Physics chapter 4 post-lecture

Physics chapter 4 post-lecture - Physics 101 Lecture 5...

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1 PHYS 101 -- Lecture 5 1 Physics 101 – Lecture 5 Dynamics Summary: • Dynamics: why things move • Newton’s first law and ‘inertia’ – inertial and non-inertial frames • Newton’s second law: F = ma – mass vs weight • Newton’s third law: action and reaction PHYS 101 -- Lecture 5 2 We’ve talked about how things move – the description of motion: kinematics. Now we will consider why things move. That study – of forces and their connection to motion - is called dynamics … and you have likely already seen it in the guise of Newton’s equations. Intuitively we know what a force is: - a push or a pull, necessary to get something moving Or: - to make it move differently from how it’s moving now. So force has both a magnitude (a nudge is different than a shove) and a direction (pushing is not pulling): force is a vector quantity .
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2 PHYS 101 -- Lecture 5 3 Motion and forces: which of the following is true? C ontinual a ppl i c ation. .. Force s not neces sa. . I don’t know 84 12 491 1. Continual application of a force is necessary for motion 2. Force is not necessary for there to be motion 3. I don’t know PHYS 101 -- Lecture 5 4 Consider a real-world example: you turn off the engine in your car at 110 km/h, and you glide to a stop. It appears force (from the motor) is clearly necessary for motion to continue. In fact, this misconception persisted for millennia, famously couched in the differing opinions of Aristotle vs. Galileo: Aristotle: the “natural state” of objects is at rest; a force is necessary to “keep them moving” Galileo: motion at constant velocity is just as “natural” a state as rest;
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3 PHYS 101 -- Lecture 5 5 Problem here: our “intuition” is developed in the real world, where friction (a hidden force) is commonplace – in fact unavoidable . So our intuition misleads us… Galileo’s genius – apart from a keen observational sense – was to realize that to idealize the world actually helps to make sense of it . If you idealize it and remove the force of friction, then you can see the “naturalness” of what we now call Newton’s first law of motion : Every object continues in its state of rest (or of uniform velocity in a straight line) unless acted upon by an outside force. The word “inertia” (and misleadingly, the term “force of inertia”) is
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Physics chapter 4 post-lecture - Physics 101 Lecture 5...

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