## Thursday, 29 May 2014

### Centripetal force vs speed of rotation

Equipment

Glass tube 15cm length with ends flamed so that they are smooth.
1.5m length of string
Hooked mass hanger for 100g slotted masses
Large rubber bung with home through centre
Marker pen
Stopclock

Method

Tie the bung to one end of the string. then measure 60cm from the centre of the bung along the string and make a mark with the pen.
Run the string through the tube and tie a loop at the end to hang the masses from.
Hang the mass hanger and spin the bung at the correct speed to keep the pen mark at the top of the tube.
Start the stopclock and time how long it takes for 10 swings of the bung. Take two more readings of the time for 10 swings to calculate an average and check for anomalies.
Add a mass to the hanger and repeat the swinging and timing being careful to ensure that the pen mark stays at the top of the tube. It is important that the radius of the swing does not change as this is another factor that affects the centripetal force needed and so will affect the speed you measure.
Continue adding masses and timing 10 swings until you have 500g on the hanger.

Calculate the centripetal force applied by multiplying the mass used by the gravitational field strength.
Calculate the average speed by taking the circumference of the circle the bung travels in and dividing by the average time for one swing.

Plot a graph of centripetal force vs speed of motion.

Risk assessment

Ensure you are working in a clear area before starting to swing. After each repeat check the string to make sure it is not beginning to fray. There is still a risk of the string breaking and the bung flying free. As such only rubber bungs should be used. Do not attach a metal mass to the swinging end. Goggles should be work to protect eyes from any possible flying bungs. As you are using a glass tube care should be taken not to grip it too tightly as this increases the chance of the tube breaking. The tube should also be well supported at the rotating string end to prevent any extra stress on the tube increasing the risk of it breaking.