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Author Any physics people?
Matty W
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10th Oct 11 at 18:40   View User's Profile U2U Member Reply With Quote
not sure how to find acceleration.

i have the values for distance, start and end velocity.

S=700mm
U=0ms-1
V=990ms-1
T= ?
A= ?

what is the correct equation to use?



[Edited on 10-10-2011 by music1990]
Matty W
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10th Oct 11 at 18:46   View User's Profile U2U Member Reply With Quote
nope.. still confused
John
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10th Oct 11 at 18:47   View User's Profile U2U Member Reply With Quote
Is this engineering at uni?
Matty W
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10th Oct 11 at 18:48   View User's Profile U2U Member Reply With Quote
yeah, foundation year :/
John
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10th Oct 11 at 18:50   View User's Profile U2U Member Reply With Quote
Ian
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10th Oct 11 at 18:55   View Garage View User's Profile U2U Member Reply With Quote
So -

Distance - 700mm
Starting velocity 0 m/s
End velocity 990 m/s

?

Assuming acceleration is linear (which means speed has an exponent of 2) you can use v=d/t to derive time.

Seems fairly short though? T is only a few seconds I think.

PS that -1 is to the power of -1, ie. metres x seconds^-1 = metres per second, don't mix that up when its written.
Matty W
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10th Oct 11 at 19:00   View User's Profile U2U Member Reply With Quote
yeah its meters per second to the power minus 1 or ms^-1

the question is
" a bullet travels 700mm along a rifle barrel, accelerating uniformly from rest ( u= 0ms^-1 ) to a muzzel velocity of 990ms^-1 ( v = 990ms^-1) , find the acceleration involved and the time the bullet was in the barrel
FlaFFy_91
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10th Oct 11 at 19:19   View Garage View User's Profile U2U Member Reply With Quote
If you find out the mass and force you can figure out the acceleration. Force = mass X acceleration
Steve
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10th Oct 11 at 19:20   View Garage View User's Profile U2U Member Reply With Quote
Paul J will tell you the answer based on an Xbox 360 Joypad
Matty W
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10th Oct 11 at 19:23   View User's Profile U2U Member Reply With Quote
i think ive worked it out. 700mm = 0.7m

0.7m/990ms = 0.000707070S

now i know time i can work out acceleration
Ian
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10th Oct 11 at 19:24   View Garage View User's Profile U2U Member Reply With Quote
Force and mass can be used but they're not relevant to that question.

We have enough to work it out

v=d/t

t=d/v

=700mm / 990

=0.7070 reoc.

Gets a bit funky because the speed increases exponentially with linear acceleration, not sure of that formula without checking it properly.
Jake
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10th Oct 11 at 19:28   View User's Profile U2U Member Reply With Quote
foundation
Ian
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10th Oct 11 at 19:29   View Garage View User's Profile U2U Member Reply With Quote
What's funny? Post your solutions.
Jake
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10th Oct 11 at 19:31   View User's Profile U2U Member Reply With Quote
i cant believe this is foundation engineering
Matty SRi
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10th Oct 11 at 19:34   View User's Profile U2U Member Reply With Quote
I have absolutely no idea what anything in this thread means
Matty W
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10th Oct 11 at 19:35   View User's Profile U2U Member Reply With Quote
well i worked out that 700mm is 0.7 meters ( because im measureing velocity in meters per second) so 0.7m devided by 990 ms^-1 = 0.0007070 (move the decimal place from your equation) then acceleration = end velocity - start velocity / time ( 990 /0.0007) = 1414285.714 ms^2

does that sound right?

sounds a bit too fast to me

[Edited on 10-10-2011 by music1990]
Ian
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10th Oct 11 at 19:38   View Garage View User's Profile U2U Member Reply With Quote
v1 = v0 + at
v1 = 0 + at
v1/t = a

990/0.0007070 = 1400282 m/s^2

1.4 million metres per second squared?

Might get it more accurate by interpolating the other equation.
Ian
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10th Oct 11 at 19:40   View Garage View User's Profile U2U Member Reply With Quote
quote:
Originally posted by music1990
sounds a bit too fast to me
It's a bullet and it's doing nearly a km per second by the time it's travelled the length of the barrel. I would go with that
Ben J
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10th Oct 11 at 19:43   View User's Profile U2U Member Reply With Quote
One of my mates is head of physics.
Ian
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10th Oct 11 at 19:43   View Garage View User's Profile U2U Member Reply With Quote
1400142.8571429

It rounds to 1400143 as you get more accurate with the seconds.
Matty W
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10th Oct 11 at 19:54   View User's Profile U2U Member Reply With Quote
nice one. im starting to get it now
Ian
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10th Oct 11 at 20:01   View Garage View User's Profile U2U Member Reply With Quote
Did they show you integration and differentiation? That's the main concept to grasp because it shows properly the relationship between acceleration and speed.

Typically in a scenario like that acceleration is linear and speed is exponential. Falls down a bit in real situations because the bullet wouldn't continue to accelerate so you can't consider anything outside of the barrel in that one, that's why I initially thought it was quite short because it makes more sense if you use cars and things which can continue to accelerate at a constant rate throughout the test.
Matty W
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10th Oct 11 at 20:10   View User's Profile U2U Member Reply With Quote
nothing on that yet. i think its just to get a hang of using the equations for now. they may go over that in the first year
allza
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10th Oct 11 at 20:26   View User's Profile U2U Member Reply With Quote
Not done this for years but i'll try. First you ned to put them in correct units so s=0.7. Then the 2 equations you need to use are v^2 = u^2 + 2as and also s = ut+1/2at^2. If you use the first you get 990^2 = 2a*0.7. 980100=1.4a a= 700,071 ms2 You then need to use this value of a into the equation s=ut + 1/2 at^2 to work out t. I'll let you do that! I think thats correct any how.

[Edited on 10-10-2011 by allza]

[Edited on 10-10-2011 by allza]
nibnob21
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10th Oct 11 at 20:30   View Garage View User's Profile U2U Member Reply With Quote
I would use:

v^2 = u^2 + 2as

to work out the acceleration.

u disappears as it is 0 so after rearranging you are left with:

a = (v^2) / (2s)
= (990^2) / (2*0.7)
= 4900500 / 7
~ 700071.43 m/s^2


Then I would use:

v = u + at

to work out the time.

Again u disappears as it is 0, so after rearranging to make time the subject you get:

t = v / a
= 990 / (4900500 / 7)
= 7 / 4950
~ 1.41*10^-3 s


Using s = ut +1/2at^2 you can check that the values are correct. The ut term goes because u=0 so we are left with s = 1/2at^2 as follows:

1/2 * (4900500 / 7) * (7 / 4950)^2 = 0.7m

Job's a goodun

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