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Chapter 2.3 & 3 Force, Mass and Acceleration Newton’s Laws

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Sir Isaac Newton An English Mathematician and Physicist that formulated the three laws of motion, law of universal gravitation and invented calculus before the age of 30 An English Mathematician and Physicist that formulated the three laws of motion, law of universal gravitation and invented calculus before the age of 30 Newton’s discoveries helped to answer many questions 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 to answer many questions 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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What is Force? Force is a push, pull or any action that has the ability to change motion (This doesn’t mean that a force will always make something move– can you think of a situation such as this?) Force is a push, pull or any action that has the ability to change motion (This doesn’t mean that a force will always make something move– can you think of a situation such as this?) 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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Force vs. Mass The Newton relates to force and motion. A force of 1 Newton causes a 1 kg mass to have an acceleration of 1 m/s 2 The Newton relates to force and motion. A force of 1 Newton causes a 1 kg mass to have an acceleration of 1 m/s 2 We call it a Newton because that is somewhat easier to say than “1 kg times 1 m/s 2 ” We call it a Newton because that is somewhat easier to say than “1 kg times 1 m/s 2 ”

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Newton’s First Law Known as the Law of Inertia Known as the 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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Inertia Inertia is dependant on mass, therefore an object with more mass would have a greater inertia Inertia is dependant on mass, therefore an object with more mass would have a greater inertia Example: Which is harder to move, a bowling ball or a beach ball? The ball with the greater mass is harder to move because it has greater inertia Example: Which is harder to move, a bowling ball or a beach ball? The ball with the greater mass is harder to move because it has greater inertia

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Newton’s 2 nd Law Relates the applied force to an object’s mass and its acceleration Relates the applied force to an object’s mass and its 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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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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Balanced vs. Unbalanced Forces 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, 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, then the object is in equilibrium and there is no chance of movement occurring because the net force is zero 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 A certain mass of something has a “weight”, which is the force acting on that mass. It varies depending on the gravity. A certain mass of something has a “weight”, which is the force acting on that mass. It varies depending on the gravity.

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Gravity, Mass and Weight Imagine that you travel to the moon. Which of these will be the same as on earth: Gravity, your mass or your weight? Imagine that you travel to the moon. Which of these will be the same as on earth: Gravity, your mass or your weight? An object will have the same mass on the moon as on earth – it still has the same amount of matter An object will have the same mass on the moon as on earth – it still has the same amount of matter 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 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 On earth, every kilogram of mass weighs 9.8 Newtons On earth, every kilogram of mass weighs 9.8 Newtons 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

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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 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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Friction Friction is a force that resists other kinds of movement Friction is a force that resists other kinds of movement Friction always works against the movement of the object– it comes from two objects moving against each other Friction always works against the movement of the object– it comes from two objects moving against each other We use the term “friction” to describe any force caused by motion and that acts to slow motion down We use the term “friction” to describe any force caused by motion and that acts to slow motion down

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Types of Friction Air friction- Air moving around objects creates an opposing force Air friction- Air moving around objects creates an opposing force Sliding friction- Caused when two surfaces rub against each other Sliding friction- Caused when two surfaces rub against each other Viscous friction- Objects that move in water or other fluids create this type. Viscous friction- Objects that move in water or other fluids create this type. Rolling friction- Caused by one object rolling over another Rolling friction- Caused by one object rolling over another

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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” In other words, for every object, there is a PAIR of forces acting on it In other words, for every object, there is a PAIR of forces acting on it 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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