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Newton’s laws

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NEWTON HIMSELF

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Newton

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Life Scholar –Brilliant mind –Became Head of Oxford Colleges in England –Royal Society of England –Master everything in his age Teacher –Help to develop whole branch of Mathematics –Newtonian physics Alchemist

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LIMITATIONS OF HIS LAWS

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Newton’s laws of motion Clearly laid down the rules that govern why things move in our world Rules can not fully explain motion on or smaller than the atomic level or movement close to the speed of light

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Review Net force Balanced Force Unbalanced forces

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NEWTON’S FIRST LAW Law of Inertia

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Rolling Ball

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A bowling ball will move with nearly constant velocity No significant friction from the ground or air resistance to help change the bowling ball’s motion Velocity of ball changes only when it hits the pins or the back wall of lane

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Newton’s first law An object at rest stays at rest or an object in motion stays in motion UNLESS acted upon by a net outside force

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How does the 1 st law relate to you as you walk down the hall?

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Newton’s first law Definition

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Why should a baby not be transported on a car in a person’s lap? Would it be ok if you put the seat belt around you and the baby?

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If Fluffy the Kitty was sleeping (without a seatbelt) in the rear window well of a sedan… Describe Fluffy’s motion if the car was involved in a head-on collision?

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How does one get whiplash? How does whiplash relate to 1 st law?

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1 st law of motion An object at ______, stays at ______ An object in motion, stays at a _______ rate Unless A net outside ________ acts upon it.

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Trebuchet

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NEWTON’S SECOND LAW The equation law

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Newton’s 2 nd law If there is a net force on an object, then the object will accelerate The amount of acceleration depends on the following relationship: A = F net m

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2 nd law A = F net / m or rearranged F net = ma Triangle set up to solve for different variables Units: acc m/s 2 F net N mass kg

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2nd law can be represented using a triangle F net = m a F net MA t

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When Solving problems using 2 nd law F net means net force or the sum of all forces on the object Force is measured in N (Newtons) Mass is measured in kg Acceleration is measured in m/s 2

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The greater amount of net force applied, the larger the acceleration of a specific object

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The greater the mass, more force is needed to produce the same acceleration

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An object will accelerate at a higher rate as more force is applied An object will accelerate if more mass is added to it

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2 nd law An object will change its motion only if a net force is applied to it (f net ≠ 0)

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If you apply the same force to roll each a long the floor, how will their motion compare?

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NEWTON’S 3 RD LAW

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Newton’s third Law

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3 rd Law For every _____________ There is an __________ but opposite __________.

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Newton’s Third Law For every action force, there is an equal reaction force applies in the opposite direction

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What are Action-Reaction forces For any force applied to an object, the object will respond with a like force back Forces are equal in size Forces that are applied to different objects Examples in Picture?

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Action and Reaction Forces Action Force: the force of most interest, initiates the action Reaction force: force applies in response to action force

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Action and reaction forces Equal and opposite Applied to different objects Applied at same time May have different effects

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If all forces are equal and opposite, how does anything move?

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Answer Though equal in size, the forces act on different objects Force of ground on box Force of box on ground

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The effect of the action reaction forces may differ on an object. By first law, objects accelerate if there is a net force on them What other piece of information is required to determine the amount of acceleration?

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If I throw a punch, why does my hand hurt?

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Consider the guys hands pressing down on desk. What are the action /reaction forces?

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Action/Reaction force related to the hand

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Action/Reaction force of the falling object

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FINDING REAL LIFE EXAMPLES OF 3 RD LAW

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Examples of 3 rd law

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Actions that can be related to 3 rd law in the classroom

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APPLICATION OF NEWTON’S 2 ND LAW

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Determining the weight of an object Weight = force due to gravity F = ma (by 2 nd law) F = mass (in Kg) times (-9.8) Determine mass by measurement on scale or conversion

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Determining mass Convert from pounds to kg by: =(weight in lbs) / 2.2

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2nd Law problem set up #1 Draw free body diagram #2 Calculate as many of the forces on the FBD as possible #3 What are the givens? #4 what do you want? #5 what equations will connect the givens to what you want #6 solve and check

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Using Newton’s laws use 2 nd law and free body diagrams as tools to determine force, acceleration and mass values Combine with con acceleration equations to solve for all of the old variables

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Finding Net force Net force is the sum of all forces on an object At the start only one direction will have unbalanced forces All forces are added, but forces in one direction are positive, and forces in opposite direction are negative

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Net force on a helium balloon Assuming the balloon goes up, how would the net force on the balloon be determined? How would you determine the acceleration of the balloon? Force of helium Weight

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Fnet = F helium – Weight By 2 nd law F net = ma

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Example Bob is holding his very favorite stuffed dog (mass 0.35 kg). Mary decides that she wants it and tries to pull it away from Bob. If Sarah pulls horizontally to the left with 100 N of force and Bob pulls horizontally to the right with 120 N, what is the horizontal acceleration of the dog?

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Helicopter problem A 4500 kg helicopter accelerates upward at 2.0 m/s 2. What is the lift force exerted by the air on the propellers?

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Elevator Problem

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Elevator problem A elevator car from the disney ride “Tower of Terror” is lifted by a cable up to the top of the shaft. If the 1200 kg car is carrying 12 people with an average mass of 64.3 kg and is lifted by the cable with a force of 54000N, what is the acceleration of the car? How do you determine if the car is moving with constant acceleration?

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info How many people = Average mass of each person = Mass of car = Acceleration upwards = Applied force needed to move the object upwards

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DETERMINE VALUES USING CONSTANT ACCELERATION EQUATIONS AND 2 ND LAW

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Combination problem A 4500kg car moves down a level highway at 20 m/s. If 1300 N of force pushes it forward and a 650 N resistance (friction and air resistance combined) pushes backward on object, find the following… –The acceleration of object –How fast object travels after 1 minute –How far it travels after 1 minute.

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Sliding into base

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Free Fall Any object that moves under the influence of only the force due to gravity Thrown, Dropped objects Acceleration = 10 m/s 2 or (9.8)

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What is the velocity and acceleration value at the following points in time of a heavy object being thrown upwards at 5.0 m/s? a)Just when it leaves the hand? b)At the apex of its flight? c)Just before it is grabbed by the hand?

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Advanced 2 nd law problems #1 Have to use the acceleration equation first then use 2 nd law #2 Have to determine F net first, then use 2 nd law

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Example Nicole slows down in her BMW from 15 m/s to 3 m/s in 42 seconds. What is the mass of her car if the net force on it is 7312 N?

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Example A small car has a mass of 957 kg rests on a level icy patch of road. If a person can apply 500 N of horizontal force, what will be the car’s acceleration?

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USING 2 ND AND 3 RD LAWS TO SOLVE QUESTIONS

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Connections How does first law explain the change of direction of the ball when hit by the bat? Concerning bat and ball, what are the action and reaction forces?

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Which undergoes a greater change of velocity, ball or bat?

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What is true about the action and reaction forces between ball and bat?

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Draw free body diagrams of the ball and bat Are both action and reaction forces effecting the ball? Are the masses of the ball and bat the same? If the same force was applied to both ball and bat would it have the same effect?

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Third law problems Interaction of forces Two boxes are placed one on top of the other on a table What can you say about the applied forces

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3 rd law problems Boxes and string problems

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Third law problems Stacked Box problem What is the force on the top box? What is the force on the table? What are the forces on the middle box?

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Third law Accelerating boxes with ropes What is true about the acceleration of both boxes? How do the forces on the ropes compare to each other?

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Rope problems Look at the whole, look at the parts Acceleration of all parts is constant

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Pulley problems Pulley is a simple machine that, at minimum, changes the direction of the force 2 blocks starting from rest. What is the magnitude of the acceleration of either?

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Pulley problems Pulley is a simple machine that, at minimum, changes the direction of the force The tension on the rope must be the same throughout Ropes do not stretch or elongate in our questions 2 blocks starting from rest. What is the magnitude of the acceleration of either?

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AUTO ACCIDENTS AND NEWTON’S LAWS

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Auto Accidents

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Have you been in an auto accident?

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In 2004, there were over 42,000 deaths involving automobiles and between 3-6 million injuries in the United States

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There may be as many as 750,000 million deaths and 24-36 million injuries resulting from auto-accidents worldwide per year

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What are the ways people get hurt if they inside the autos during the accident?

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Be thrown through windshield Crushed Injured by steering wheel, or dashboard Injured by loose material in car Injured by objects outside of cars

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Frontal accidents Which way does the person travel after impact? How about stuff on the seat? How about stuff in the person’s hands?

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Can you explain, using Newton’s Laws why the face is against the air bag instead of the headrest?

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Rear accidents Which way does a person’s head go upon impact from behind? What would happen to the head without a headrest? Should the airbag deploy in a rear-end collision?

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How does speed relate to auto accidents?

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At low speeds

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At somewhat higher speeds

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Collisions and the laws Be able to use the laws of motion to explain what happens in different types of collisions Front crash Rear crash At least 2 reasons for head rests Does the bus and the car exert the same amount of force on each other? Why are people less injured on the bus?

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Collisions and the laws Seat-belt configurations What is wrong with just lap-belts? Why are seat-belts flat and wide? What are uses for EKG’s, MRI’s and Xrays?

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Newton’s second law Triangle Metric units of force

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Wrap up of forces Any questions about the material over the last 2 weeks Questions on seat belt paper Wall questions Yellow booklet (due Wednesday) pages: –2,3,14-19 (only pre-lab questions on page 19), 21,22,45

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To review Chapters 10,11 (mostly 11) What is a force? Types of forces? What are Newton’s 3 Laws of motion? Examples of the 3 laws Free-body diagrams Calculating with the 2 nd law Gravity, Weight, Black holes

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Newton’s 3 laws !st 2 nd 3 rd When does an object change its motion (accelerate)?

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Newton’s 3 laws 1 st : Things will continue move or not move as they have unless a net external force is applied to them 2 nd : F net = ma 3 rd : For every action force, there is an equal and opposite reaction force

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