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**Newton’s Laws of Motion**

(Chapter 4) Newton’s Laws of Motion Fill in the blanks on your guided notes during the PowerPoint

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**Newton’s Laws of Motion**

To quote Bill Nye: “Consider the Following”: 1. Lifting a backpack: When you lift a backpack, you exert a force on it that causes it to move (pull it up) The backpack initially was at rest (not in motion) The force you exerted caused the backpack’s velocity to change.

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**Newton’s Laws of Motion**

2. Pushing on a table: Push down on a table and the table does not move. The force you applied did not cause the velocity of the table to change.

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**Newton’s Laws of Motion**

Sir Isaac Newton ( ) came up with three laws to explain how motion and forces are related:

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**First Law of Motion First Law of Motion aka The Law of Inertia:**

An object at rest will remain at rest and an object in motion will remain in motion with a constant speed & direction (velocity) unless acted upon by an external unbalanced force. (Gravity and Friction are unbalanced forces)

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First Law of Motion If an object is not pushed or pulled upon, it’s speed & direction (velocity) will naturally remain constant….which means: Once moving at a steady speed in a straight line…it will continue to move at a steady speed in a straight line. Once standing still…it will stay still. (Note: a motionless object is maintaining a constant velocity of 0 m/s)

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**First Law of Motion Example:**

A skateboard is at rest and doesn’t move unless you give it a push and increases its velocity while you push it. After the skateboard leaves your hand, it slows down and stops because friction acts on it as it rolls.

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First Law of Motion Inertia: the tendency of an object to resist a change in motion. Objects want to remain in uniform motion in a straight line so they resist changes in speed or direction

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**First Law of Motion (Inertia Continued)**

Depends on mass: Increase Mass, Increase Inertia Heavier objects are harder to move because they have more inertia or more resistance to change in motion. Pushing a heavier person on a swing is harder than pushing a lighter person.

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**First Law of Motion Example:**

A bowling ball and a volleyball are rolling towards you. You would have to exert a greater force on the bowling ball to make it stop. The bowling ball has more inertia than the volleyball because it has more mass.

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**First Law of Motion Example:**

You are pushing a cart with a box on top and come to a sudden stop, causing the box to slide off the cart. The inertia of the boxes causes them to keep moving even after the cart stops.

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First Law of Motion Example: A car traveling at 85 mph crashes head-on with something solid, crumples, slows and finally stops within 0.1s. Anyone in the car not wearing a seatbelt continues to move forward at the same speed the car was traveling (85 mph), slamming into the windshield, dashboard etc. Note: 85mph is about the speed falling from a three-story building. Bonus Lesson: Always wear your seatbelt!

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**Second Law of Motion Second Law of Motion:**

The acceleration of an object is dependent upon the force acting upon the object and the mass of the object.

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**Second Law of Motion 1N = 1kg.m/s2**

Newton’s Second Law can be written as the equation: F = ma or Force = mass x acceleration F = force (N) m= mass (kg) a = acceleration (m/s/s or m/s2) 1N = 1kg.m/s2

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Second Law of Motion This equation demonstrates the following two important relationships between force, mass and acceleration:

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**(Increase Force, Increase Acceleration)**

Second Law of Motion 1. The greater the force on an object, the greater the acceleration of an object. (Increase Force, Increase Acceleration) Example: In baseball, if I hit a ball as hard as I can, the ball accelerates more than if I was trying to bunt the ball.

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Second Law of Motion 2. The greater the mass of an object, the smaller the acceleration. (Increase Mass, Decrease Acceleration) Example: In baseball, if I hit a 50 gram ball and a 25 gram ball with the same force, the 50-gram ball would go slower than the 25 gram ball because it has a greater mass.

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Second Law of Motion Acceleration is directly proportional to the force: *you push twice as hard and it accelerates twice as much. (Increase Force, Increase Acceleration)

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**(Increase Mass, Decrease Acceleration)**

Second Law of Motion Acceleration is inversely proportional to the mass: *If it gets twice the mass, it accelerates half as much. (Increase Mass, Decrease Acceleration)

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Third Law of Motion Third Law of Motion: *For every action there is an equal and opposite reaction*

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Third Law of Motion When one object exerts a force on a second object, the second object exerts the same amount of force back on the first object but in the opposite direction. If a student were to jump off of a desk onto the floor, their feet would sting. This is because the student’s feet would exert a force on the ground but the ground exerts a force back on the student’s feet, causing their feet to sting. You jump on a trampoline and exert a downward force while the trampoline exerts an equal force upward, sending you high in the air.

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**m = mass (kg) V= velocity (m/s)**

Momentum Momentum: a property of a moving object that equals its mass x velocity. Formula: P = mv m = mass (kg) V= velocity (m/s) P=Momentum (Kg.m/s)

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**Law of Conservation of Momentum**

Law of Conservation of Momentum: total momentum is conserved unless an outside force acts on the objects. Friction is an outside force.

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**Law of Conservation of Momentum**

The momentum of an object doesn’t change unless its mass, velocity or both change. If a group of objects exerts forces only on each other, their total momentum doesn’t change. Momentum can be transferred from one object to another, but none is lost.

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Gravity Gravity: the attractive force between any two objects that depends on the mass of the objects and the distance between them.

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Gravity The earth is close enough and has a large enough mass that you can feel its gravitational attraction. The sun has much more mass than the earth, but it is too far away to notice a gravitational attraction to you.

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Gravity The Law of Universal Gravitation: The gravitational force between two masses decreases rapidly as the distance between the masses increases. i.e Increase Distance, Decrease Gravitational Attraction

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Gravity Gravitational Force Equation: F=mg Gravitational Force (N) = mass (kg) x gravitational acceleration (m/s2) g=9.8 m/s2

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**mass (kg) x gravitational acceleration (m/s2)**

Gravity Weight: the gravitational force exerted on an object W=mg Weight (N) = mass (kg) x gravitational acceleration (m/s2)

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Gravity Weight and Mass are NOT the same thing! Weight is a force and mass is a measure of the amount of matter in an object. You weigh less on the moon because gravity is 1.6 m/s2. Your mass is the same.

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