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Ch. 8.3 Newton’s Laws of Motion Motion and Forces

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Section 8.3 Objectives State Newton’s three laws of motion, and apply them to physical situations. Calculate force, mass, and acceleration with Newton’s second law. Recognize that the free-fall acceleration near Earth’s surface is independent of the mass of the falling object. Explain the difference between mass and weight. Identify paired forces on interacting objects.

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Newton’s First Law Newton's 1 st Law States : An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force. Newton’s 1 st Law is also known as the Law of Inertia. Inertia is the tendency of an object to remain at rest or in motion with a constant velocity.

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Newton’s Second Law Newton’s 2 nd Law States: The unbalanced force acting on an object equals the object’s mass times its acceleration. This means : force = mass x acceleration F = ma Where : F is in Newton’s (N) m is in kilograms ( kg) a is in meters per second squared (m/s 2 )

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Free fall and Weight Free fall is the motion of a body when only the force of gravity is acting on it. Free fall acceleration near Earth’s surface constant. Disregarding air resistance, all objects on Earth’s surface (regardless of their mass) accelerate at 9.8 m/s 2. Weight is the force on an object due to gravity. Weight equals mass times free fall acceleration or w = mg. Weight is a force and has SI units of Newtons (N).

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Terminal Velocity Terminal velocity is the maximum velocity reached by a falling object that occurs when the resistance of the medium is equal to the force due to gravity. At terminal velocity, air resistance and force of gravity are equal. Sky divers reach a terminal velocity of about 320 km/h (200 mi/h).

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Newton’s Third Law Newton’s Third Law states: For every action force, there is an equal and opposite reaction force. Forces always occur in pairs but act on different objects. The upward push on the rocket equals the downward push on the exhaust gases.

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Section 8.3 Summary An object at rest remains at rest and an object in motion maintains a constant velocity unless it experiences an unbalanced force (Newton’s 1st law). The unbalanced force acting on an object equals the object’s mass times its acceleration, or F = ma (Newton’s 2 nd Law). The SI unit for force is the Newton (N). Weight equals mass times free fall acceleration, or W = mg. For every action force, there is an equal and opposite reaction force (Newton’s 3rd law).

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