lecture 10

lecture 10 - Physics 2A Lecture 10: Jan 30 Vivek Sharma...

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Physics 2A Lecture 10: Jan 30 Vivek Sharma UCSD Physics
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More on Force F ma is NOT the of Force! Equation DEFINITION reactio shows the of the object to a for n ce = G G SI Unit of force is the NEWTON 1 Newton is the amount of net force that gives an acceleration a =1 m/s 2 to an object of mass m=1.0 kg 1 N = 1kg . m/s 2 2- 5 2 In CGS units: 1 dyne = 1 g.cm/s = 10 Newton In British units: 1 Pound=1 slug.ft/s 4.45N = Your Quarterpounder is really a 1 Newtoner Geek joke ! don’t laugh
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Weight on Earth & Moon Standard KG weighs 9.80N on earth Standard KG weighs 1.6N on moon Want to lose weight? Go to the moon!
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Measuring Mass & Weight On earth, an easy way to measure mass is by measuring its weight and comparing with the standard kg In Zero gravity you weigh nothing so to measure mass, use 2 nd law Apply a force & measure object's acceleration m F / a = G G 11 2 2 ma =
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A Consequence of The 2 nd Law Large F implies large a and vice-verca Human body not capable of handling large F To stop an accelerating body safely, need to decelerate slowly and over a “long” period of time. This concept crucial for airbag design, fighter pilot ejector seats, bungee cords & even saving Lois Lane !
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Ejection Seat In Fighter Planes rocket to slowly accelerate pilot vertical to flight direction used to be explosives, high acc. over short time= BAD
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Superman Saves or Kills Lois Lane ? Lois Lane thrown off high tower by bad guy, superman comes to the rescue, scoops her up just as she is about to hit ground Did he save her or kill her ? Magnitude of deceleration (> 6g) as Superman stopped her free fall could well have killed her (depending on how long she was in free fall ) ….Had Superman taken PHYS2A he would have known not to wait till the last moment to scoop her
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Newton’s Third Law
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Newton’s 3 rd Law • Force acting on a body is always a result of its interaction with another body. Forces always come in pairs • Newton’s third law: –When a force due to object B acts on object A, then an equal & opposite force acts on B BA AB F F =− G G this force acts on A this force acts on B action-reaction pair choice of which one is action or reaction is arbitrary ! Forces are equal &opposite
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Action-Reaction Pair The pair of forces are simultaneous, equal & opposite and act on different bodies
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Newton’s Third Law Action Equals Reaction, Acts On Different Bodies Demo: The Ethanol Rocket !
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Hazards of Space Walk Dangerous space walk to fix Discovery’s heat shields Why was it so dangerous ?
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lecture 10 - Physics 2A Lecture 10: Jan 30 Vivek Sharma...

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