To measure the centripetal force by whirling it around a horizontal circle, then compare the result with theoretical value FC = m(2r.
1Glass tube (About 15 cm long)
1Slotted weights, with hanger 12 × 0.02 kg
1Nylon thread 1.5 m
1.Attach one end of a 1.5 m length of nylon thread to a rubber bung and thread the other end through a glass tube, a paper marker and a number of weights as shown.
2.First adjust the position of the marker so that it is about 1 cm near one end of the glass tube, and the length of the thread L from the other end of the glass tube to the rubber bung is, say, 0.8 m. Fix the position of the marker using adhesive tape if necessary. First start with M = 0.16 kg (i.e. 160 g).
3.Holding the glass tube vertically, whirl the bung around above your head in a horizontal circle. Increase the speed of the bung gradually and allow it to move out (i.e. let L increases) until the marker is about 1 cm below the lower end of the glass tube. 4.Try to keep the angular speed constant so that the marker is always about 1 cm below the tube throughout. Ask your partner to time 20 revolutions of the bung using a stop watch. Remember to start the stop watch at 0 and stop it at 20. Take one more confirmatory reading and obtain the mean time for 20 revolutions.
5.The horizontal component of the tension T of the string provides the centripetal acceleration of the rubber bung.
As there is no vertical motion, the vertical component of tension(T ) is balanced by the weight of the bung: Tcos[pic]=mg_____ (1) The horizontal component of the tension provides the net centripetal force: Tsin[pic]=mr(2_____ (2)
Substituting r= l sin@ into equation (2), we can find the tension(T ) in the string. Tsin[pic]=m(L sin[pic])(2
The tension (T ) is provided by the weight(Mg).
By comparing the two values of tension, the expression of centripetal force F =mr(2 can be verified.
Precaution & Safety Measures
◆ Wear the safety goggles during experiment
◆ The angular speed of the motion should be kept constant so that the marker was always about 1 cm below the tube. ◆ Have all observations by standing away as the demonstrator rotates the rubber bung ◆ Make sure that you have plenty of room and you may rotate horizontal circular path. Also, switch off the fans. ◆ Ensure the rubber bung hold tightly with the string.
◆ Ensure the paper marker at the same position during experiment. ◆ Count the numbers of turns begin at 0.
Measure the mass m of the rubber bung.
m = 0.0316kg
Length of the string= 0.8m
|M / kg |Mg / N |Time for 20 revolutions 20T / s |Angular speed |m(2L / N | | | | |(=[pic] / rad s-1 | | | | |1st trial |2nd trial |Mean | | | |0.12 |1.18 |16.03 |16.47 |16.25 |7.73 |1.511 | |0.16 |1.57 |15.22 |15.41 |15.32 |8.20 |1.700 | |0.20 |1.96 |14.54 |15.13 |14.84 |8.46 |1.809 | |0.24 |2.35 |13.50 |13.56 |13.53 |9.29 |2.182 |
Mean m[pic]L: [pic]
II) Calculations(% of Error)
For 0.12kg, For 0.16kg
For 0.20kg For 0.24kg...