# HELP Semester Exam on Monday

HELP!! Semester Exam on Monday!!

## Homework Statement

A missile is fired horizontally from an aircraft carrier in the ocean. Assuming the missile was shot from a height of 50m and a horizontal velocity of 200m/s...
a. How long is it in the air?
b. How far away will the missile land?
c. Explain which direction of motion is constant and which is accelerated. Why is this true?

Thank You
Allison :)

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BruceW
Homework Helper

This forum has a few rules on not helping too much. So I'm hoping you already know the answers, but just need a bit of help!

Start with what you know about projectile motion. What laws/principles does it obey? Also, are you meant to derive the answer using calculus, or just use the equations of motion (a.k.a. SUVAT equations)?

turbo
Gold Member

Allison, the inclusion of "urgent" or "asap" into thread titles, are pretty good indicators that the poster hasn't done enough work to keep himself/herself well-founded in their field of study. PF is a wonderful resource. Please use it when you are seeking clarification and and trying to advance your knowledge. If you jam in here looking for a bail-out from some imminent disaster, that's not a problem that people want to help you with. PF is all-volunteer, with some of us willing to help Greg with expenses (the gold people). We are usually quite willing to help any student that has made an honest effort and can see that their work is somehow faulty.

I do know that we are to use the equations of motion and...
displacement= (initial velocity)(change in time) + 1/2 (acceleration)(change in time)^2

Physics doesn't come easily to me, I am more of a Literature/History minded individual, but I am here to clarify things that will be on the Exam that I am unfamaliar with.

First step: deep breath and then draw a picture.
When I was in HS I thought my intelligence was being insulted when this advice was given to me, but as a physics undergrad I start every test this way.

When you get to the math problems, just split it into two directions. If you are unsure, just pretend you are 50m tall and can throw a dart at 200m/s and think about what happens (carefully).

For clarification, is this a problem that will be handed in and graded? or is it a practice problem? Just like in Lit/Hist you need to give your own answer before somebody else can critique your work. Describe (using words and or math) what you might try in order to get an answer. No matter how lost you think you are, I bet you can figure it out.

Haha, I cannot draw worth a darn, but I will do my best. This is Exam Review, practice problems.

The direction of motion that's constant would be to horizontal, since the vertical changes when the missile declines?.... As for the acceleration, it would be verticle? Being that it rises/declines?...

(I'm not even sure what the correct terminology would be)

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Picture doesn't have to be good. Give it a shot and post what you have... and next time give yourself more than a few days!

Thanks, and I would have had I not only discovered PF this AM

How long does it take for a missile to fall 50m with an initial y velocity of 0m/s?

you know the distance d, you know the initial y velocity and you know gravity

Figure out which equation to use

When you have found the time it takes how do you find the distance traveled in the x direction when you know two things the initial velocity of the missile and the time it travels for and there is no acceleration in the x direction

okay, I think I've got it....

a)
h=50
v=200
a=9.8
t=?

h=(1/2)agt^2
50=(.5)(9.8)t^2
50=4.9t^2
√10.20 = √t^2
t=3.19s

b)r=?
R=vt
R=(200)(3.19)
R=638m

c) Horizontal motion is constant because there aren't horizontal forces acting on the missile
Verical motion is accelerated because of gravity pulling the missle down
These are true because only gravity is acting on the missile

THOUGHTS?..

How long does it take for a missile to fall 50m with an initial y velocity of 0m/s?

you know the distance d, you know the initial y velocity and you know gravity

Figure out which equation to use

When you have found the time it takes how do you find the distance traveled in the x direction when you know two things the initial velocity of the missile and the time it travels for and there is no acceleration in the x direction
Thank you :) that was a BIG help!

Looks good

BruceW
Homework Helper

THOUGHTS?..
Looks correct to me. Nice work. A lot of people don't realise that it is OK to separate the horizontal and vertical motion. (for example, me, when I was first learning this stuff).

Y'all are the best!!