Math help please
Soo I'm having trouble doing this math problem:
A flare is launched from the deck of a lifeboat 4ft above the water surface. The initial upward velocity is 80 ft/s. After how many seconds will the flare be 100 ft above the water surface?
Please help, thanks
April 10, 2013
17 Comments • Newest first
^ topic starter doing this in math, not physics.
@NameHerexD I just learned this today, I'm still kind of confused on it, but still sort of getting how to do my homework.
@Florpe Thanks man, sometimes I do the silliest mistakes
no problemo
needed to brush up on my quadratics myself
@kooner729 OOOOH thanks! I factored the trinomial incorrectly, thanks for helping, everybody xD.
@NameHerexD: I know, just saying XD What if he copy & pasted and then forgot to change ball into flare? :o
Who needs math when you have swag?
h(t)=-16t^2+vt+c
h(t) is the final height
t is the time
v is the initial velocity
and c is the initial height
the formula is showing h as being a function of t. you can think of it on a graph like t being x and h(t) being f(x) or y.
100=-16t^2+80t+4
take everything to the other side so that t^2 has a positive coefficient.
16t^2-80t+96=0
divide the equation by 16
t^2-5t+6=0
and factor that as a trinomial, giving
(t-2)(t-3)=0
therefore t=2,3
meaning at 2 seconds, the flare will be at 100 ft going up, and at 3 seconds it will be coming down.
@kooner729 Could you maybe show me how you got 2 and 3, I tried doing it myself but I kept getting wrong answers.
@NameHerexD Your work kind of confused me (all of it)
^It's a flare, not a ball
I just wish your problem was in SI units, not US units. Conversions are a (female dog).
@PurplePickles Oh, I did it using physics formulas haha
Oh I see it now. Plugging the known values into the equation:
100=-16t^2+80t+4
t=2, 3
[quote=PurplePickles]@Florpe Uhh, I'm not quite sure how you got 9.8t^2, explain please?[/quote]
9.8meters per squared seconds is the universally accepted value of acceleration due to Earth's gravitational pull. If the flare were falling to earth, the 9.8m/(s^2) would be a positive value. The person that first responded with a viable equation has the acceleration due to gravity as a negative value because the flare is flying against the gravitational pull.
Edit: the values you have are in ft/s while the person I'm referring to mixed the equation to have feet and meters per second squared. He is wrong for now.
@KittyKawaii Yes, it is a velocity problem.
@kooner729 The homework tells me to use the equation h(t)= -16t^2 + vt +c
And I'm not quite sure the answer is correct, since it isn't one of those choices on the homework.
oh sorry, I'm Canadian ^^
didn't read the ft part
@kooner729 If the velocity is in ft/sec, I think you'd use -32ft/sec^2, not -9.8m/sec^2
Vf^2=Vi^2+2ad
Vf^2=80^2+2(-32)(96)
Vf=16
Vf=Vi+at
16=80+(-32)t
t=2.0s
@Florpe Uhh, I'm not quite sure how you got 9.8t^2, explain please?