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why bjorn is not a car seat (youtube video)
- Thread starter ntrenary
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LISmama810
Admin - CPS Technician
Those are airplane seats, huh? No wonder carriers aren't approved for takeoff and landing. Of course holding the baby on your lap is fine. 
BeckC
Well-known member
Great visual.
As a geek, I have a bone to pick. I *love* that they said that force = mass x acceleration (instead of force = weight x speed which I've heard before and is not true). AND, their calculations do turn out correctly. It is true that a 20 lb object experiencing 9Gs is equal to about 180 lbs of force. BUT, it's not because 20 lb x 9G = 180 lbs. That equation makes no sense, the units do not work. Anyway, like I said, nerdy bone to pick. That's all.
I really do like the video though. It turned my stomach to see the baby go flying.
As a geek, I have a bone to pick. I *love* that they said that force = mass x acceleration (instead of force = weight x speed which I've heard before and is not true). AND, their calculations do turn out correctly. It is true that a 20 lb object experiencing 9Gs is equal to about 180 lbs of force. BUT, it's not because 20 lb x 9G = 180 lbs. That equation makes no sense, the units do not work. Anyway, like I said, nerdy bone to pick. That's all.
I really do like the video though. It turned my stomach to see the baby go flying.
Those are airplane seats, huh? No wonder carriers aren't approved for takeoff and landing. Of course holding the baby on your lap is fine.
:yeahthat:
Considering I don't even weigh 180lbs, whadaya think the chances of me holding my 20lb (or 180lb baby @ 9g's) baby are in a crash? lol
urchin_grey
New member
Ooh neat. I'm reposting on FB.
TechnoGranola
Forum Ambassador
Great visual.
As a geek, I have a bone to pick. I *love* that they said that force = mass x acceleration (instead of force = weight x speed which I've heard before and is not true). AND, their calculations do turn out correctly. It is true that a 20 lb object experiencing 9Gs is equal to about 180 lbs of force. BUT, it's not because 20 lb x 9G = 180 lbs. That equation makes no sense, the units do not work. Anyway, like I said, nerdy bone to pick. That's all.
I really do like the video though. It turned my stomach to see the baby go flying.
The unit of measure for a G is normally m/s² and G-force is an object's acceleration. But she should have used kg for the mass then and her results answer should be in Newtons. I'm not sure her calculation is quite accurate in the end. But I can't be bothered to figure it out right now as I was up late doing homework and my brain is mush.
ETA: I'll assume she was using ft/s² for the G's, and isn't 1G = 32 ft/s²?
Last edited:
BeckC
Well-known member
The unit of measure for a G is normally m/s² and G-force is an object's acceleration. But she should have used kg for the mass then and her results answer should be in Newtons. I'm not sure her calculation is quite accurate in the end. But I can't be bothered to figure it out right now as I was up late doing homework and my brain is mush.
ETA: I'll assume she was using ft/s² for the G's, and isn't 1G = 32 ft/s²?
I converted the lbs to kg, then got the result in Newtons and converted back to lbs. Her results are accurate (within a couple pounds to account for round numbers). I did bother to check it because I have no life and I'm that nerdy. lol
I have not yet bothered to figure out WHY the numbers work out when the units don't. But ya, her calculations are correct. The equation she shows just makes no sense.
ETA - Warning: Science Content (I feel like a Mythbuster)
20 lb = 9.07 kg. 9.07kg x 9 x 9.8 m/s2 = 799.97 N = 179.9 lb
180 lb = 77.11 kg. 77.11kg x 9 x 9.8 m/s2 = 6801 N = 1529 lb
TechnoGranola
Forum Ambassador
That's right. I know what my brain was doing wrong, I was doing 20 lb * 9 * 32 ft/s² = 5760 and thinking that result was in pound-force but lb * ft/s² is NOT pound-force, it is POUNDALS. ARGH. So, if I would have multiplied the poundals by the conversion of .031081 I would have got 179.03 pound-force as a result. Which is slightly different than yours because I rounded the G's to 32 ft/s² instead of using 32.152 in which case I would have got the same answer as you.
See, told you my brain was mush today. But I will blame that on the fact that I use metric so starting out with non-metric messed with me because I am used to dealing in Newtons and not this pound-force and poundal stuff.
Should have just converted like you did, but I was trying to make it work with the units she was showing.But her equation lacks that 1G = 32.15 2 ft/s² and that the end result would need to be converted from poundals to pound-force.
It's no wonder people don't understand math/physics when you have people propagating inaccurate equations in order to "simplify" for people. It's a lot easier for me to explain an equation to someone when it's the right equation rather than having to make sense of it in my head and redo calculations so it's right and then try to explain to someone why that simple equation they say isn't accurate and will not always give them the correct answer.
BeckC
Well-known member
That's right. I know what my brain was doing wrong, I was doing 20 lb * 9 * 32 ft/s² = 5760 and thinking that result was in pound-force but lb * ft/s² is NOT pound-force, it is POUNDALS. ARGH. So, if I would have multiplied the poundals by the conversion of .031081 I would have got 179.03 pound-force as a result. Which is slightly different than yours because I rounded the G's to 32 ft/s² instead of using 32.152 in which case I would have got the same answer as you.
See, told you my brain was mush today. But I will blame that on the fact that I use metric so starting out with non-metric messed with me because I am used to dealing in Newtons and not this pound-force and poundal stuff.
But her equation lacks that 1G = 32.15 2 ft/s² and that the end result would need to be converted from poundals to pound-force.
It's no wonder people don't understand math/physics when you have people propagating inaccurate equations in order to "simplify" for people. It's a lot easier for me to explain an equation to someone when it's the right equation rather than having to make sense of it in my head and redo calculations so it's right and then try to explain to someone why that simple equation they say isn't accurate and will not always give them the correct answer.
YES! (To your last thing). Not to mention, could we PLEASE just use metric so that we could tell people F=ma. Either that or measure our mass instead of our weight? I think it trips people up that our weight is a force, because it takes the force of the acceleration due to gravity into account ALREADY. A lb-m and a lb-f are NOT the same thing. GAH!
BeckC
Well-known member
Thank you captain obvious! I'm not saying that to be sarcastic to you, but to myself, because it should have been obvious to me and it was not. I was over thinking it with "Lbf = lbmft/s2 so blah blah". Dur, I say to myself. Also, I hate the English system. lol
So it works out because. 20Lbf = 20Lbm at 1g. So she's really multiplying 20 Lbm by 9g, to get 180 Lbf.
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