# How to Build a Newton Car

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A Newton car demonstrates Newton’s third law of motion, namely the law of interaction: For each action, there is an equal and opposite reaction. The car does this by hurling a weight off the back, forcing itself forward. This is a demonstration of how rockets are propelled in space, ejecting something that was initially internal. (This experiment also illustrates his second law of motion: Force equals mass times acceleration.)

Use the plane to round out the front lower edge of the block of wood.

This will be one of the 10 cm edges. (Picture the block lying flat.) Doing so will allow the car to travel over the straws laid in its path.

Screw the three screws into one of the two broad sides of the board, so as to form a triangle centered on the board.

Two screws will be close to the corners at the end of the board opposite the end that was planed. The third will be at the other, planed end of the board, some 20 cm away, centered between two corners on its own end.

Loop a rubber band over the back pair of screws, then pull it forward and tie its center to the third screw with a piece of string.

Make a track for the car with equally spaced straws in front of its planed end.

Place the film container onto the board, inside the V-shape made by the rubber band. Fill it with weights, e.g., pennies.

Place the board flat on top of the straws, screw and weight side up.

Cut or burn the string to release the rubber band, sending the film container and the car in opposite directions, having exerted equal and opposite forces on each other. The car will accelerate over the straws, which will provide a near frictionless path.

#### Things You'll Need

• 25 straight straws (not flexi)
• rubber bands
• string
• block of wood, about 10x20x2.5 cm
• 3 3-inch #10 wood screws
• wood plane
• scissors or matches
• plastic film container
• screwdriver
• pennies or other small weights