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Unformatted text preview: 1 Lecture 6 Mass and Weight Free-Body Diagram Application of Newtons Laws Friction Physics 111, Summer 2011, June 21, Lecture 6 Summary A unit of force: System International: the newton (N) British System: a unit of force is the pound (lb) 2 Force has the magnitude and direction and is a vector quantity. There are two groups of forces: (1) contact forces (a pull, push, tension, friction, and (2) non-contact forces (gravitational force, electric force, magnetic force, forces acting in subatomic world). Newtons first law of motion: An object remains at rest or in motion with constant velocity as long as no net force acts on it. Newtons second law of motion: an object accelerates if the net external force acts on it. The acceleration equals to the force divided by the objects mass. Newtons third law of motion: whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. m F a = r r m F a = man on wall on F F r r = Physics 111, Summer 2011, June 21, Lecture 6 Mass and Weight Mass is a characteristic of inertial properties of the object (quantity of matter): Weight ( W ) of an object is the force of the gravity acting on it: mass force on accelerati = on accelerati force mass = Mass (m) g m F W or a m F G r r r r r = = = 2 8 9 : Earth On the s m/ . g = g m F W G r r r = = g r 2 / 6 . 1 : Moon On the s m g = Mass of a certain object is constant! Weight is determined by mass and acceleration due to gravity. 3 Physics 111, Summer 2011, June 21, Lecture 6 Mass and Weight on Earth and Moon Mass of some man on the Earth is 80 kg. What are his mass and weight on the Moon? 4 1) 80 kg and 784 N 2) 50 kg and 490 N 3) 80 kg and 128 N 4) 50 kg and 128 N N s m kg mg F W G 128 ) / 6 . 1 )( 80 ( : Moon on the eight W 2 = = = = 2 8 9 : Earth On the s m/ . g = 2 / 6 . 1 : Moon On the s m g = Mass of a certain object is constant!...
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