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# lecture7 - Today's Lectures The Laws of Gravity Speed...

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Today’s Lectures: The Laws of Gravity Speed, velocity, acceleration, force Mass, momentum and inertia Newton’s threelaws of motion The Force of Gravity

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Before Newton Kepler understood the phenomenology of planetary motions, I.e. the three laws, but never understood *why* planets move the way they do He thought of magnetic force from the Sun He even thought (and dismissed) that Angels pull planets along their orbits Galileo understood inertia, force and acceleration very well, but fell short of understand the causal relationship between force and acceleration. He also did not propose a universal law for the force of gravity

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Describing Motion Motion is when the position of an object changes in time If position does not change, the object is at rest The describe motions we need to monitor position and time The rate at which an objects covers a given amount of space in a given amount of time is called speed v = s/t
Vectors, or things with a sense of direction There are physical quantities in nature for which only one number, their intensity , tells the whole story: e.g. temperature, mass, luminosity, color Other quantities need both an intensity and a direction to be fully described: e.g. speed, force, acceleration. These quantities with a “direction” are called VECTORS

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Galileo and theConcept of Inertia Aristotle held that objects at rest remained at rest unless a force acted on them, but that objects in motion did not remain in motion unless a force acted constantly on them: F = v WRONG . Galileo concluded that an object in a state of motion possesses an ``inertia'' that causes it to remain in that state of motion unless an external force acts on it RIGHT .
Galileo’s Law of Inertia But velocity cannot not the whole story: we need a more general concept to describe motions: momentum P = m·v An object maintains its “quantity of motion” ( momentum ), unless a force is acted upon it Only a force can change the “quantity of motion” ( momentum ) of an object, either by changing its speed, or by changing its direction of motion, or both

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More on Momentum
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lecture7 - Today's Lectures The Laws of Gravity Speed...

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