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09/01/10 - Why do things move the way they do and why do they stop?
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 | What happens when there is no net force acting on an object?
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 | Note that no net force includes friction and air resistance.
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| This is now called Newton's First Law.
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| Galileo first identified this principle, called inertia. If an object has no net force acting on it, then it's velocity remains unchanged. That is, if an object is at rest and no net force acts on it, then it will continue to be at rest. If an object has a velocity and no net force acts on it, then its velocity will remain unchanged. It will move at a constant speed in a straight line.
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| This is why it's hard to push a car and start it moving, but once it's rolling, it's not so hard to keep it moving on flat ground. In the same way, once it starts moving, it's hard to stop it.
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| The mass of an object is a measure of its inertia. The more mass an object has, the harder it is to change its velocity.
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| Note that wheels are a way to reduce the frictional force acting on an object.
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| What happens when there is a net force acting on an object?
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| If there is a net force acting on an object, then its velocity changes. That is, it accelerates. It either speeds up, slows down or changes direction. The direction of the change in velocity is the same as the direction of the net force. The magnitude of the acceleration depends directly on the size of the force F and inversely on the mass of the object: a = F/m. The SI unit of force is the Newton (N). A force of 1 N will accelerate a mass of 1 kg by 1 m/s/s.
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| This is called Newton's Second Law.
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| What happens when you exert a force on another object?
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| This is the principle of action and reaction. It says that when one object exerts a force F on another object, the second object exerts an equal and opposite force on the first object. In other words, if two people are on skateboards (to reduce friction), and one pushes the other, they will both move apart. Alternatively, if one person on a skateboard pushes against a wall, the wall pushes back and causes the person on the skateboard to roll away from the wall.
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| This is called Newton's Third Law.
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| Read the first two sections of Chapter 2, pp. 38-48.
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