6.1.2 Lab Activity - Going In Circles

Purpose: How does radius, mass and period influence centripetal force?

Safety Note:Safety concerns are minimal; however, common sense should prevail while doing this activity. Eye protection is recommended.

Materials: Ball point pen tube, 2 m of string, paper/plastic cup, paper clip, 30 metal washers, stop watch, tape, meter stick, candy (wrapped is better – in case you spill, you can still eat it without invoking the 5 second rule!)

Procedure:

A. Assemble the Centripetal Force Apparatus:

1. Pull the string through the empty pen tube until a little over a meter passes through the tube.

2. Attach the cup to one end of the string by punching three holes, each 1/3 the distance around, and forming a “handle” to tie the string to.

3. Take the other end of the string and run it through a stack of 5 washers and attach a paper clip that has had the middle pulled up to create an inverted “T”. Let the washers slide over the middle of the “T” and rest on the head of the “T”.

4. Measure out a radius of 30.0 cm of string starting from the cup and ending at the top of the tube. Place a piece of tape securely on the string about 1 or 3 cm below the other end of the pen tube.

5. Place some candy (the mass) in the cup, about ¾ full will do just fine.

B. Practice spinning the centripetal force apparatus:

1. Grasp the pen tube with the cup of candy at the top with one hand and with the other hand, hold the paper clip of washers.

2. Hold the tube vertically above your head and get the cup of candy moving in a circle parallel to the floor.

3. Let go of the paper clip of washers and move your hand in smaller and smaller circles until you can keep the cup going in circles by only moving your hand in tiny circles. Important note: Keep the tape mark slightly below the end of the pen tube as you spin the cup. Be sure not to let the tape touch the pen tube as you are spinning the cup of candy.

C. Experimenting with the centripetal force apparatus with a constant mass:

1. Get the apparatus spinning and looking straight ahead, measure the time it takes the cup to complete 30 revolutions. Make sure to keep the tape mark just below the end of the tube at all times. Remember to start the stopwatch or have someone time for you. Record the radius, the time, and the number of washers (the force) in Data Table 1.

2. Add 5 more washers and repeat step 1. Repeat this procedure 4 more times, each time adding 5 washers until the total force is 30 washers.

3. Increase the radius to 60 cm and repeat steps 1-2 recording the collected data in Data Table 2.

4. Increase the radius to 90 cm and repeat steps 1-2 recording the collected data in Data Table 3.

Observations:

Going In Circles Data Table 1

30 cm Radius

Force
(# of washers) / Radius
(m) / Time
(s) / Time for 1 revolution / Velocity
(m / s) / Velocity2
(m / s)2
5
10
15
20
25
30

Going In Circles Data Table 2

60 cm Radius

Force
(# of washers) / Radius
(m) / Time
(s) / Time for 1 revolution / Velocity
(m / s) / Velocity2
(m / s)2
5
10
15
20
25
30

Going In Circles Data Table 3

90 cm Radius

Force
(# of washers) / Radius
(m) / Time
(s) / Time for 1 revolution / Velocity
(m / s) / Velocity2
(m / s)2
5
10
15
20
25
30

Calculations:

  1. Calculate the time for 1 revolution and determine the velocity and the velocity squared for each trial. Record this data in the data tables. PSYW
  2. In this investigation, you varied the force (number of washers) and kept the mass (the candy) constant. Plot a graph with force on the vertical axis and velocity on the horizontal axis. Plot the three sets of data on the same graph.
  1. Plot a graph with force on the vertical axis and velocity squared on the horizontal axis. Plot the three sets of data on the same graph.

Helpful website

The National Center for Education Statistics created this online tool so that anyone can make graphs and print them out or download the image to a computer and email the graph.

“Create a graph”

Interpretations and Analysis –

  1. What does the shape of the curve (line) of the graph for force vs. velocity reveal about the relationship of the 2 variables?
  2. What does the shape of the curve (line) of the graph for force vs. velocity squared reveal about the relationship of the 2 variables?
  3. Does centripetal force depend directly on the velocity or on the velocity squared? Explain.
  4. Do the results of your investigation support the equation for centripetal force? Explain.

Submit your completed report to your lab notebook. Include the data collected, the graphs, and the interpretations/analysis.

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