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Unit 3 – Newton’s Laws of Motion! Hand back test Lecture on History of Motion Introduction to Newton’s 1 st & 2 nd Law

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Today you should learn… What is mass? What is inertia? What did Aristotle think the natural state of objects was? What is Newton’s 1 st Law? If you were in outer space and you threw a ball 10 m/s when would it stop?

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A Brief History of Motion:

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Aristotle’s Thoughts It’s natural for objects to stop moving.

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Aristotle continues to think… A force is needed to move an object (which is against nature) Ipso Facto: if the force stops, the motion stops.

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Aristotle, Aristotle, Aristotle… Some natural motion exists (up and down), Violent motion are caused by an external force.

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Belief held for 2000 years. What do you think?

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Galileo’s Thoughts: Every object resists a change to its present state of motion.

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Inertia An object’s tendency to keep moving… Thanks to Galileo!

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Then came Newton… Restates Galileo’s idea Things keep on doing what they are already doing.

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If at rest – resists moving

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If in motion – resists changing that motion!

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Newton’s First Law – Law of Inertia Objects that are at rest stay at rest until a force acts on it. An object in motion will stay in motion, until a force acts on it.

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Back to Inertia Resistance to change in velocity is caused by mass. Big mass – takes a lot to change its velocity!

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Different inertias The larger mass … Larger resistance to change in velocity

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A ball is rolling on a counter… And slowly comes to a stop. What would Aristotle have said was the cause? What would Galileo and Newton have said? And you? What do you say?

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And force is … A push Or a pull In the SI unit of measurements A Newton (N)

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“Net” Force Takes into account ALL the forces acting on an object! If you have a 6 N force pushing and a 4 N force pulling The net force is a 2 N force pushing

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Mass versus volume Mass = amount of matter Volume = how much space the object takes up.

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Mass versus Weight Mass = amount of matter. Weight = force of gravity on an object.

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SI Units Mass = kilogram Weight = Newton So, what is a pound? Mass or weight?

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Calculating weight Weight = mass X acceleration of gravity = mass x (9.8 m/s 2 ) 1 kg = 9.8 N

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Check your understanding… What is mass? What is inertia? What did Aristotle think the natural state of objects was? What is Newton’s 1 st Law? If you were in outer space and you threw a ball 10 m/s when would it stop?

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Newton’s Second Law: A net force acting on an object will cause the object to accelerate in the direction of the net force. Force = mass X acceleration

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Free Body Diagrams: The size of the arrow in a free- body diagram reflects the size of the force. The direction of the arrow shows the direction the force is acting.

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Forces Weight Tension Friction Normal Springs (later)

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October 26 Newton’s 2 nd Law Lab II Finish with carts and graph.

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Inertia What is Inertia? Resistance to change in current state of motion (or lack of motion).

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Inertia determined by… Amount of mass.

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What is Newton’s 1 st Law? An object in motion will remain in motion until an outside force acts on it. An object at rest will remain at rest until an outside object acts on it.

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Forces Weight: W = mg W = mass x 9.8 m/s 2 Friction Tension Normal

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Newton’s 2 nd Law A force acting on an object will cause the object to accelerate in the direction of the force.

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F = ma Force – Newtons Mass – kilograms Acceleration – m/s 2

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Force = mass x acceleration

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Mass = Force / Acceleration

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Acceleration = force / mass

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A 95 kg person starts running and accelerates at a rate of 6.5 m/s 2. How much force is he applying to get moving? An engineer walks into a bar…

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And a one, and a two… I push a curling stone (mass = 35 kg) with a force of 300 N across a frictionless ice rink. What is the acceleration of the ball?

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Why did the chicken fall out of the sky? What is the mass of chicken that hits the floor while accelerating at 9.8 m/s 2, if the force is measured to be 540 N?

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A tree starts to walk out of a bar “leafing so soon?” A van strikes a tree with a force of 10,500 N. It has a mass of 2100 kg. What was its acceleration?

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Force is directly proportional to acceleration. As Force gets bigger Acceleration gets bigger Assuming mass stays the same…

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Mass is inversely proportional to acceleration

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For today’s lab Our force comes from weight W = mg We’re keeping mass constant… So what does that tell us about force and acceleration?

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Acceleration and time a = 2d/t 2 If we “ignore” distance… Acceleration is proportional to 1/t 2 Or if acceleration increases… 1/t 2 increases

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If we apply more force, we should see “1/t 2 ” increase. f = (1/t 2 )

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Let’s see if we can prove it in lab…

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October 28 Lecture over Newton’s 3 rd Law Friction Lab

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Newton’s 3 rd Law Notes

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3 rd Law Whenever one body exerts a force on a second body… The second body exerts an equal and opposite force on the first.

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Newton’s 3 rd law Or… For every action There is an equal and opposite reaction

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Forces come in pairs You lean against a wall… You sit in your seat… You stand up and walk forward in a canoe… You jump up from the earth… The earth pulls you back down…

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More pairs In space – you throw a ball It goes forward You go backwards On earth – you pull a trailer with your car It pulls back with the same force So how do you move forwards?

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Newton’s 3 rd law Whenever object A exerts a force on object B, object B exerts an equal and opposite force on object A. Or… For every action There is an equal and opposite reaction

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Pairs of Forces One is called the ACTION FORCE. The other is called the REACTION FORCE

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You tell me… Come up with an example of a pair of reaction forces related to riding a bike.

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The “Great Misconception” If forces are equal and opposite… HOW CAN ANYTHING MOVE?

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Because… A… B… ? The action force acts on Body A, while the reaction force acts on Body B.

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Remember that differing mass can cause objects to accelerate at differing rates.

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If force is the same… Small mass = high acceleration Large mass = low acceleration

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Lab… Lots of action/reaction pairs And some friction, too!

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October 30 Friction notes Friction Lab

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Forces? Friction Tension Weight Spring “Normal”

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Net Force? What are we talking about?

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What forces are acting… On a falling rock? On a book pushed across the floor? On a monkey swinging from a rope?

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F = ma If you double the mass… What happens to the acceleration? If you double the force… What happens to the acceleration? What is your acceleration… If you move with a constant velocity? What is the net force if that is true?

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The Force of Friction

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Determining factors: Does surface area matter?

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Other factors… Does the type of surface matter? If so, which one?

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More factors??? How about the amount of force between the two surfaces (often caused by gravity).

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Just what is going on with friction? Surfaces are not totally smooth Pits and bumps Atom to Atom interaction Electromagnetic forces Either attract or repel.

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Static? Moving? This atom to atom attraction is called static friction. Once object is moving, it is called sliding friction.

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Once you get it moving… Electrostatic has less effect Just bumping along Moving friction is less than the maximum static friction

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Coefficient of Friction: Force friction = µ Force normal µ is just a constant determined by the 2 surfaces involved. force normal = weight in our case.

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This lab… Is a brick…

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November 3 POTD Friction Friction Lab

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The Force of Friction

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Determining factors: Does surface area matter?

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Other factors… Does the type of surface matter? If so, which one?

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More factors??? How about the amount of force between the two surfaces (often caused by gravity).

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Just what is going on with friction? Surfaces are not totally smooth Pits and bumps Atom to Atom interaction Electromagnetic forces Either attract or repel.

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Static… sliding… This atom to atom attraction is called static friction. Harder to get an object moving Than to keep it moving. If it is moving, it is called sliding friction.

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Once you get it moving… Electrostatic has less effect Just bumping along Moving friction is less than the maximum static friction

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Coefficient of Friction: Force friction = µ Force normal µ is just a constant determined by the 2 surfaces involved. force normal = weight in our case.

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F = ma = More Newton November 5, 2006

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Today F = ma again! Short review/worksheet

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Velocity is constant: F = ma What are the forces? Tug of war Pushing a box over the ground Human Dominos

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Velocity is changing: F = ma Free fall: What is different? A piece of hail Or a bowling ball What happens if you slide down a hill? What forces are different than free fall?

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Sledding in K. Falls 30 seconds to go 300 meters What was our acceleration? a = 2x/t 2 What if you know the initial velocity and the final velocity? a = (v f – v i )/t What if the slope wasn’t constant?

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Parachuting What is your acceleration if the air drag is equal to ½ your weight?

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Worksheet Work on your own first. When I let you know – work with your lab partners to compare answers Come and get your answers checked Fix your answers and turn in

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