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# Download Chapter 3: Forces and Motion

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Transcript

Chapter 3: Forces and Motion First law of motion: -first concluded by Galileo, stated here in Newton’s words *An object at rest will remain at rest and an object in motion will continue in motion at a constant velocity (constant speed in a straight line) in the absence of any interaction with something else inertia – the apparent resistance of an object to change its state of rest or uniform motion mass – a measure of the inertia of an object at rest *the more something resists being set in motion the greater its mass. Compare the mass of a bowling ball to a basketball - an experiment of 2 different masses both attached to the same spring will move away with different velocities - if different springs are used the ratios of velocities will stay the same ma = va *the object of lower mass will always have greater speed mb vb the next step would be to have something like a standard unit of measure. In France there is the 1kg Pt cylinder that defines 1 kg Why do we measure mass this way? What does mass measure? *amount of matter - somewhat vague definition - spring method is much more convenient Force ex hitting a ball with a bat, the result is a change in velocity (direction) *an interaction can lead to a change in magnitude or direction A force is any influence that can change the velocity of an object. *this definition agrees with the idea of forces as “pushes” or “pulls” contact force arise from physical contact pushing, pulling, hitting, friction field forces (action-at-a distance) when forces exert forces on each other even though they are not touching. Magnetic and electric fields Gravity ?? Would something move if there were 2 exactly opposite forces acting on it?? a net force is needed for a velocity change. also called and unbalanced force equilibrium is when an object is acted on by a set of forces whose resultant is zero. these forces all cancel. the first laws restated in terms of net force: *if no net force acts on it, an object will remain at rest and an object in motion will stay in motion at constant velocity Second Law of Motion *the net force on an object equals the product of the mass and the acceleration of the object. The direction of the force is the same direction of the acceleration. F = ma (net force) = (mass) * (acceleration) Forces are vectors, therefore they must be added “vectorally” ΣFx = max = net forcex ΣFy = max = net forcey Honors Homework: Read Chapter 4 Sections 4.1-4.5 Problems: Pg 125 1 & pg 127 3-10