ASU KIN 335 - Basic Kinetic Concepts

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1Basic Kinetic Concepts• Inertia:– Natural property of a body to resist a change in state of motion (i.e., state of motion defined by velocity, ∆ state of motion = acceleration)Newton’s 1st Law: Inertia• A body will stay at rest...• A body will continue to move with the same velocity unless...• A net force acts on the body– e.g., to speed up or slow down an object’s motion, and/or to change an object’s direction of motion, and object’s inertia must be overcome.2Mass• Quantity of matter of which a body is composed (a measure of “stuff”)• Direct measure of a body’s resistance to change in linear motion– e.g., it is more difficult to change the motion of a large object (large mass) than a small object– large mass (e.g., shot) vs. small mass (e.g., tennis ball). Which requires more effort (i.e., force)? Definitions• Force:– Effect of one body on another– A push or a pull applied to an object– That which is needed to change the state of motion (i.e., velocity) of an object• Acceleration:– Rate of change of velocity of an object with respect to time.• Describes the change in state of motion of an object (i.e., ∆ velocity)3Newton’s 2nd Law: ΣF = ma ΣF = “Net force” = Sum of all forcesm = mass (a measure of inertia)a = linear accelerationExample• What average net force must a baseball catcher apply to an 80 mph (35.8 m/s) pitch to stop it over a time of .025 s? (mass = 4.5 oz = 0.13 kg)– Know:– Need:–Use:– Answer:F = ma = 0.13 kg × (0 m/s – 35.8 m/s )/0.025 s = –186.2 Nm = .13 kg; t = 0.025 s; vi= 35.8 m/s; vf= 0 m/sFF = ma...but…we must find a first and a = (vf–vi)/t4• Things to consider:– The meaning of a negative force– Average v. instantaneous force– The effect of ∆t on F• Important interpretations of the ΣF = marelationship:– Cause (ΣF) and effect (a) relationship: the most important concept in this class! • a directly proportional to and in the same direction as ΣF• To produce a given a, it takes a larger ΣF for a more massive objectNewton’s 3rd Law: Action/Reaction• Action-reaction: describes how objects interact with one another.– If one body exerts a force on a second body, the second exerts back on the first a force of equal magnitude but opposite direction– example: Vertical jumping• “Action” force is applied by person (via muscles) and acts on ground.• “Reaction” force is applied by ground and acts on person.5Newton’s Law of Gravitation• A fundamental physical principle that describes the concept of gravity...• Any two particles of matter (any objects or bodies) attract one another with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them (i.e., distance between their centers).G = gravitational constant = 6.7 x 10-11 N⋅m2/kg2• Like it or not, there is a force of attraction between you and the person sitting next to you. – However, this force is so small that you don’t notice it.– When one of the objects is the earth (with its huge mass), the force of attraction (i.e. gravity) is very significant.FGmml=⋅⋅()122Example• Two students sitting 1.5 m apartFGmml=⋅⋅()122()FxNmkgkg kgmFx xN=⋅⋅==−−67 1070 5010422 10 104 1011 2 2211 7./ (1.5-()()).()FxNmkgxkgkgxmFx N=⋅⋅==67 10598 10 504 92 10 49111 2 22421./ (6.38 10 -6(. )( )).• Attraction between earth and student6Weight• Question: Is earth’s gravitational attraction the same for all objects on or near the earth’s surface?•Answer: NO...this force is dependent on the involved masses and the distance between the CM of the object and the CM of the earth.Weight vs. Mass• Are weight and mass the same thing? NO (Why or why not)– An objects weight represents the force of attraction between the earth and the object. Mass represents the quantity of matter or stuff of which a body is composed.• Simplified relationship for the link between weight and mass on earth:W =


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ASU KIN 335 - Basic Kinetic Concepts

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