01 Tablecloth Pull

01 Tablecloth Pull#

Aim#

To show an example of Newton’s first law

Subjects#

  • 1F20 (Inertia of Rest)

Diagram#

../../../../../_images/figure_021.png

Fig. 59 .#

Equipment#

  • Sheet of paper.

  • Different objects placed on the sheet of paper.

Presentation#

Set the table as shown in Diagram (light a candle etc.). Our tablecloth is not really a tablecloth. We use a sheet of paper (see Diagram). Take the protruding free end of the paper in both hands and give a sharp downward jerk. The sheet of paper comes out from under the glasses and they are hardly moved. (Even the water in the glasses is not disturbed!)

Explanation#

This is one of the many possible demonstrations to show the validity of Newton’s first law.

Also Newton’s second law can be used to explain this demonstration:

The effect of a given force between the sheet of paper and the glasses depend on the impulse of that force ( \(F \Delta t)\). The impulse is small when the sheet of paper moves away quickly ( \(\Delta t\) is small). The resulting horizontal displacement will then be very small. Analysis shows that the horizontal displacement \(d\) of a mass on the sheet equals:

\(d=\frac{1}{2} k_{1} g \Delta t^{2}\left(1+\frac{k_{1}}{k_{2}}\right)\), where \(k_{1}\) is the coefficient of friction between sheet and glass and \(k_{2}\) is the coefficient of friction between glass and table and \(\Delta t\) the time to pull the sheet from beneath the glasses. So \(d\) is very sensitive to \(\Delta t\) !

Remarks#

  • After lecture students like to try the demonstration by themselves.

  • This demonstration needs trying it before you show it!

Sources#

  • Ehrlich, Robert, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, pag. 21

  • Sutton, Richard Manliffe, Demonstration experiments in Physics, pag. 46-49

  • Jones, Evan, The Physics Teacher, Vol. 15, pag. 389

  • Perez, Joseph, The Physics Teacher, Vol. 15, pag. 242

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