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# I need an equation

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I''ll be frank, this is a homeowrk problem, but I don''t want the answer. The problem states: A parachutist desceding at a speed of 10 m/s drops a camera from an altitude of 50m. (a) How long does it take the camera to reach the ground? (b) What is the velocity of the camera just before it hits the ground? I know this is an easy problem, but I don''t know what equation I use to determine the time in part ''a''. I have a bunch of equations, but none that solve for time. What equaton do I use?

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let me c if i remember

v0 = 10
d = 50
t = ?
acc = 10

try this for (a):
d = v0*t+0.5*acc*t^2

(b):

v^2 = v0^2+2*acc*d

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i''ve been doing allot of this stuff lately. it feels pretty fresh, but nevertheless i could be wrong.

acceleration (t) = -9.8
velocity (t) = -9.8t -10
position (t) = -4.9t^2 -10t +50

you can make these equations using calculus.
in case you wanted an explanation:

acceleration = -9.8 (for gravity)

integral of -9.8 with respect to t is -9.8t, add your initial velocity to that and you get -9.8t -10 (the velocity equation)

integral of the velocity equation plus the initial position will be your position equation.

for part a set the position equation equal to 0 and solve for t (looks like use the quadratic formula is required)
for part b set the time to whatever you got in part a and solve for v (but you knew that already)

hope this helps

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To begin, list what information you have. From the problem you have the beginning velocity, 10m/s, the displacement, 50m, and the acceleration (gravity). You''re looking for time.

Find the equation that uses time, displacement, beginning velocity, and acceleration and then plug in the values.

The hackers must have gotten into the system through the hyperlink!!

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Do it yourself. Giving you the equation is like giving you the solution.

A few weeks ago, I would have given you the solution myself, but with the moderator''s recent posts, I won''t. I agree with Graham that this is not the place to ask for homework questions. To all of you hurrying to post before the thread is closed: by doing so, you''re basically expressing your disagreement with the FAQ. I think that we should all stop answering these questions once and for all, even if the thread hasn''t been closed yet...

Cédric

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Just a friendly admonishment. Your question is a very basic physics question. Instead of asking for the equation, you should make sure you understand *why* the equation works. Once you understand the concepts, the equation is extremely easy to figure out. In a sense, you''re asking the wrong question, both for this forum and for your own education. Unfortunately, many schools promote the "just give me the equation" mentality.

A suggestion - ask your teacher to explain the basics (again?). Also, if you *must*, I bet Graham would be more forgiving of a "Explain why Newtonian physics works" type of question rather than "do my homework for me".

My two cents...

Thank you.

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Also, if this *is* a homework question, your teacher was extremely flaky in not teaching you the formula first. What kind of math class is this?!

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If I''d seen this sooner I would have closed it sooner. As cedricl stated, giving the equation is giving the answer. Part of the understanding involved in solving physics/maths problems like the one described is in knowning which equations are applicable and which one should be used.

To those that provided the solution, please consider what it is that you are doing. Certainly you''re showing everyone that you know some maths/physics. That''s nice, but we probably already knew that anyway! Sure you''re helping the question asker by providing them with information/the solution, but that doesn''t reallly help them when they come to an exam and they don''t have someone else around to ask for help from. I know it sounds harsh, but NOT giving the solution is the better course of action and leads to a better understanding on the part of the student. Directing them to a resource (such as MathWorld) encourages them to develop research skills of their own which will always help them with future problems.

I would hope that everyone who hasn''t done so recently will review the Forum FAQ.

Thanks,

Timkin

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