Friday, 23 May 2014

Factors affecting circular motion & centripetal force

This is a series of three short experiments which examine how a different factors affect the size of the centripetal force needed to maintain motion.

The three factors you will investigate are; Mass of object, Radius of circle, Speed of object.

Equipment

Plastic ballpoint pen tube sanded to smooth off the ends
1.5 m length of string marked every 10 cm
Slotted masses on a hanger 100g/division and 10g/division
Large and small rubber bung with a hole through the centre
Stopclock

Diagram



Method 1 - Effect of mass.

Set up the equipment as shown using the small bung with a 50cm length between tube and bung centre. Maintain this distance throughout
Add 100g mass to the end of the string and swing the bung so that the 50cm mark stays in position.
Time 10 swings and record the time. Stop and start again and record the time for 10 swings. Repeat this once more and take an average of the three readings.
Replace the small bung with a large one.
Start to swing the bung at the same rate as before (same time for 10 swings).
If the bung moves outwards and will not maintain the 50cm radius when swung at this rate add more masses to the hanger. If it slides towards the ballpoint tube remove masses from the hanger.
Keep adding/subtracting masses until the larger bung will swing at the same distance and at the same rate as the smaller bung. Note down the new mass needed and check three times as before that it is swinging at the same rate as for the small bung.

Does a larger bung need a larger or smaller centripetal force to make it swing at the same rate and distance?

Method 2 - Effect of radius

Using the small bung start with 100g mass and a 10cm radius. Sing the bung to maintain the 10cm radius and time 10 swings. Repeat as above twice more and calculate the average time at this radius.
Increase the radius by 10cm.
Start to swing the bung at the same rate as before (same time for 10 swings).
If the bung moves outwards and will not maintain the 20cm radius when swung at this rate add more masses to the hanger. If it slides towards the ballpoint tube remove masses from the hanger.Keep adding/subtracting masses until the  bung will swing at the correct distance and at the same rate as for 10cm. Note down the new mass needed and check three times as before that it is swinging at the same rate as for 10cm.
Repeat the process at 10cm intervals up to 50cm radius.

How does the radius of the swing affect the centripetal force required to make it swing at the same rate for the same mass object?

Method 3 - Effect of speed

Using the small bung start with 100g mass and a 50cm radius. Sing the bung to maintain the 50cm radius and time 10 swings. Repeat as in method 1 twice more and calculate the average time at this radius.
Add a new mass to the hanger and repeat the above, timing 10 swings.
Repeat for one more addition of mass.

How does the speed affect the centripetal force required to keep a fixed mass moving at a constant radius?

In each experiment it is important you only change one variable and control the others so that you can see the effect it has on the centripetal force. Do not for example change the bung, the radius and the speed of swing all at once as you will not be able to tell which factor is affecting the mass required to provide teh centripetal force.

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.