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physics_chapter7examples

# physics_chapter7examples - Height of a baseball from energy...

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Height of a baseball from energy conservation You throw a 0.145-kg baseball straight up in the air, giving it an initial upward velocity of magnitude 20.0 m/s. Find how high it goes, ignoring air resistance. + = + KE1 PE1 KE2 PE2 + = + 12mv12 mgh1 12mv22 mgh2 . . + = + . . 120 145 kg20 0 ms2 0 J 0 J 0 145 kg9 8 ms2h = . h 20 4 m Work and energy in throwing a baseball In Example 7.1, suppose your hand moves up 0.50 m while you are throwing the ball, which leaves your hand with an upward velocity of 20.0 m/s. Again ignore air resistance. (a) Assuming that your hand exerts a constant upward force on the ball, find the magnitude of that force. + + = + 12mv12 mgh1 Wother 12mv22 mgh2 + . . - . + = . . + 0 J 0 145 kg9 8 ms2 0 50 m Wother 120 145 kg20 0 ms2 0 J = . Wother 29 7 J = Wother Fs . = ( . ) 29 7 J F 0 50 m = . F 59 4 N (b) Find the speed of the ball at a point 15.0 m above the point where it leaves your hand.

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