View Full Version : Quadratic Eqn Word Problem: projectile shot up @ 928 ft/s

afritts1987

10-30-2008, 08:52 PM

You fire a rifle straight up. Your bullet leaves your gun at a velocity of 928 feet per second. Ignore air resistance. Consider the muzzle of your gun to be at height zero. The bullet will reach its maximum altitude of _______ feet after ______ seconds.

I know in order to find time I have so far with the equation....

h(t) = -16t^2+928t but thats as far as I got. I'm not sure how to do the rest. I tried the quadratic equation using just that and it didn't work. I got 58 or 57 point something and the answer is wrong. The teacher just said we had to solve this using a quadratic equation.

Subhotosh Khan

10-30-2008, 09:16 PM

You fire a rifle straight up. Your bullet leaves your gun at a velocity of 928 feet per second. Ignore air resistance. Consider the muzzle of your gun to be at height zero. The bullet will reach its maximum altitude of _______ feet after ______ seconds.

I know in order to find time I have so far with the equation....

h(t) = -16t^2+928t but thats as far as I got. I'm not sure how to do the rest. I tried the quadratic equation using just that and it didn't work. I got 58 or 57 point something and the answer is wrong. The teacher just said we had to solve this using a quadratic equation.

Please share with us your work - so that we know where to begin to help you.

Mrspi

10-30-2008, 09:44 PM

You fire a rifle straight up. Your bullet leaves your gun at a velocity of 928 feet per second. Ignore air resistance. Consider the muzzle of your gun to be at height zero. The bullet will reach its maximum altitude of _______ feet after ______ seconds.

I know in order to find time I have so far with the equation....

h(t) = -16t^2+928t but thats as far as I got. I'm not sure how to do the rest. I tried the quadratic equation using just that and it didn't work. I got 58 or 57 point something and the answer is wrong. The teacher just said we had to solve this using a quadratic equation.

Ok, you are given this equation:

h(t) = -16t^2 + 928t

If you look at the expression on the right side of the equals sign, you should recognize it as a parabola, and the maximum or minimum value on a parabola occurs at its vertex.

h(t) = -16t^2 + 928t

has a graph that is a parabola that opens DOWNWARD, and the vertex of the parabola represents the maximum value for the height (h).

You should have learned a couple of methods for finding the vertex of a parabola. Until you let us know which methods you know, and SHOW US YOUR WORK, then it is difficult to give you more help.

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