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Lecture 7

Lecture 7

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PHYS 183
Tracy Webb

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st Lecture 7: January 21 , 2013 4.1 Describing Motion  precise definitions to describe motion o speed: rate at which an object moves; speed = distance/time, ie/ 100 m/s  not useful in physics  doesn’t mention direction o velocity: speed and direction; 10m/s, due East o acceleration: any change in velocity; acceleration = velocity/time, ie/ 50 m/s  positive acceleration: when velocity is increasing  positive acceleration: when direction changes but speed constant  negative acceleration: when velocity is decreasing  acceleration due to gravity  most common form of acceleration on earth & in universe o in absence of friction (air resistance) all falling objects accelerate at same rate 2 o on earth this acceleration is 9.8 m/s (known as g) o object increases its speed every second by ~10 m  momentum = mass x velocity o it’s hard to get something moving when it has a lot of mass o it’s easy to get something moving when it has little mass  a net force changes momentum, which generally means an acceleration (change in velocity) o if you roll a ball and then put a force on it to slowly stop it, you change the momentum and in turn velocity  rotational momentum of a spinning or orbiting object is known as angular momentum  thought question: if there net force? o a car coming to a stop – yes, you apply a force to stop the car (brakes) o a bus speeding up – yes, you apply a force to make bus go faster (pedal) o an elevator moving up at a constant speed – no, it is moving at a constant speed, not accelerating o a bicycle going around a curve – yes, it change
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