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Mechanics Kinematics – how things move vs Dynamics – why things move one reason: forces. And now for a bit of history… Around 350 BC – Aristotle described 2 types of motion Aristotle’s 2 Types of Motion Natural – things that just naturally moved the way they do a heavy object falls a light material rises a heavenly body circles and, most commonly, objects come to or stay at rest, their “natural” state, including Earth – it wasn’t moving. Violent – any motion that required a force to make it occur most notable, any object that keeps moving, would require a force to make it so But now we know better! Nicolaus Copernicus worked through the early 1500’s to try to explain that it was actually the Earth that moved around the sun, but fear of persecution by The Church, meant he kept this a secret Until Galileo Galilei, in the late 1500’s and early 1600’s, not only publicly supported Copernicus, but had a few ideas of his own that would shatter our 2000 year old understanding of why things move… He demolished the notion that a force is necessary to keep an object in motion, by defining and explaining friction. 1st: a force is a push or a pull 2nd: friction is a force that acts between 2 touching surfaces as they try to move relative to each other. It opposes this relative motion, slowing the objects down. 3rd: he was able to envision a world without friction – then once an object was pushed or pulled, it would move forever without any additional forces acting 4th: Consider What happens when a ball is rolled up or down a ramp – then what if there was no ramp, only a horizontal plane? then it will roll forever What happens on the double sided ramp – if the ball tries to reach its original height, then what if the other side of the ramp was flat? then it will roll forever Galileo defined a new natural state as whatever the object was already doing, that’s what it would continue to do unless a force acted to change it. Since then, we named this idea… inertia – the tendency of an object to resist a change in its state of motion Do all objects have the same amount of inertia? Does a wadded up ball of paper have the same tendency to resist a change in its state of motion as an 18 wheeler truck? No, the more mass an object has, the harder it is to get it going if it is stopped and stopped if it is going – the more inertia. Mass – the amount of matter in an object Inertia – the tendency for an object to resist a change in its state of motion As it turns out, (and it took about 200 years for scientist to figure this out!) mass and inertia are 2 different ways to describe the exact same property of an object Back to some history: Newton was born the very same year that Galileo died, and within 25 years (1666), he was the next scientist to carry on the torch of enlightenment in England at a time when the public was much more receptive to these ideas, so this time, they stuck… Newton’s First Law of Motion: Every object continues in a state of rest , or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it. In other words, objects will do whatever they’re already doing if stopped, then stay stopped if moving, then keep moving – with constant velocity unless acted upon by an external force Also called the law of inertia Now let’s look at some demos of this… Nail into Wood with Cup & Book When just the cup, it’s crushed: A force is applied to the wood, which applies a force to the cup and changes it’s motion – from rest to crushed. When the book is in there, the cup is safe: The large inertia (mass) of the book is at rest & stays at rest, so it protects the cup underneath Like street performers that mash melons on the chest of a buddy… but 1st they put a concrete block on their chest… Quarter, Card & Cup Flicking the card applies a force to it, so it changes it’s state of motion – was at rest, now moving. But if you flick the card straight & fast, no force is applied to the quarter, so it’s inertia (mass) stays at rest. The proof is that it drops into the cup. Same idea as Tablecloth Trick! Pull straight out – with enough room for entire cloth Pull hard/quickly Plates should have good inertia (mass) Reduce friction with smooth cloth & plates Green Truck & Guy With a quick start forward, you FEEL pushed back… But really the car applies a force to move forward while you’re inertia (mass) wants to stay at rest. Safety device to avoid injury? Headrests! With a sudden stop, you FEEL thrown forward… But really the car applies a force to slow down while you’re inertia (mass) wants to keep moving. Safety device to avoid injury? Seatbelt! Newspaper Breaks Board The large column of air above the sheet of newspaper is at rest. When a sharp blow is delivered to the end of the board, it changes that end of the board’s motion. But it can’t change the other end of the board – the one that’s under the newpaper – because of the large inertia (mass) of air sitting on it, which the force wasn’t applied to, so it remains at rest, causing the board to break.