document.write( "Question 49724: a ball is thrown upward from the roof of a building 100m tall with an initial velocity of 20m/s when will the ball reach a height of 80m \n" ); document.write( "
Algebra.Com's Answer #32989 by AnlytcPhil(1806)\"\" \"About 
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document.write( "A ball is thrown upward from the roof of a \r\n" );
document.write( "building 100m tall with an initial \r\n" );
document.write( "velocity of 20m/s when will the ball reach \r\n" );
document.write( "a height of 80m\r\n" );
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document.write( "The formula is\r\n" );
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document.write( "s = sO + vOt + atē/2\r\n" );
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document.write( "where\r\n" );
document.write( "s = h = the height of the ball off the ground in meters\r\n" );
document.write( "sO = hO = the initial height = 100 m\r\n" );
document.write( "vO = the initial velocity = 20 m/s\r\n" );
document.write( "t = the number of seconds\r\n" );
document.write( "a = g = -9.8 m/sē (the acceleration of gravity)\r\n" );
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document.write( "Substituting:\r\n" );
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document.write( "h = (100) + (20)t + (-9.8)tē/2\r\n" );
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document.write( "h = 100 + 20t - 4.9tē\r\n" );
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document.write( "That's the formula for the height h in meters \r\n" );
document.write( "at any time t seconds.\r\n" );
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document.write( "Plug in h = 80m and solve for t\r\n" );
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document.write( "80 = 100 + 20t - 4.9tē \r\n" );
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document.write( "4.9tē - 20t - 20 = 0\r\n" );
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document.write( "If you like, clear of decimals by multiplying \r\n" );
document.write( "thru by 10\r\n" );
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document.write( "49tē - 200t - 200 = 0\r\n" );
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document.write( "Use the quadratic formula and you'll get\r\n" );
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document.write( "t = -.83 and 4.92, approximately\r\n" );
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document.write( "Discard the negative answer.  The legitimate answer is\r\n" );
document.write( "4.92 seconds.\r\n" );
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document.write( "Analysis:\r\n" );
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document.write( "at 0 seconds, the height is 100 m\r\n" );
document.write( "at 1 second, the height is 115.1 m\r\n" );
document.write( "at 2 seconds the height is 120.4 m\r\n" );
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document.write( "then it starts falling\r\n" );
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document.write( "at 3 seconds, its height is 115.9 m\r\n" );
document.write( "at 4 seconds its height is 101.6 m\r\n" );
document.write( "at 4.92 seconds its height is 80m\r\n" );
document.write( "at 5 seconds its height is 77.5 m\r\n" );
document.write( "at 6 seconds its height is 43.6 m\r\n" );
document.write( "at 7 seconds its height is 0, i.e., it hits the ground.\r\n" );
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document.write( "Edwin
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