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Unformatted text preview: Lecture 10 Applying Newton’s Laws ACT: Bowling on the Moon An astronaut on Earth kicks a bowling ball horizontally and hurts his foot. A year later, the same astronaut kicks a bowling ball on the moon with the same force. His foot hurts: A. More B. Less C. The same Ouch! F B,A F A,B Movie from Apollo 17 “String Theory String Theory ” Tension: magnitude of the force acting across a crosssection of the rope/string/cable at a given position (it’s the force you would measure if you cut the rope and grabbed the ends). Consider a segment with mass m of a rope with an acceleration a to the right. If we neglect gravity, the forces on the segment are: m a T 1 T 2 2 1 2 1 T T ma T T − = ⇒ > 2 1 2 1 If 0, 0 m T T T T = − = ⇒ = (also, then the weight of the segment really is negligible) We’ll assume ideal (constant length), massless strings (i.e, mass much smaller than the rest of the masses in the system). T T Massless string: • The tension is the same throughout the string. • It can only pull in the direction of its length. Constant length string: • All objects attached to it move together (same acceleration and velocity) This makes our lives a lot easier (and it is a good approximation most of the time)....
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 Fall '06
 Johnson
 Physics, Force, Mass, Mg 3

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