06 Recoil of a Water Jet

Aim¶

To show an example of Newton’s third law.

Subjects¶

• 1H10 (Action and Reaction)

• 1N22 (Rockets)

Diagram¶

Equipment¶

• Tube with a T-junction, $\varnothing 10$

• Rubber hose (about 2 meters)

• Faucet

• A tray to catch the water

Presentation¶

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

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¶

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 3).

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 to 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