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C H A P T E R 4 Forces and Newton's Laws of Motion

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4.1 The Concepts of Force and Mass

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In common usage, a force is a push or a pull. Forces can be categorized as, Contact forces and Non-Contact forces.

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4.1 The Concepts of Force and Mass In common usage, a force is a push or a pull. Forces can be categorized as, Contact forces and Non-Contact forces.

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Mass The word mass is just as familiar as the word force. A massive supertanker, for instance, is one that contains an enormous amount of mass. In comparison, a penny does not contain much mass. Mass is a scalar quantity.

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Mass Mass is a measure of the amount of matter contained in an object.

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Newton's First Law Of Motion

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An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force.

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Newton's First Law Of Motion An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force. The net force is the vector sum of all of the forces acting on the object.

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Arrows are used to represent forces. The length of the arrow is proportional to the magnitude of the force. 15 N 5 N Forces

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The net force on an object is the vector sum of all forces acting on that object. Individual ForcesNet Force 10 N4 N 6 N Net Force

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Individual Forces Net Force 3 N 4 N 5 N Net Force

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Mathematically, the net force is written as where the Greek letter sigma denotes the vector sum. Net Force

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Inertia and Mass

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Inertia is the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line.

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Inertia and Mass Inertia is the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line. The mass of an object is a quantitative measure of inertia. SI Unit of mass: kilogram (kg)

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The Physics of Seat Belts

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Inertial Reference Frame

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An inertial reference frame is one in which Newton's laws of motion are valid.

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Inertial Reference Frame An inertial reference frame is one in which Newton's laws of motion are valid. The acceleration of an inertial reference frame is zero, so it moves with a constant velocity.

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Inertial Reference Frame An inertial reference frame is one in which Newton's laws of motion are valid. The acceleration of an inertial reference frame is zero, so it moves with a constant velocity. Earth can be considered as an inertial reference frame.

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Newton's Second Law of Motion

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Newton’s second law is a relationship between acceleration, forces, and mass.

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Newton's Second Law of Motion Newton’s second law is a relationship between acceleration, forces, and mass. When a net external force acts on an object of mass m, the acceleration a that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the net force.

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Newton's Second Law of Motion Newton’s second law is a relationship between acceleration, forces, and mass. When a net external force acts on an object of mass m, the acceleration a that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the net force. SI Unit of Force: : kg · m/s 2 = newton (N)

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Units

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4.4 The Vector Nature of Newton's Second Law of Motion

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Pushing a Stalled Car Two people are pushing a stalled car, as the figure indicates. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other applies a force of 395 N. Both forces act in the same direction. A third force of 560 N also acts on the car, but in a direction opposite to that in which the people are pushing. This force arises because of friction and the extent to which the pavement opposes the motion of the tires. Find the acceleration of the car.

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