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"a projectile is fired straight up at an initial velocity of 58.8 m/s. How high above the ground is it 1.5 seconds after firing?"

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- Thread starter KingNothing
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- #1

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"a projectile is fired straight up at an initial velocity of 58.8 m/s. How high above the ground is it 1.5 seconds after firing?"

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Doc Al

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This should be in the homework help section.Originally posted by Decker

I am not sure how to do this. I keep coming out with the wrong answer.

Since you didn't show your work, we can't tell where you went wrong.

Ignoring the complication of air resistance, this is a straightforward application of the kinematics of constant acceleration:

[tex]y = y_0 + v_0t + \frac{1}{2}at^2[/tex]

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russ_watters

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For the sake of a high school physics problem (which this appears to be), none of that is relevant (except the effect of gravity, which is known).Originally posted by Rog

I always split these problems into two parts for easier swallowing. First, use the deceleration due to gravity to find the speed of the projectile after 1.5 sec. Then multiply the time by the

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Doc Al

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Nothing wrong with that. For many students, it's the best approach.Originally posted by russ_watters

I always split these problems into two parts for easier swallowing. First, use the deceleration due to gravity to find the speed of the projectile after 1.5 sec. Then multiply the time by theaveragespeed.

I encourage students who are comfortable with the algebra to know (or derive!) the relationships between the three variables: distance, time, and speed. One should be able to produce at will a formula connecting any two of these variables (for constant acceleration). And understand how to apply them.

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Yeah, I think thats the approach I ended up taking (I had to do this problem today, but sadly I didn't get to check this forum in time to see if there were any replies.)

Now this is really jsut pissing me off. I can't remember how I did it. AHHH!!!!! I hate that feeling. I want to say what I got for an answer to see if you all think its reasonable, but I can't even remember. If its not too much trouble, could someone give me a good or reasonable answer? I don't know why my memory chose to fail me now.

Scratch the above idea...let me see if I can remember.

What is y in that equation?

Now this is really jsut pissing me off. I can't remember how I did it. AHHH!!!!! I hate that feeling. I want to say what I got for an answer to see if you all think its reasonable, but I can't even remember. If its not too much trouble, could someone give me a good or reasonable answer? I don't know why my memory chose to fail me now.

Scratch the above idea...let me see if I can remember.

What is y in that equation?

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y is referring to distance or displacement. [itex]y_0[/itex] is the initial displacement, which in this case is zero.

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I got D=77.175m. Can anyone confirm this?

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That agrees with my answer.

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