03 Centripetal Force

03 Centripetal Force#

Aim#

To show an example of centripetal force.

Subjects#

  • 1D50 (Central Forces)

Diagram#

../../../../../_images/figure_08.png

Fig. 22 .#

Equipment#

  • Conical beaker, \(2 \text{l}\), filled with water.

  • Rubber stop.

  • Ping-pong ball tied to rubber stop (see Diagram).

Safety#

  • The conical beaker filled with water is quite heavy (\(m \approx 2\text{kg}\)). Hold it firmly!

Presentation#

Hold the conical beaker filled with water upside-down in your hands. The ping-pong ball stands vertically above the rubber stop. Make yourself turn in a circle and while turning observe the ping-pong ball (see Figure 23).

../../../../../_images/figure_16.png

Fig. 23 .#

The ping-pong ball is displaced towards you.

Explanation#

The ping-pong ball being completely immersed in water experiences an upward thrust \(F_{u}\) that is larger than its weight \(m\cdot g\). The net force (\(F_{u}-m\cdot g\) ) is directed upwards. The tension \(T\) in the string prevents that the ping-pong ball floats upwards (see Figure 24a).

../../../../../_images/figure_23.png

Fig. 24 .#

When turning around in circles the ping-pong ball is forced to move in a circle. A centripetal force is needed for that. Figure 24b shows the new situation of equilibrium: the net upward force and tension are compensated by a centripetal force \(F_{c}\). Any other position of the ping-pong ball is not a situation of equilibrium (drawing a free body diagram of the forces will show this).

Remarks#

  • When an air-bubble is trapped in the conical beaker filled with water, this bubble will behave in the same way as the ping-pong ball does.

  • When you move the system from left to right, the acceleration on the left side and the deceleration on the right side can be observed. In general: the system can be used as an acceleration-meter.

Sources#

  • Ehrlich, Robert, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, pag. 31-32.

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