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Chapter 3 Force, Mass and Acceleration Newton’s Laws

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Sir Isaac Newton 1642-1727 English Mathematician and Physicist English Mathematician and Physicist Formulated the three laws of motion and law of universal gravitation Formulated the three laws of motion and law of universal gravitation Invented calculus Invented calculus All before the age of 30 All before the age of 30 Newton’s discoveries helped scientists understand ideas such as: what causes tides, how do the planets move and why do objects of different masses fall at the same rate? Newton’s discoveries helped scientists understand ideas such as: what causes tides, how do the planets move and why do objects of different masses fall at the same rate?

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A Newton Newton came up with a formula to help scientists understand force. Newton came up with a formula to help scientists understand force. Force is measured in Kg.m/s 2 this is called a Newton (named after guess who? – Newton) Force is measured in Kg.m/s 2 this is called a Newton (named after guess who? – Newton) There are 4.48 N in 1 lb There are 4.48 N in 1 lb

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What is Force? Force is a push, pull or any action that has the ability to change motion Force is a push, pull or any action that has the ability to change motion Will a force always cause motion?Will a force always cause motion? There are two units that are commonly used to measure force, Newtons and Pounds. Scientists prefer Newtons(N) There are two units that are commonly used to measure force, Newtons and Pounds. Scientists prefer Newtons(N)

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Newton’s First Law Law of Inertia Law of Inertia “An object at rest will stay at rest until acted upon by an unbalanced force, an object in motion will continue with constant speed and direction until acted upon by an unbalanced force” “An object at rest will stay at rest until acted upon by an unbalanced force, an object in motion will continue with constant speed and direction until acted upon by an unbalanced force” Basically – An object at rest remains at rest and an object in motion remains in motion unless acted on by an unbalanced force. Basically – An object at rest remains at rest and an object in motion remains in motion unless acted on by an unbalanced force.

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Newton’s 1 st Law

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Inertia Objects ability to resist changing its state of motion (Keep on doing what it is doing) Objects ability to resist changing its state of motion (Keep on doing what it is doing) Inertia depends on mass, so an object with more mass would have a more inertia Inertia depends on mass, so an object with more mass would have a more inertia

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Inertia Which is harder to move, a bowling ball or a beach ball? Which is harder to move, a bowling ball or a beach ball?

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Newton’s 2 nd Law Relates force to an object’s mass and acceleration Relates force to an object’s mass and acceleration It says that force causes acceleration, mass resists acceleration and the acceleration is a ratio of force over mass It says that force causes acceleration, mass resists acceleration and the acceleration is a ratio of force over mass Common sense: If something has more mass, it would take more force to cause the object to accelerate Common sense: If something has more mass, it would take more force to cause the object to accelerate

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Newton’s 2 nd Law F = ma F = ma

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Using Newton’s 2 nd Law F = ma F M A An object’s Force equals its mass times its acceleration

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2 nd Law A car engine produces a force and you can calculate the acceleration of the car when you know this force and the mass of the car A car engine produces a force and you can calculate the acceleration of the car when you know this force and the mass of the car If you want to increase the acceleration of the car you must decrease the cars mass or increase the force of the engine If you want to increase the acceleration of the car you must decrease the cars mass or increase the force of the engine

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Balanced vs. Unbalanced Forces Net Force – the sum of all forces acting on an object Net Force – the sum of all forces acting on an object The motion of an object depends on the total of all forces acting on it The motion of an object depends on the total of all forces acting on it If the forces are balanced(net force =0), then the object is in equilibrium and there is no chance of movement occurring because the net force is zero If the forces are balanced(net force =0), then the object is in equilibrium and there is no chance of movement occurring because the net force is zero

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Balanced Forces If the forces are balanced(net force =0), then the object is in equilibrium and there is no chance of movement occurring because the net force is zero If the forces are balanced(net force =0), then the object is in equilibrium and there is no chance of movement occurring because the net force is zero

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Unbalanced Forces If there is a net force, then the forces are unbalanced, and movement occurs If there is a net force, then the forces are unbalanced, and movement occurs

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Weight vs. Mass Mass is the amount of matter in an object, usually measured in kilograms Mass is the amount of matter in an object, usually measured in kilograms Weight is the force of gravity acting on a certain mass, measured in Newtons or pounds Weight is the force of gravity acting on a certain mass, measured in Newtons or pounds Weight changes with gravity Weight changes with gravity Mass is the same everywhere Mass is the same everywhere

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Gravity, Mass and Weight An object weighs less on the moon but has the same mass. It weighs less because there is less gravity. An object weighs less on the moon but has the same mass. It weighs less because there is less gravity. You would weigh more on Jupiter than Earth because Jupiter has more gravity than Earth You would weigh more on Jupiter than Earth because Jupiter has more gravity than Earth

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Gravity A force that pulls every mass toward another mass A force that pulls every mass toward another mass Earth is the biggest mass for us so other masses are pulled toward earth’s center Earth is the biggest mass for us so other masses are pulled toward earth’s center On earth, every kilogram of mass weighs 9.8 Newtons On earth, every kilogram of mass weighs 9.8 Newtons

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Weight is a force You can figure out the weight by multiplying the mass times the acceleration of gravity You can figure out the weight by multiplying the mass times the acceleration of gravity F= m X a F= m X a Weight is a force (pulled to earth’s center) Weight is a force (pulled to earth’s center) The acceleration due to gravity on Earth is 9.8 m/s 2 The acceleration due to gravity on Earth is 9.8 m/s 2

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Gravity, Cont. On Earth, every object will fall at the same rate (not counting air friction) On Earth, every object will fall at the same rate (not counting air friction) The Acceleration of gravity is 9.8 m/s 2 meaning that every second, a falling object accelerates 9.8 m/s The Acceleration of gravity is 9.8 m/s 2 meaning that every second, a falling object accelerates 9.8 m/s In other words, every second something is falling it is moving 9.8 m/s faster In other words, every second something is falling it is moving 9.8 m/s faster

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Gravity with no resistance If you drop a bowling ball and a match box car at the same time, they will hit the ground at the same time If you drop a bowling ball and a match box car at the same time, they will hit the ground at the same time

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Newton’s Third Law “For every action, there is an equal and opposite reaction” “For every action, there is an equal and opposite reaction” There must be a PAIR of objects There must be a PAIR of objects The action and reaction act on separate objects The action and reaction act on separate objects

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Newton’s Third Law If you press down on the table, you are exerting a force on it. Is the table exerting a force on your hand? If you press down on the table, you are exerting a force on it. Is the table exerting a force on your hand? Your force on the table is action force, the table’s force on you is reaction force Your force on the table is action force, the table’s force on you is reaction force

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3 rd Law Skateboard – foot on ground is action – earth against skateboard is reaction Skateboard – foot on ground is action – earth against skateboard is reaction Throw ball on a skateboard Throw ball on a skateboard Recoil – backward acceleration from reaction force Recoil – backward acceleration from reaction force

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Law of Universal Gravitation The force of attraction (gravity) between 2 objects depends on the mass of the object and the distance between them The force of attraction (gravity) between 2 objects depends on the mass of the object and the distance between them More noticable between large objects (plants and stars) More noticable between large objects (plants and stars) Force = G x mass 1 x mass 2 Force = G x mass 1 x mass 2 d 2 d 2 G = 6.67 x 10-11

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Friction Forces that work against the motion of the object Forces that work against the motion of the object Force of friction is always in the opposite direction of the motion Force of friction is always in the opposite direction of the motion Must consider when finding net force Must consider when finding net force Will cause motion to stop eventually unless force is continually supplied Will cause motion to stop eventually unless force is continually supplied

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Air Friction Caused by air moving around objects Caused by air moving around objects

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Sliding Friction Two objects sliding against one another Two objects sliding against one another

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Rolling Friction One object rolling over another object One object rolling over another object

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Viscous Friction Object moving in water or fluid Object moving in water or fluid

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