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Momentum

This is going to be the final chapter on motion. When we discussed Newton’s Laws we already talked about how we are trying to find a cause for the motion. However in with forces we only found out what causes motion to change, the question when does motion stop or why does it keep going seems simpler to answer then figuring out what causes object to change their motion.

After Aristotle, who talked about the natural order of things being the cause for motion, it was a Persian philosopher who looked further into motion.  In the 11th century Ibn Sina studied the motion of projectiles and assumed that moving objects contain a quality that is related to their heaviness and given to them by the person or thing that made the object move in the first place. He called this quality “mayl” and for him it was the things that “helped carry the motion”. It was a quality of the object itself.

Three hundred years later this idea was changed a little with Jean Buridan (who only lived to be 30 years old) talking about motion as being a property that is given to the object. Much like a coat of paint that is put on a ball. This paint will fade but only if wind or sun or rain will make it erode. He called this property “impetus”. Impetus is given to an object when it is made to move. Air resistance and wind ca decrease the impetus and gravity can increase the impetus. Both of these are outside forces. Without outside forces the impetus would continue forever. Heaviness of the object can also change its impetus.

This relation of mass to motion (or velocity) is important because so far we have thought of motion as independent of mass. The combination of mass and velocity we now call momentum, the Latin term for movement. In the lab we did we figured out that momentum is a quantity that can be transferred form one object to the next if these two objects come in contact, that is, if they collide. When a big (moving) ball collides with a small ball at rest some or al of the motion gets transferred from the big to the small ball. This can also work the other way if a small ball collides with a big ball, but it is easiest to see when two of the same balls collide. In the lab we actually found a relationship between the momenta of the two objects and turned it into a mathematical equation, something that was far out of reach for geniuses like Newton or Galileo, because algebra was not a tool used to proof concepts back then.

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We can think about this whole concept of motion and momentum from a different angle as well. Let’s think about how the term momentum is used in every day life. A soccer team can have a lot of momentum, or a stock on the stock market. There are momentum plays on the stock exchange. A football player can have a lot of momentum. What all of these have in common is that there motion will be hard to stop (or change direction in the stock example). In physics “momentum” can be interpreted in the same way. We could ask ourselves how hard it would be to stop an object. This depends on both mass and velocity of the object and therefore agrees what with what we discussed earlier.

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When we look at stopping of an object more carefully we will see that there are two different ways to stop an object. Both involve force. Without force an object cannot be stopped, because stopping is changing motion and changing motion can only be accomplished with a net force.  You can stop a cruise ship for instance, by applying a small force (friction) or by applying a big force (running it aground). The two forces are different (big and small) but something else involved in the process of stopping the ship is also different – the small force has to be applied over a long time, the big force can be applied over a short amount of time. There are many applications of this concepts from airbags in cars o how to handle a ball in lacrosse.

The force slows down the ship, therefore changing its velocity and because of that the momentum changes. This change in momentum, caused by a force, is called the impulse.

 

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