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Newton’s 1 st Law: The Law of Inertia “Every object maintains a state of rest or uniform motion in a straight line unless acted upon by an unbalanced external force.” Copy this note!

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Newton’s 1 st Law: The Law of Inertia Inertia –Depends on the mass of an object –Inertia is a resistance to a change in motion –Large mass = Large inertia –Small mass = Small inertia Copy this note!

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Newton’s 1 st Law: The Law of Inertia Simply put: –Objects like to stay where they are or to keep moving as they have been –The heavier an object is, the harder it is to change its state of motion. –Think of inertia as Lazy “I’m a keep on doing what I’m doing” Copy this note!

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Newton’s 1 st Law The Law of Inertia Video

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Newton’s 1 st Law: The Law of Inertia If the net force acting on an object is zero, the object will stay at rest or continue to move at constant velocity Example: –A computer sitting on a desk (at rest) Copy this note! “At rest” means Velocity = 0

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Newton’s 1 st Law: The Law of Inertia –A hockey puck moving across smooth ice at a constant velocity –A body in a vehicle keeps moving forward when the car stops suddenly The seatbelt applies a force in the opposite direction to stop the body from going forward Copy this note! “At rest” means Velocity = 0

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Note: Be safe and always wear your seatbelt when driving or as a passenger!

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Newton’s 2 nd Law “An unbalanced force causes an object to accelerate” “If the net external force on an object is not zero, the object will accelerate in the direction of this net force” Copy this note!

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Newton’s 2 nd Law An object’s acceleration (a) is: –Directly proportional to the applied unbalanced force (F) –inversely proportional to the object’s mass (m) F = ma (N) (kg) (m/s 2) F - NET FORCE in Newtons (N = kg m/s 2 ) m - mass is in kg a - net acceleration is in m/s 2 Copy this note! Highlight me!

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Newton’s 2 nd Law Newton’s 2 nd Law tells us –The heavier an object is (i.e. more massive), the greater the force is required to accelerate it Example: a pickup truck requires a much larger engine (larger force) compared to a small compact car. –To increase the rate of acceleration requires a greater force as well Example: consider the acceleration of a Smart Car compared to a Lamborghini. –The Lamborghini’s bigger engine (bigger force) means it has bigger acceleration F = ma

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What do you think?

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Newton’s 2 nd Law Example 1 Find the net force acting on a 10 kg object that is accelerating at 5.0m/s 2 south. Draw a FBD Copy this note! +N +E - S -W Label Directions

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Newton’s 2 nd Law Example 2 Find the acceleration of a 15 kg object that is experiencing a net force of 30 N to the right. Draw a FBD Copy this note! +up +right - down -left Label Directions

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Newton’s 2 nd Law Example 3 Determine the net force on the following object. Copy this note! +y +x - y -x Label Directions 7.0 N 5.0 N

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Newton’s 2 nd Law Example 4 Determine the net force on the following object. Copy this note! +y +x - y -x Label Directions 1.0 N 5.0 N

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Newton’s 2 nd Law Example 5 Determine the net force on the following object. Copy this note! +y +x - y -x Label Directions 1.0 N 4.0 N 5.0 N7.0 N NET is assumed Ѳ tan Ѳ = F y / F x Ѳ = tan -1 (F y / F x ) Ѳ = tan -1 (3 /2) Ѳ = 56 ̊ Use Pythagoras & SOH CAH TOA

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Important Announcements Tuesday April 5 th –Automotive Safety Report (due Monday April 11 th ) Wednesday April 6 th –Review Thursday April 7 th –Forces Unit Test Friday April 8 th –PA Day!

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Coefficient of Friction Tables

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Newton’s 1 st Law: The Law of Inertia “Every object maintains a state of rest or uniform motion in a straight line unless acted upon by an unbalanced external force.” From Yesterday

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Newton’s 2 nd Law An object’s acceleration (a) is: –Directly proportional to the applied unbalanced force (F) –inversely proportional to the object’s mass (m) F = ma (N) (kg) (m/s 2) F - NET FORCE in Newtons (N = kg m/s 2 ) m - mass is in kg a - net acceleration is in m/s 2 From Yesterday Highlight me!

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Newton’s 2 nd Law Example 6 Determine the net force on the following object. Copy this note! +y +x - y -x Label Directions Draw a right angle triangle: F y points +y. F x points -x. tan Ѳ = F y / F x Ѳ = tan -1 (F y / F x ) Ѳ = tan -1 (3.0 /3.0) Ѳ = 45 ̊ 4.0 N 7.0 N 3.0 N Ѳ For the tan Ѳ calculation, use positive F X, F Y numbers and then work out [L45 ̊ U]

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Newton’s 3 rd Law: Action-Reaction Copy this note!

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Newton’s 3 rd Law: Action-Reaction If you exert 10N of force on an object, the object will exert 10N of force back on you! You punch a wall, it punches back with an equal but opposite force A shotgun recoils. The gun exerts a force on the bullet to move it forward. The bullet exerts and equal but opposite force against the gun, causing it to recoil madly! Copy this note!

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Newton’s 3 rd Law Action-Reaction Video

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Problem Solving With Newton’s Laws A box of mass 40 kg is pushed horizontally to the right across a floor with a force of 185N. μ K = a) Determine the weight of the box? Copy this note! +U +R - D -L FBD!

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Problem Solving With Newton’s Laws b) Determine the size of the normal force. Copy this note! +U +R - D -L FBD!

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Problem Solving With Newton’s Laws c) Determine the size of the frictional force. +U +R - D -L FBD! Copy this note!

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Problem Solving With Newton’s Laws d) Determine the resulting acceleration of the box. +U +R - D -L Copy this note!

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Newton’s 3 rd Law Example Standing on perfectly smooth ice, Eric pushes Andrew with a force of 20N to the right. Eric’s mass is 75kg. Andrew’s mass is 65kg. Find: a)The reaction force that Andrew applies against Eric b)Andrew’s acceleration c)Eric’s Acceleration Copy this note!

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Newton’s 3 rd Law Example Copy this note! +N +E - S -W Label Directions

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Two people are pushing on a boat of mass 200 kg. They both push with a force of 300 N at an angle of 25 ̊ to each side of the boat. Sketch a FBD and calculate the net force and net acceleration of the boat. Copy this note ! N 25 ̊ 300 N 25 ̊ 300 N 25 ̊ Ѳ Ѳ = 180 ̊ - 25 ̊ - 25 ̊ Ѳ = 130 ̊ Forces at Different Angles Newton’s 2 nd Law 25 ̊ uses the Z Rule

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Forces at Different Angles Newton’s 2 nd Law (cont’d) Copy this note ! 300 N 25 ̊ 130 ̊ F NET a = 300 Nb = 300 N 25 ̊ C =130 ̊ c = F NET

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A person of mass 70 kg is sitting on a 20 kg toboggan. If two people are pulling her with two different ropes, find the person`s net force and net acceleration. The force of person 1 is 50N [E40 ̊ N] and the force of person 2 is 60N[E25 ̊ S] as seen from the top. Copy this note ! N+ E+ 50 N 60 N 40 ̊ 25 ̊ Forces at Different Angles & Newton’s 2 nd Law

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Copy this note ! N+ E+ 50 N 60 N 40 ̊ 25 ̊ 40 ̊ is due to z rule C=180 ̊ -40 ̊ -25 ̊ C =115 ̊ Forces at Different Angles Newton’s 2 nd Law C 25 ̊

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Copy this note ! C = 115 ̊ c = a= = b

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2 nd Law Worksheet Garfield #3 Copy this note!

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2 nd Law Worksheet Garfield #3 (cont`d) Copy this note!

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2 nd Law Worksheet Garfield #4 Copy this note!

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2 nd Law Worksheet Garfield #4 (cont`d) Copy this note!

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Solving Newton Problems F=ma Ѳ Often one of Fy or Fx is 0 so we don’t’ need to do this.

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