Joe85 Posted June 6, 2018 Posted June 6, 2018 Glorious Math guru's, please shower me with your superior intellect. A ball is thrown down at 20ms^-1 speed from the top of a building. The building is 125 meters tall. The distance travelled before it reaches the ground is as follows:S=U0t + 1/2gt^2U0 = 20ms^-1g = 10ms^-2t= timea) Find the time for the ball to drop a fifth of the height of the buildingb) Find the time for the ball to reach the groundNow, i feel fairly confident that i have solved this myself. Having converted this into a quadratic equation i have:a) t = 1sB) t = 3.385sHowever, when checking my answer against an online caluclator (https://www.calculatorsoup.com/calculators/physics/displacement_v_a_t.php) it states the answer is a complex conjugate. Now, i'm struggling to see how they have come up with a complex number here (i thought for it to be a complex number, the number you find the square root of (#^2- 4ac) had to be negative ie J = -1) but part of the course i am doing does involve complex numbers and i'm slightly worried they are trying to catch you out a bit.Any help for a dunce would be appreciated. Quote
megawatt Posted June 6, 2018 Posted June 6, 2018 I may have answered this if it had said Chinese 125 dropped from top of building. But it doesn't, so I don't care. Quote
Joe85 Posted June 6, 2018 Author Posted June 6, 2018 I may have answered this if it had said Chinese 125 dropped from top of building. But it doesn't, so I don't care. Noted for next time glorious mathematician. Quote
Westbeef Posted June 6, 2018 Posted June 6, 2018 Ignore them calculators if they give you a quadratic answer and you’re not expecting it, they go mental sometimes. Quote
bonio Posted June 6, 2018 Posted June 6, 2018 I think there's an error in the calculator.If I put the speed and acceleration as negative values, then it gives the right answerBut, putting your numbers for part a (with t = 1) into the equation s = ut + 1/2gt^2 you get25 = 20 + 0.5*10which is correct.My guess is that the person writing the page confused themselves by writing the equation as ut + 1/2gt^2 - s = 0, which led them to messed up the signs. Quote
TimR Posted June 7, 2018 Posted June 7, 2018 My answer is .Change the values to the following ...https://www.justanswer.com/math-homework/6ssh9-ball-thrown-down-72-km-h-1-speed-top.html Quote
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