Investigating Circular Motion (OCR A Level Physics)

Equipment & Method

• Circular motion can be investigated using the following setup:
  • Tie a bung of mass m, to a piece of string, which sits horizontally
  • Thread it though a glass tube and a paper clip, which sits vertically
  • At the other end of the string a heavier mass, M is suspended vertically
  • This provides the centripetal force, F = Mg when the tension in the string is constant
• The string is spun in a circle:
  • The time taken for several rotations is recorded and repeated to remove any random errors
• The masses in the experimental set up are changed before the experiment is repeated again

Explanation

• As the bung is swung in a circle the bigger suspended mass, M will remain stationary
  • When the force it provides is equal to the centripetal force, Mg 
  • This is the centripetal force required to make the bung travel in a circular path
• The weight, and hence the centripetal force, required for different masses, radii and speeds can be investigated

• The forces acting on the bung are:
  • The tension in the string
  • The weight of the bung downwards

• If the centripetal force required is greater than its weight then the suspended mass moves upwards
• If the centripetal force required is less than its weight then the suspended mass moves downwards
  • The paperclip will move accordingly to make this movement clearer

• As the bung moves around the circle, the direction of the tension will change continuously
• The magnitude of the tension will also vary continuously, reaching a maximum value at the bottom and a minimum value at the top
  • This is because the direction of the weight of the bung never changes, so the resultant force will vary depending on the position of the bung in the circle

• At the bottom of the circle, the tension must overcome the weight, this can be written as:

• As a result, the acceleration, and hence, the speed of the bung will be slower at the top
• At the top of the circle, the tension and weight act in the same direction, this can be written as:

• As a result, the acceleration, and hence, the speed of the bung will be faster at the bottom