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Objective To measure the centripetal force for whirling a mass round a horizontal circle and compare the result with the theoretical value given by F= mw2r .

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Canossa College




Hazel Chow Ho Ying

Class no:






Centripetal force


To measure the centripetal force for whirling a mass round a horizontal circle and compare the result with the theoretical value given by F= mω2r .


  • rubber bung
  • glass tube
  • screw nuts
  • Wire hook
  • 1.5m of Nylon string
  • Small paper marker
  • metre rule
  • stop-watch


When a mass m attached to a string is whirled round a horizontal circle of radius r, the centripetal force for maintaining the circular motion is given by

F = mω2r   where ωis the angular velocity of the circular motion.

This force is provided by the tension of the string.

The formula can also be expressed in the terms of the velocity v of the mass, where ω=v/r .

Substituting ω=v/r into the formula for F , F = mv2/r

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Time 50 revolutions of the bung and calculate the angular velocity ω.Repeat several times using different lengths L of the string.


Tabulate the results as follows:

        Mass of rubber bung m =   0.03491   kg

        Mass of screw nuts M =   0.13   kg

 Tension in string T = Mg =   0.13   × 9.8 N =   1.274   N

Length of string L/m





Time for 50 revolutions 50 t/s





ω = image00.png/rad s–1










        Mean mω2L =   1.264   N


Form the results, we can find that the length of the string L is increasing, the value of angular velocity ω is decreasing. The length of the string is indirectly proportional to the value of angular velocity.

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ω2L .

They are almost equal.

Possible sources of errors

  1. Friction exists between the glass tube and the string.
  2. The rubber bung is not set into a horizontal circular path.
  3. The rubber bung does not move with constant speed.
  4. The length of the string beyond the upper opening is not constant.

θ increases with ω

Vertical components is T cosθ

Horizontal components is T sinθ

The system has no vertical acceleration

T cosθ = mg

The horizontal component of tension provides the centripetal acceleration

T sinθ = mrω2

Let L be the length of the string

i.e.  r = L sinθ

T sinθ = mrω2

T sinθ = m(L sinθ)ω2

T = mLω2

 mg/cosθ= mLω2

θ increases with ω

When the rubber bung is whirled around with a higher angular velocity ω , the angleθ becomes larger.


  1. Level practical physics for TAS p. 28 - 30
  2. Physics Beyond 2000 p. 40
  3. http://en.wikipedia.org/wiki/Centripetal_force
  4. http://www.greenandwhite.net/~chbut/centripetal_force.htm

...read more.

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