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November 01, 2025, 03:11:38 pm
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Author Topic: Dynamics  (Read 960 times)  Share 

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soccerboi

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Dynamics
« on: August 22, 2012, 08:43:11 pm »
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Is the normal reaction force always equal to the weight force? Are there situations where they differ?
Thanks
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pi

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Re: Dynamics
« Reply #1 on: August 22, 2012, 08:45:25 pm »
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Quote from: soccerboi on August 22, 2012, 08:43:11 pm
Is the normal reaction force always equal to the weight force? Are there situations where they differ?
Thanks

Think about slopes :)
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Jenny_2108

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Re: Dynamics
« Reply #2 on: August 22, 2012, 08:45:51 pm »
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Quote from: soccerboi on August 22, 2012, 08:43:11 pm
Is the normal reaction force always equal to the weight force? Are there situations where they differ?
Thanks

No. They differ when the surface isnt horizontal
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soccerboi

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Re: Dynamics
« Reply #3 on: August 22, 2012, 08:56:16 pm »
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Oh yeh lol. Thanks guys. But when the surface is horizontal they will always be equal right? just making sure..
So when we know the weight force but dont know the normal reaction force, we can safely assume that they equal without showing any working out to find the R value yeh? Cause i saw a problem where it was a horizontal surface and they just assumed it without any working out.
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Bhootnike

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Re: Dynamics
« Reply #4 on: August 22, 2012, 09:01:26 pm »
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Quote from: soccerboi on August 22, 2012, 08:56:16 pm
Oh yeh lol. Thanks guys. But when the surface is horizontal they will always be equal right? just making sure..
So when we know the weight force but dont know the normal reaction force, we can safely assume that they equal without showing any working out to find the R value yeh? Cause i saw a problem where it was a horizontal surface and they just assumed it without any working out.
if theres no rope or anything pulling on it. so if its just an object sitting there:
i guess so..
its an assumption as per newtons 3rd law as you know, so i guess all you need to write is something like, Newton Third Law: Fw = Fn
« Last Edit: August 22, 2012, 11:37:23 pm by Bhootnike »
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monkeywantsabanana

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Re: Dynamics
« Reply #5 on: August 22, 2012, 09:02:11 pm »
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not always when there's a force acting upon it
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TrueTears

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Re: Dynamics
« Reply #6 on: August 22, 2012, 11:00:19 pm »
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Quote from: soccerboi on August 22, 2012, 08:56:16 pm
Oh yeh lol. Thanks guys. But when the surface is horizontal they will always be equal right? just making sure..
nope, what if you have a rope pulling a box along a perfectly horizontal ground at an angle to the horizontal?
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Re: Dynamics
« Reply #7 on: August 22, 2012, 11:03:48 pm »
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As a general rule I'd advise you to not assume that N = mg. Instead, create a j axis and sum all the components, equate to zero (unless it's accelerating upwards, as in flying) and solve for N. This should usually take you roughly 30 seconds, and then you're certain you are correct.

EDIT: Being a grammar nazi.
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