Lab: Centripetal Force
Purpose:
The purpose of this lab is to investigate the relationship between the speed of an object in uniform circular motion and the centripetal force on the object.
Variables:
· Radius of circular motion (r): The horizontal distance measured from the top of the tube to the stopper when the stopper is swinging in a circle. Radius must be measured in meters.
· Period (T): The time in second for the satellite (rubber stopper or ball) to do one complete circle.
· Centripetal Force: The force causing the satellite to travel around in a circle. In this case, the satellite is attached to a string which has weights weighing it down on the other end. Without the weights, there would be nothing to counteract the satellite. Therefore, the force on gravity on the weights must be supplying the centripetal force for the satellite.
· Linear Speed: The linear distance the stopper travels divided by the time it takes to travel that distance. V = 2pr/T.
Procedure: Apparatus:
1. Place one weight on the bottom clip of the apparatus (or tie it on). Record this total mass. The weight of this mass provides the centripetal force.
2. Pick a target radius. Measure out the target radius from the satellite mass to the top of the glass tube. Then attached a paper clip the string a little below the bottom of the glass tube.
3. Practice whirling the stopper until you can keep the paper clip a short distance below the tube. IMPORTANT! If the clip touches the bottom of the glass tube, the weights are no longer supplying the centripetal force. If the clip raises or falls appreciable as the stopper whirls, the radius of the circle is changing. Practice!
4. Use a stopwatch to measure the time taken for a reasonable number of revolutions (20 – 30 perhaps). Record your data.
5. Change the number of weights on the bottom clip.
6. Repeat steps 3 – 5 for at least 3 different weights.
7. Now change the target radius. Repeat the experiment for this radius. Be sure to indicate where the radius changed in your data table.
8. Change the radius a final time and repeat the experiment. By the time you are done, you should have data for at least three different radii.
9. If you have time, you might try to determine the relationship between satellite mass and centripetal force. In order to do this, you need to keep both the radius of the circle and the speed constant while you vary the satellite (stopper) mass and the centripetal force. You can design another data table for this. ~ OR ~ You could investigate the relationship between the radius and the centripetal force.
Data Table:
Using Excel, create a data table to adequately hold the needed data: radius (m), mass (kg), centripetal force (N), Total Time (sec), Period (sec), velocity (m/s). {Hint: The weight of the masses at the bottom are creating the centripetal force what you need to calculate. Also, velocity = distance traveled/time. The distance traveled around a circle is the circumference of the circle, 2pr. The time it takes to go all the way around is the period. Therefore, velocity = 2pr/T}
Calculations:
Show an example calculation any data that needed to be calculated (either in your head, with a calculator or with Excel) before placing it in the data table (centripetal force, period, velocity)
Graphing:
Using Excel, create a graph of centripetal force vs. linear velocity. Remember, the independent variable (the variable “I” change, centripetal force in this case) goes on the x-axis. The dependent variable (the variable that changes on its own, speed in this case) goes on the y-axis. Use Excel to make a POWER trendline fitted to the data.
Percent Error:
The correct equation is: or
Show your percent error calculations using the exponents for centripetal force. The experimental value will be from the graph, the real value is 0.5.
Conclusion:
· Restate the purpose of this lab.
· Summarize your procedure.
· State your findings (include your equations with the appropriate variables: v a Fc?) and percent error.
· Explain three sources of unavoidable error while doing the lab.
· How would this error effect your data (would it make your data too large, too small, ect)
Your Lab Report (47 pts)
· Title Page (1 pt) (include title of the lab, your name, lab group members, date)
· Abstract (4 pts) (brief summary of purpose, procedure, and findings w/ percent error)
· Purpose (1 pt)
· Theory (10 pts) (What is centripetal force, why is it needed to produce circular motion, what is producing the centripetal force in this lab; what is a period, how did you calculate it in this lab; what is tangential velocity and how was it calculated in this lab)
· Data Table (5 pts) use Excel to make a data table. Remember to include units and a title
· Sample Calculations (6 pts) (provide a sample calculation for each variable calculated before putting it in the data table: centripetal force, period, velocity)
· Graph (5 pts) (use Ecel to create a graph with the appropriate variable on each labeled axis, title, POWER trendline with equation)
· Percent error calculation (2 pts) (v a Fc0.5)
· Conclusion (10 pts) (summarize purpose & procedure, state findings and percent error, explain three sources of error and how that would effect your data.
· Presentation (3 pts): (Typed, neat, and in order)