Function Word Problem: A ball is thrown upward from....

[mathmash]

I don't understand the textbook answers at ALL! I really would appreciate some help!

A ball is thrown upward from the roof of a building 60m tall. The ball reaches a height of 80m above the ground after 2seconds and hits the ground 6 seconds after being thrown.
I made a parabola graph with 80m and 2 seconds being the vertex, the y intercept is 60m and the x intercept is 6 seconds.

The back of the text book says the equation is y= -5t+20t+60.
I understand that the -5 is the slope but what is the 20 and 60?

I thought the equation would be y= -5(x-2)squared+80
Because 2 and 80 are the vertex. and -5 is the slope.

Can anyone explain? Thanks for any help!

 

[fasteddie65]

Re: Function Word Problem

"A ball is thrown upward from the roof of a building 60m tall. The ball reaches a height of 80m above the ground after 2seconds and hits the ground 6 seconds after being thrown."

With metric units, the equation usually used with projectile motion is h = h(0) + v(0)t - 4.9t^2, where h(0) is the initial height and v(0) is the initial velocity.

Try substituting 80 for h and 2 for t, then 0 for h and 6 for t. You have two variables h(0) and v(0), and two equations, so you should be able to solve it.

 

[mathmash]

Re: Function Word Problem

Thanks for replying but actually the chapter isn't about velocity, it's just about how to make y=af[k(x-d)] +c equations. So I can't use the velocity equation you wrote. Thanks anyways though!

 

[mathmash]

Re: Function Word Problem

Thanks for any help! I think I actually figured it out on my own!