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April 22nd, 2016, 12:53 AM   #1
Joined: Apr 2016
From: Singapore

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Velocity of a projectile

Hi Guys,

First time post, I hope this is the correct location to post my 2 questions?

1/ I am trying to work out the horizontal distance a projectile will travel on level ground:

Resultant Velocity: 930m/s
Angle: 45deg
a: -9.81m/s2

I determined that Vx & Vy were 657m/s each.
I determined that tUp 67s, therefore tTotal was 134s

To calc the horizontal distance it should be just a matter of 134s x 657m/s (tTotal x Vx). Correct?

That gives me = 88,119m or 88km... this seems way off?

2/ Same projectile accelerates vertically (at 90deg from horizontal):

Total height
= sqr [ Vr2 / 2(-9.81m/s2) ]
= sqr [ (-930m/s)2 / ( -19.62m/s2) ]
= sqr [ -864,900m2/s2 / -19.62m/s2 ]
= sqr 44,082m
= 210m
Dist = 420m (seems too little to me??)

tUp = 930m/s / -8.91m/s2 = 94.8s
tTotal = 189.6s (3min 3s)

Again it only travels 420m vertically but takes 3min & 3s to travel...?

I cannot see any errors but the values don't look like they line up. I have been staring at this for 20m.

Any help would be really appreciated before I tear all my hair out. Thanks.

Last edited by skipjack; April 22nd, 2016 at 02:42 AM.
BioMechMan is offline  
April 22nd, 2016, 02:37 AM   #2
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Joined: Apr 2014
From: UK

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1) Looks correct

2) After 1 second, the speed has dropped from 930m/s to 920.19m/s, so it must have travelled at least 920m in the first second alone, so yes, I think 420m is incorrect.
I see 44082m in your calculations, I get this number if I do 94.8 x 930/2 (flight time X average speed)
weirddave is offline  

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