Physics 20 Name: Lab: Speed and Radius Question: For an object moving with uniform circular motion, what relationship exists between the radius of its path and its speed? (Assume the object is experiencing a constant centripetal force.) Hypothesis: Variables Controlled: Manipulated: Responding: Materials and Equipment 1-hole rubber stopper (mass ≤ 25 g) Metre stick 1.5 m of string or fishing line Felt marker Small-diameter plastic tube Safety goggles 100-g mass Stopwatch CAUTION: Remember to swing the rubber stopper over your head and in a place clear of obstructions Procedure 1. Secure the rubber stopper to one end of the string. 2. Run the other end of the string through the plastic tube and attach the 100-g mass to it. 3. Hold the end of the string attached to the stopper at the zero mark of a metre-stick laid on a table. The zero mark of the ruler should line up with the centre of the stopper. While holding the stopper in position, pull the string taut along the ruler 4. With a felt marker, mark the string at 20, 25, 30, 35, 40, 45, 50, 55, 60, and 65 cm. 5. Adjust the string’s position so that the 20-cm mark is positioned on the lip of the plastic tube. Record 20 cm in the “Radius” column of the table. 6. Grasp the plastic tube in one hand and pinch the string to the lip of the tube using your thumb or forefinger. Physics 20 Name: 7. Put on your safety goggles. Begin spinning the rubber stopper in a horizontal circle above your head as you release the string. Make sure the 100-g mass is hanging freely. At first, you may have to pull the string up or down using your other hand to position the mark as the stopper is spinning. 8. Adjust the rate at which you swing the rubber stopper so that the string does not slip up or down and the mark stays in position at the lip of the tube. Once you have reached a steady rate, your partner can begin timing. Do not pull the string up or down with the other hand. 9. Your partner should time 20 complete revolutions of the rubber stopper using a stopwatch. While your partner does this, be sure to monitor the speed of the stopper so that the mark does not move off the lip. Record the time in the “Time 1” column of the table. 10. Repeat step 9 and record the time in the “Time 2” column of the table. 11. Increase the radius by 5 cm, and record this radius in the “Radius” column of the table. Repeat steps 6 - 11 until all radii are used. Data for Speed and Radius Lab Radius (m) Time for 20 Revolutions (s) Time 1 Time 2 Average Time (s) Period (s) Speed (m/s) Physics 20 Name: Analysis 1. For each trial average the two time and place the result in the “Average Time” column. Show a sample calculation. 2. For each trial, divide the average time by the number of revolutions the stopper made. Record the values for the time it takes to make one revolution in the “Period” column. Show a sample calculation. 3. For each trial, determine the speed of the stopper. The distance travelled is the circumference of the circle and the time is the period. Record your values in the “Speed” column. Show a sample calculation. 4. Identify the force that acted as the centripetal force, and determine its value. 5. Plot a graph of speed vs. radius. Remember to plot the manipulated variable on the horizontal axis and the responding variable on the vertical axis. 6. Plot a straightened graph, develop an equation for your original line. In your conclusion make sure to answer the following: 7. Complete the statement, “The speed varies with…” 8. Was your hypothesis accurate? If not, how can you modify it to reflect your observations from this lab? 9. What are some possible sources of error that could have affected your results? As well, what could you do if anything, to correct for next time?
