Use an empty pill bottle or fill with water, washers, sand or other objects to change the mass in the bottle. Place the bottle in the nook of the rubber band(s), making sure that band(s) are centered on the posts. Distribute straws below the car, making sure they are parallel and placed evenly across a smooth surface.
It is helpful to mark the positions of the straws to enable accurate measurement of the distance moved by the car during various trials of the experiment. Place the car on top of one end of the straw “road,” with the end of the car with looped string facing toward the straws.
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