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Chapter 4 Newton’s Laws of Motion

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Newton’s First Law of Motion Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

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In simpler terms: An object in motion (in a straight line) stays in motion (in a straight line); An object at rest stays at rest; UNLESS that object is acted upon by an outside force In simpler terms: An object in motion (in a straight line) stays in motion (in a straight line); An object at rest stays at rest; UNLESS that object is acted upon by an outside force

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Newton’s 1st law of motion is also called the “Law of Inertia” Inertia - tendency of an object to resist changes in motion (the “laziness” of an object). The inertia and object has is related to mass of that object (more mass ~ more inertia) Newton’s 1st law of motion is also called the “Law of Inertia” Inertia - tendency of an object to resist changes in motion (the “laziness” of an object). The inertia and object has is related to mass of that object (more mass ~ more inertia)

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Mass - the amount of matter an object has Weight - the force on an object due to gravity Mass - the amount of matter an object has Weight - the force on an object due to gravity

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Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. Acceleration = net force ------------- mass The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. Acceleration = net force ------------- mass

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Net force is a vector quantity (just like velocity and acceleration), and can be calculated in the same manner given two or more applied forces. Same direction - add magnitude Opposite direction - subtract magnitude At an angle - find the resultant using parallelogram method, Pythagorean theorem, 3-4-5, etc. Net force is a vector quantity (just like velocity and acceleration), and can be calculated in the same manner given two or more applied forces. Same direction - add magnitude Opposite direction - subtract magnitude At an angle - find the resultant using parallelogram method, Pythagorean theorem, 3-4-5, etc.

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When acceleration is zero - Equilibrium Static equilibrium - the object is not in motion due to equal but opposite forces Example - book lying on a table the book pushes down on the table the table pushes up on the book Example - using a bathroom scale to measure your weight Static equilibrium - the object is not in motion due to equal but opposite forces Example - book lying on a table the book pushes down on the table the table pushes up on the book Example - using a bathroom scale to measure your weight

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Dynamic equilibrium - the object is in constant, straight line motion Example - a hockey puck sliding across the ice with constant velocity Dynamic equilibrium - the object is in constant, straight line motion Example - a hockey puck sliding across the ice with constant velocity

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Friction Occurs between surfaces that are in contact with each other. Always acts in a direction that is opposite to the direction of the motion. Examples: static friction - the force needed to cause an object to start moving Occurs between surfaces that are in contact with each other. Always acts in a direction that is opposite to the direction of the motion. Examples: static friction - the force needed to cause an object to start moving

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sliding friction - friction between a sliding object and the surface rolling friction - friction between a rolling object and the surface Friction between solids does not depend on: speed area of contact sliding friction - friction between a sliding object and the surface rolling friction - friction between a rolling object and the surface Friction between solids does not depend on: speed area of contact

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Friction in fluids does depend on speed does depend on area of contact Friction in fluids does depend on speed does depend on area of contact

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When a = g (free fall) Why does gravity affect falling objects the same? Gravity pulls on a more massive object more than on a less massive object. A more massive object has more inertia than a less massive object. The greater pull of gravity is cancelled out by the greater amount of inertia. Why does gravity affect falling objects the same? Gravity pulls on a more massive object more than on a less massive object. A more massive object has more inertia than a less massive object. The greater pull of gravity is cancelled out by the greater amount of inertia.

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When a < g (nonfree fall) Air resistance is factored in….. Air resistance depends on: the size of the object the speed of the object Air resistance is factored in….. Air resistance depends on: the size of the object the speed of the object

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Terminal speed (velocity) when acceleration stops (the falling object stops getting faster each second) net force equals zero air resistance equals weight of falling object Terminal speed (velocity) when acceleration stops (the falling object stops getting faster each second) net force equals zero air resistance equals weight of falling object

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Newton’s Third Law of Motion Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Action-reaction law Forces occur in pairs Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Action-reaction law Forces occur in pairs

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Each force of the pair acts on a different object (A pushes on B, B pushes on A) The force cancels out only if A pushes on C and B pushes on C with same size, but opposite direction, of force. Explains: guns rockets Each force of the pair acts on a different object (A pushes on B, B pushes on A) The force cancels out only if A pushes on C and B pushes on C with same size, but opposite direction, of force. Explains: guns rockets

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