LAB EXERCISE 4

PhysicsLab4, Monday, August 9, 2010 NAME __________________
Dr Dave Menke, Instructor
I Title: Centripetal Acceleration

II Purpose: To study centripetal acceleration and have fun
Theory: Planets, like Earth, travel around the Sun similar to how a weight on a string travels in a circular path if you swing it around. For the Sun and the planets, there is no “string,” but the force is Gravity. The planets are like weights. Each planet has a velocity or speed and an acceleration. You will notice that the force in this lab is a central force, so that the acceleration is a central one, i.e., ac.

III Equipment
- White String
- Metal weight (washer, nut, whatever)
- Wooden metric ruler
- stopwatch
- Scissors

IV Procedure
1. Select a weight
2. Obtain approximately a 1.0-meter length of string
3. Attach the weight to one end of the string, just like before
4. Suspend the (string + weight) by holding the top of the string tightly at one end. This is your friend, “plumb bob.” He’s related to Sponge Bob, Bob the Builder, and 97.5 Bob FM. He’s the same guy as last time,
5. Measure exactly the length, l, of Bob (from your fingers to the middle of the weight) in meters.
6. Leave the classroom and to find an open area reasonably clear of muggles*.
7. Have one lab partner to practice - carefully - swinging Bob in a circle until he/she/it has achieved a relative constant velocity. Don’t hit anyone. Some students swing it overhead, like a lasso. It is not likely that you will hit anyone who is walking on the ceiling.
8. Have another lab partner practice using the stop watch.
9. When ready, have the swinging partner (SP) begin swinging Bob in circles at a constant rate. When ready, have the stopwatcher lab partner (SWLP) click the stop watch and count 10 cycles, then have the SWLP stop the watch. Record. The SP can keep swinging or not. Personal preference.
10. Repeat this three times to get an average amount of time for each 10-cycle period. Record. Now stop the SP if he/she/it hasn’t already.

*muggle (1) common, ordinary, ignorant person; (2) someone with NO magical powers – from the Harry Potter series of books; (3) a marijuana “joint” – from the 1920’s New Orleans

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Lab 3, page 2, August 9 Name _____

11. Return to the classroom, and encourage the SP and SWLP to join you. Put away your toys, and write up your report.

12. Divide your average cycle time by 10 to get the period, P, of one cycle. Record.

13. Find the circumference, c, of the orbital path. Do this by multiplying Bob’s length that you found in #5, l, by the number 2 pi or 2p = 2(3.14). Record.

14. Calculate the average linear velocity, v, of the mass. Do this by dividing the circumference that you found in #13 by the period (time) that you found in #12. Record.

15. Calculate the mean centripetal acceleration, ac, of the mass. Do this by squaring the velocity, v2 (multiply it by itself) that you found in #14 and dividing that by the length of the string that you find in #5, l. Record.

V Data & Calculations (This is where you put your data)
1. Bob’s length, l, in meters: _____________________
2. Trials and Times


TRIAL NUMBER of 10 Cycles TIME IN SECONDS of each 10 Cycles
1
2
3
AVE

3. Period of one cycle, P (divide the average of 10 cycles by 10) ______ s

4. The circumference of the orbital path, 2 p l = ______________ m

5. The average linear velocity of the mass, v = _______________m/s

6. The mean centripetal acceleration of the mass, ac = __________m/s2

VI Results
“The purpose of the lab was to go Bob-Bob-Bobbin’ along.” No, for “reals” it was to study orbital revolutions and have fun, and it (was, was not) [circle one] achieved because …


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Lab 3, page 3, August 9 Name _____

VII Error Analysis
A. Quantitative Error – NA

B. Qualitative Error:
1. Personal
2. Systematic
3. Random
VIII Questions
1. Find the circumference of Earth’s orbit around Sun (in meters) if Bob’s length, l, (the radius of Earth’s orbit) is 150,000,000 km, just like you did in Procedure #13 above.
2. Find the period of the Earth’s orbit (in seconds). Do this by multiplying the number of seconds in a day, 86,400, by the number of days in a year, 365.
3. Find the linear velocity of Earth (in m/s). Do this by dividing what you found in Question #1 with what you found in Question #2.
4. Find the centripetal acceleration of Earth around the Sun (in m/s2). Do this by squaring the velocity that you found in Question #3 and then dividing it with the radius of Earth’s orbit, 150,000,000 km.
5. There is no number 5.