212 – Propulsion by recoil

Boat with water propulsion  

Rocket principle  

Introduction to:  

• Conservation of momentum  
• Recoil  

Material:  

• Food storage box made of polystyrene / plastic plate
• Cups (light, e.g. polystyrene cups, plastic cups)  
• Straw  
• Glue / Hot Glue (optional)
• Scissors / Drills  
• Vessel with water  
• Tub with water / natural water surface  

Setup:  

• Use the scissors to drill a hole the diameter of the straw on the side of the lower part of the cup  
• Glue the straw into the hole  
• Drill a hole in the side of the food storage box and pass the straw through it  
• Glue the straw into the hole

Execution: 

• Put the boat in the tub of water  
• Carefully fill the cup halfway with water  

Observation:  

• The water flows from the cup through the straw into the tub  
• The boat accelerates in the opposite direction  

Explication:  

• Due to the pressure at the bottom of the cup, the outflowing water receives an impulse, backwards 
• Due to the conservation of momentum, an opposite impulse is created on the boat, forward (recoil)

Didactic note:  

• The propulsion system works on the same principle as a rocket