06 Recoil of a Water Jet

06 Recoil of a Water Jet#

Aim#

To show an example of Newton’s third law.

Subjects#

  • 1H10 (Action and Reaction)

  • 1N22 (Rockets)

Diagram#

../../../../../_images/figure_034.png

Fig. 80 .#

Equipment#

  • tube with a T-junction, \(\varnothing 10\)

  • rubber hose (about 2 meters)

  • faucet

  • tray to catch the water

Presentation#

The rubber hose with T-junction hangs vertically down. Opening the faucet makes the end of the hose move away.

../../../../../_images/figure_124.png

Fig. 81 .#

When a plate is held in the water-jet nothing changes.

When the plate is fixed to the end of one side of the T-junction the hose stays vertically in its position.

Explanation#

In order to convert a downward water flow into a sideways water flow, the T-junction has to exert a force on the water. The reaction to this force is responsible for the recoil to the other side. When a plate is placed in the outgoing water stream, it also exerts a force on the plate. When this plate is fixed to the T-junction, these two forces cancel, so there is no recoil

Remarks#

This demonstration can be performed by the students themselves, by giving each of them a flexible soda straw, giving it a \(90^{\circ}\) bend (Figure 183).

../../../../../_images/figure_210.png

Fig. 82 .#

Blowing hard in the long part of the straw, the free end recoils. The no-recoil can be observed when a small plastic bag is attached at the end of the straw.

Sources#

  • Friedrich, Artur, Handbuch der experimentellen Schulphysik, part 2, Mechanik der festen Körper, pag. 169

  • Ehrlich, R., Why Toast Lands Jelly-Side Down: Zen and the Art of Physics Demonstrations, pag. 34

  • Ehrlich, Robert, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, pag. 35

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