the tube) Make sure you are recording the radius in meters, not in cm. Use this radius to calculate the circumference and record it in your lab notebook.

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What happened here?

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SAFETY WARNING If one of these rubber stoppers were to strike a person, it could cause serious injury. YOU WILL WEAR EYE PROTECTION. You will not twirl the stopper until after everyone in the room is aware of your experiment and is properly protected with safety goggles. Horseplay and non-participation will not be tolerated.

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Procedure Use a force sensor to determine the mass of the rubber stopper. This will most likely be measured in grams; divide by 1000 to get the mass in kilograms. Record the mass of the rubber stopper in kilograms in your notebook.

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Procedure Hang a brass weight from the bottom paper clip. Now whirl the stopper overhead just hard enough to make the weight stay in one place. Do not let the top paper clip touch the tube.

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Procedure Have one team member twirl the stopper, keeping a close eye on the top paper clip. Have two other team members count the exact number of revolutions the stopper makes in 15 seconds. (One should watch the clock, while the other watches the stopper.) Record the number of revolutions it made in 15 seconds.

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Procedure Now divide the number of revolutions you counted by 15. The result is the number of revolutions the stopper makes each second. Record this as the rotational speed of the stopper in revolutions per second (rps).

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Procedure Now use the rotational speed of the stopper ( that you just calculated ) and the circumference of the path ( the pre-lab calculation ) to determine the linear speed of the stopper: linear speed v ( in m/s ) = (circumference) x (rotational speed). ____________m/s Calculate this and record it in the notebook.

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Procedure The equation for centripetal force is: We know the mass of the stopper, its linear speed, and the radius of the circle it is moving in. We can calculate the amount of force necessary to make the stopper spin at the speed you had it going. Calculate that now and record it!

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Procedure You have just calculated how strong the centripetal force is. But what causes the pull on the rubber stopper?

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Procedure You have just calculated how strong the centripetal force is. But what causes the pull on the rubber stopper? Hint – there is one part of the lab equipment that if it were removed, the stopper and string would fly off.

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Procedure What is the centrifugal force pulling on? How strong is the centrifugal force?

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Procedure Draw a free body diagram of the brass weight on the bottom of the string. This means you have to draw and label all forces that are acting on the weight. Record how strong these two forces are. If you have to perform a calculation, then be sure to show the work in your lab notebook.

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Conclusions In your conclusions section for this lab, I want you to address the relationship between the centripetal force and the centrifugal force. How are the values related? Why is this so? Predict how your results would change if you were to perform this experiment again with a heavier brass weight. If after this lab, you do not understand centripetal force and centrifugal force, it would be a very good idea for you to see me outside of class for help – before the test!

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