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June 24th, 2009, 06:31 PM
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| |  Car crash, Seatbetls, impulse
Also ive been trying to get my head around an equation for seatbelts. Hyperphysics has an auto calculator and ive been trying to do it algebraically and still cannot get the same answer.
Here is what i have done...
In relation to a car travelling at 50mph with a driver which weighs 75kg and stops with in 1 foot. Car Crash Example m = 75 kg, initially travelling at 50kmph = 13.88888889m/s d= 30 cm Distance time interval =
dt = 0.3 m / 13.88888889 m/s = 0.022 s
During this 220th’s of a second it is decelerated from 13.88888889 m/s to zero, so
a = dv/dt = 13.88888889 m/s / 0.022 s = 631 m/s^2
So the impact force is
F = m x a = 75 kg x 631 m/s^2 = 47348N .... But it is completely different answer |
June 24th, 2009, 07:03 PM
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Quote:
Originally Posted by Dingo  Also ive been trying to get my head around an equation for seatbelts. Hyperphysics has an auto calculator and ive been trying to do it algebraically and still cannot get the same answer.
Here is what i have done...
In relation to a car travelling at 50mph with a driver which weighs 75kg and stops with in 1 foot. Car Crash Example m = 75 kg, initially travelling at 50kmph = 13.88888889m/s d= 30 cm Distance time interval =
dt = 0.3 m / 13.88888889 m/s = 0.022 s
During this 220th’s of a second it is decelerated from 13.88888889 m/s to zero, so
a = dv/dt = 13.88888889 m/s / 0.022 s = 631 m/s^2
So the impact force is
F = m x a = 75 kg x 631 m/s^2 = 47348N .... But it is completely different answer | your acceleration calculation is incorrect ... should be about 316 m/s^2 |
June 24th, 2009, 07:20 PM
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Quote:
Originally Posted by skeeter your acceleration calculation is incorrect ... should be about 316 m/s^2 | How?
or how else should i calculate it? |
June 24th, 2009, 07:24 PM
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June 24th, 2009, 07:30 PM
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Quote:
Originally Posted by skeeter | Where Vf and V0 represent?
and what forumla is this? |
June 24th, 2009, 07:39 PM
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Quote:
Originally Posted by Dingo Where Vf and V0 represent?
and what forumla is this? |  is final velocity ... 0  is initial velocity ... 50 kph
this is the well-known "no time" kinematics equation. | | The following users thank skeeter for this useful post: | |  |
June 24th, 2009, 08:08 PM
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I get to.... Ive been working on this all night and now swear im brain dead...
0 = 192.9 + 2*A*0.3
do i...
0 =192.9 + 2*A*0.3 -2*A*0.3
- 2*A*0.3 = 192.9
-2*A*0.3/0.3 = 192.9/0.3
-2*A = 643
-2*A/-2*A = 643/-2*A
0 = 643 / -2*A/A
0/A = 643 /-2
A = 643/-2
A = 643 / -2
A = 321.5 ms^2 (- Negative due to deceleration... |
June 24th, 2009, 08:19 PM
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50 kph = 125/9 m/s
1 ft = .3048 m
0 = (125/9)^2 + 2a(.3048)
a = -(125/9)^2 / [2(.3048)] = -316.439 m/s^2 | | The following users thank skeeter for this useful post: | | | | Thread Tools | | | | Display Modes |  Linear Mode | 
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