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Newton’s Laws Section 7.3

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Objectives Know the definitions of Newton’s Three Laws and know how to apply them Calculate force, mass, and acceleration with Newton’s 2 nd Law Introduce “g” and know what it means Know the difference between mass and weight Identify paired forces on interacting objects.

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Newton’s Laws 1 st Law: An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force. 2 nd Law: The unbalanced force on an object is equal to the mass of the object times its acceleration. 3 rd Law: For every action force, there is an equal and opposite reaction force

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Newton’s 1 st Law In a nutshell: objects are lazy and don’t want to change their velocity unless “forced” to. Take note! A change in velocity means acceleration! Objects are lazy and don’t want to accelerate unless “forced” to.

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Newton’s 2 nd Law In a nutshell: F = ma ◦F◦F is an unbalanced (or net) force. UUnits of Newtons 11 Newton = about 2.2 pounds of force ◦I◦If no unbalanced force exists, NNo acceleration NNo change in velocity WWhatever the velocity was, the velocity is

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Example A 5.0 kg bicycle experiences an unbalanced (or net) force of 50 N. Find the bicycle’s acceleration. F = ma a = F / m a = 50 N / 5.0 kg = 10 m/s 2

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Newton’s 3 rd Law In a nutshell: You can’t push without being pushed (or pull without being pulled) Forces are equal and opposite, but why don’t they cancel out? The forces do not act on the same object!

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200 N 150 N

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Special Case of Newton’s 2 nd Law All objects near the surface of earth fall with the same acceleration, “g” g = 9.8 m/s 2 downward Net force = ma Force due to gravity = mg = “weight” Weight is also measured in units of Newtons.

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Example: An object has a mass of 45 kg. What is its weight? F = ma Weight = mg Weight = 45 kg * 9.8 m/s 2 Weight = 440 N

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