01 Tablecloth Pull

Aim¶

To show an example of Newton’s first law

Subjects¶

• 1F20 (Inertia of Rest)

Diagram¶

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 technically not 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 depends 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 the lecture students like to try the demonstration by themselves.

• This demonstration needs trying it before you show it!

Video Rhett Allain¶

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