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D. Roberts PHYS 121 University of Maryland Physic² 121: Fundament°ls of Phy²ics I September 20, 2006
D. Roberts PHYS 121 University of Maryland Outline Newton’s Laws of Motion –In Chapter 4 Forces
D. Roberts PHYS 121 University of Maryland Newton’s First Law An object at that is at rest will remain at rest, or an object this is moving will continue to move in a straight line with constant velocity, if and only if the net force acting on the object is zero New ideas we need to understand: –Force –Net Force This Law basically says no cause is needed for an object to move. –Uniform (constant) motion is an object’s natural state Translation from original Latin: “Every body perseveres in its state of rest, or of uniform motion in a right (straight) line, unless it is compelled to change that state by forces impressed thereon”
D. Roberts PHYS 121 University of Maryland Forces Usually think of a force as a push or pull Vector quantity May be a contact force or a field force –Contact forces result from physical contact between two objects –Field forces act between disconnected objects Also called “action at a distance” More generally: –A force is what causes an object to change its velocity (accelerate) –Converse: If an object is accelerating, there must be a non-zero net force acting on it
D. Roberts PHYS 121 University of Maryland Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. –F and a are both vectors Can also be applied three-dimensionally
D. Roberts PHYS 121 University of Maryland Units of Force SI unit of force is a Newton (N) US Customary unit of force is a pound (lb) –1 N = 0.225 lb See table 4.1
? A small bug get splatted on the teeth of a smiling motorcycle rider. Which of the two has the largest force acting on it? 12345 1.The Bug 2.The Biker 3.They’re the same 4.“Force” has nothing to do with this 5.I have no clue
D. Roberts PHYS 121 University of Maryland Newton’s Third Law If object 1 and object 2 interact, the force exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force exerted by object 2 on object 1. Equivalent to saying a single isolated force cannot exist
D. Roberts PHYS 121 University of Maryland Newton’s Third Law cont. F 12 may be called the action force and F 21 the reaction force –Actually, either force can be the action or the reaction force The action and reaction forces act on different objects
D. Roberts PHYS 121 University of Maryland Some Action-Reaction Pairs – is the normal force, the force the table exerts on the TV – is always perpendicular to the surface – is the reaction – the TV on the table –
D. Roberts PHYS 121 University of Maryland More Action-Reaction pairs – is the force the Earth exerts on the object – is the force the object exerts on the earth –
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