Lecture4

Lecture4 - Making Sense of the Universe: Understanding...

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1 Making Sense of the Universe: Understanding Motion, Energy, and Gravity If I have seen farther than others, it is because I have stood on the shoulders of giants .” Sir Isaac Newton (1642 – 1727) Describing Motion In the previous lecture we learned that the ancient Greeks thought that it was natural for everything to be in a state of rest. Galileo Galilei’s experiments showed that this was not true. Our goals for learning: How do we describe motion? How is mass different from weight ? In this chapter we will discuss the laws that govern motion and energy in the universe. By understanding these laws, you will be able to make sense of the wide range of phenomena that we will encounter. Remember, our goal is to understand the “big picture” and to get the “essential” cosmic perspective . How do we describe motion? Precise definitions to describe motion: speed : rate at which object moves speed = distance time units of m s example: speed of 10 m/s velocity : speed and direction example: 10 m/s, due east acceleration : any change in velocity (either rate or direction) Units must be speed/time (m/s 2 ) Something that has a size and direction is called a VECTOR Momentum and Force We need to recognize a few more ideas before we can pull all this together : • (Linear) momentum = mass velocity (p = mv) • Newton found that a net force changes momentum • Since mass is usually constant, this means an acceleration (because of a change in the velocity part of mv ) – Studying collisions helps to grasp the concept of momentum – The mere presence of a force is not enough. Forces can be in balance. You need an imbalance or net force to get a change in momentum
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2 Thought Question: Is there a net force? Y/N 1. A car coming to a stop. 2. A bus speeding up. 3. An elevator moving up at constant speed. 4. A bicycle going around a curve. 5. A moon orbiting Jupiter. Is there a net force? Y/N 1. A car coming to a stop. Y 2. A bus speeding up. Y 3. An elevator moving at constant speed . N 4. A bicycle going around a curve. Y 5. A moon orbiting Jupiter. Y How is mass different from weight? mass
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Lecture4 - Making Sense of the Universe: Understanding...

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