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Unformatted text preview: Lecture 3 Newtons 1 st and 2 nd Laws 1 Review from Last Time Review relations between position, velocity, acceleration. Examples 2 Law of Inertia (Newtons First Law) Galileo (1564-1642) is famous for his application of the telescope and challenging the church, but he also studied mechanics. In fact, what we now call Newtons 1st Law actually came from an idea posed by Galileo. Before, Galileo, popular belief was still with Aristotle who said that the natural state of an object is at rest and all objects tend toward this state. This seems to agree with our everyday experience since, if I roll a ball, it will eventually stop (DEMO: cart on airtrack with air off). Galileo did experiments to show this is not true. He argued that another force (friction) causes the object to slow down and stop. We can easily see this in winter - objects go much further on ice because the frictional force is less. The Law of Inertia states, Left on its own, an object in motion will remain in motion. This is the basis for Newtons 1st Law which says, In the absence of a net (unbalanced) external force, an object moves with a constant velocity. As well see in a bit, Newtons 2nd Law goes one step further and tells us what happens when there is a net force acting on an object. 3 The Momentum Principle (Newtons Second Law) Now lets turn to the second law. I am going to introduce this law in a more general form than appears in Chapter 4 of your text (the reading for todays lecture). First, we need to define the momentum . 3.1 Momentum How do objects respond to interactions with other objects? If I hit a tennis ball with a racquet, clearly the ball will fly across the court. What if I hit a bus just as hard with my racquet? Not much (except that Ill probably break the racquet trying to make the bus move!) Why? 10 Mass matters! Mass describes how hard it is to accelerate an object (the inertia). A bus is very massive so doesnt accelerate much when subjected to the same force as the tennis ball. Now a bullet has a relatively small mass. So why is it so hard to stop or deflect a bullet compared to a tennis ball? (I could easily stop a tennis ball with my hand, but not a bullet!) Because velocity matters too! A bullet moves very fast (500 m/s) so its harder to stop....
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