Remember when you were young? Your parents crammed you and your sibling units into the family car. When one parent drove in a circle the force you experienced from the seat, seat belt, door, or sibling's body (not including the punching force or sticky ice cream force) was a centripetal force. Without the centripetal force you would have kept traveling in a straight line. Centripetal forces are always directed toward the center of a circle and keep objects from flying off in a straight line. Finally, keep in mind: Centripetal force happens—centrifugal force doesn't.

Purpose

To measure the centripetal force on a rotating object.

To calculate the mass of a rotating object.

Procedure

1. Measure and record the mass (m) of the stopper on the next page.

1. Tie the stopper to one end of the fishing line. Thread the fishing line through the tube (if one end of the tube is rough, have that end point toward the weight!). Leave 0.5m of fishing line between the stopper and tube.
2. Make a loop in the other end of the fishing line.
3. Draw a permanent mark on the fishing line near the bottom of the tube.
4. Measure and record in the data table the weight of one of the weights. (Use F_g=mg if necessary.) Make sure you record the weight in Newtons. Slide the weight over the loop.
5. Stand in an open area. Hold the tube in one hand. Start rotating the rubber stopper. Adjust the speed of the stopper until the mark on the fishing line stays in the same spot near the bottom of the tube with you holding only the tube NOT the string or mass.
6. Have one partner measure the time for 20 rotations. Record the time in the data table.
7. Hook another mass on the other. Record the total hanging weight in the data table. This is trial 2. Measure and record the time for 20 rotations. Make sure the fishing line mark stays in the same place and the radius stays constant.
8. Repeat step 7 four more times with additional masses.
9. Measure and record the radius (r) between the stopper and tube on the next page.

   Due Date __/__/__

   Materials

   Goggles

   Small plastic tube (body of a pen, pen cap, etc.)

   1-m sturdy fishing line

   Permanent marker

   Rubber stopper

   Small weights to hang

   Balance

   Stop Watch

   Safety

   Wear goggles.

   Keep the rotating stopper from hitting other students.

   Do not use your head as a stopper break.

Data and Calculations

Mass of stopper (m) = __________________ radius (r) = _________________________

Time for 1 rotation [s]

Speed [m/s]

Data Table Calculations Table

TrialWeight [N]Time for 20 rotations [s]123456Circumference (C) = _______________________

SHOW ONE SAMPLE OF THE COMPLETE WORK IN EACH STEPLater, you will calculate the velocity of the stopper. But first, you need to know the distance the stopper travels in one rotation. The distance for one rotation is the circumference, (C), of a circle. Calculate the circumference using C=2pr. Record C above. You recorded the time for 20 rotations in the data table for each trial. Just divide this time by 20 to get the time for one rotation. Do this for all seven trials and record your answers in the calculations table.Divide the circumference by the time for one revolution to find the speed (v) of the stopper for each trial. Record your answers in the calculations table.Application and Critical Thinking4. Suppose your stopper broke free while it was rotating. Describe the path it would follow immediately after breaking. ________________________________________________________________________________ ________________________________________________________________________________5. Obviously, the weight of the weights provides the tension in the fishing line and is equal to the centripetal force (F) on the stopper. When you added more weight, you had to spin the stopper faster to maintain the same radius. Plot v (y-axis) versus F (x-axis) on the graph below or plot it Microsoft Excel. Draw the best-fit line through your points. 6. Is your best-fit line straight? Should it be? Explain. ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________7. Well, below is a nice formula for centripetal force that we happened to meet on the way home one day. Let's do something with it. You measured the theoretical (actual) mass of the stopper with a balance. Use the nice formula and the data from one of your best trials to find an experimental value for the mass (m) of the stopper. Record your experimental mass answer below.8. Calculate the percent error between the theoretical mass and experimental mass of the stopper. Use the friendly formula below. Record your answer below. 9. If you know the distance to a planet and the centripetal force on it, could you calculate the mass of the planet? Explain. ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ 10. Why do people go through all the trouble of using hard formulas to calculate the mass of a planet? Would it not be easier to use a bathroom scale to do the job? Explain. ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________11. If Earth's mass suddenly doubled, what would immediately happen to the gravitational force between the sun and earth? The centripetal force? Explain. ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________