Answer to Question #131590 in Physics for Alba Isturiz

Question #131590

A ball is dropped from rest at the top of a 100 m tall building. At the same instant, a second ball is thrown upward from the base of the building with an initial velocity of 50 m/s. When will the two balls collide, and at what distance above the street? 


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Expert's answer
2020-09-03T14:06:35-0400

The law of motion of the first ball

y1=y0+v0ytgt22=1009.8t22=1004.9t2y_1=y_0+v_{0y}t-\frac{gt^2}{2}\\ =100-\frac{9.8t^2}{2}=100-4.9t^2

The law of motion of the second ball

y2=y0+v0ytgt22=50t9.8t22=50t4.9t2y_2=y_0+v_{0y}t-\frac{gt^2}{2}\\ =50t-\frac{9.8t^2}{2}=50t-4.9t^2

At the instant when balls collide y1=y2,y_1=y_2, so


1004.9t2=50t4.9t2100-4.9t^2=50t-4.9t^2t=2st=2\:\rm s

The distance above the street

d=y1(t)=y2(t)=50×24.9×22=80.4md=y_1(t)=y_2(t)=50\times 2-4.9\times 2^2=80.4\:\rm m

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