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Newton Car - Conservation of momentum

 
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pwozniak



Joined: 12 Apr 2010
Posts: 1
Location: Pennsylvania

PostPosted: Mon Apr 12, 2010 10:01 pm    Post subject: Newton Car - Conservation of momentum Reply with quote

There is something that I think is not explained well in the classic Newton Car experiment. In that experiment, a block is shot off the back of a car (using rubber bands) in one direction, causing the car to roll in the opposite direction. This experiment explains Newton’s Second Law:

Force = Mass x Acceleration

So for this experiment:

Force (rubber band) = Mass (block) x Acceleration (block)

It also explains Newton’s Third Law of equal and opposite reaction: The action force of the block equals the reaction force exerted on the car.

Mass (car) x Acceleration (car) = Mass (block) x Acceleration (block)

There is one series of trials in the experiment where the number of rubber bands are increased to create a greater force and the distance the car rolls is measured. With a greater force, the car rolls further. That is understandable.

The question I have comes about when the mass of the block is increased. In the experiment, weights are added to the block and the distance the car rolls is measured. As weights are added, the car rolls further. The explanation given is “Newton's second law states that a larger mass causes a larger force.” But why is that? My thinking is the force is really caused by the rubber band. That force remains the same (assuming same stretch point, etc.). If the mass of the block is increased, the acceleration of the block should decrease according to Newton’s Second Law.

Acceleration (block) = Force (rubber band) / Mass (block)

The force, then, exerted on the car in the opposite direction, should remain the same. But the experiment shows the car going further with a greater mass of the block. Why doesn’t this equation hold?

Mass (car) x Acceleration (car) = Mass (block) x Acceleration (block)

The results prove that a greater force is somehow being exerted with a larger mass. But how can a “greater force” be “created” when the force (energy) of the rubber band is the same?

So something else must be going on that is not apparent. Thank you for your help.
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hepcj
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Joined: 23 Jun 2007
Posts: 125

PostPosted: Sun Apr 18, 2010 6:06 pm    Post subject: Reply with quote

While I am not sure I understood the set up of the experiment. I think your question boils down to if you increase the mass of the block, how is it that the car can travel further with the same force applied by the elastic bands.

The force is shared between the mass and the block but the greater mass of the block means it will not accelerate as much as the car and so the car goes further.
I think you can understand the concept more easily when you think about what happens when pushing away from a swimming pool. When you push on the wall of the pool, it does not move and so you are able to move in the opposite direction. This would not be the case if the pool wall was able to move away as you pushed it with your feet.
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