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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: pHysiX on November 12, 2008, 05:23:38 pm

Title: Inclined plane + Inclined force. Normal reaction force = ?
Post by: pHysiX on November 12, 2008, 05:23:38 pm: I've just had an exam and I'm sure i got this question wrong. But i'd like to know:

the normal reaction force and the tension/Force F.

The question is: There is this object that has a weight of 20N. It is on an inclined plane that is 30 degrees and it is held by a force 30 degrees to the plane. Determine the normal reaction force and the tension in the string.

Please give me as much relevant working as possible. Thank you for your help!

I have attached the diagram.
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: ell on November 12, 2008, 05:40:26 pm: I got $F=\frac{20\sqrt{3}}{3}$ and $N=\frac{20\sqrt{3}}{3}$

I'm no good at Physics though, can someone confirm this?
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: pHysiX on November 12, 2008, 05:42:16 pm: yea i think that was it *i hope coz my friend told me that the tension n normal reaction force were the same :-[*....wow...can u post ur working please?
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: ell on November 12, 2008, 05:48:51 pm: Quote from: pHysiX on November 12, 2008, 05:42:16 pm
yea i think that was it *i hope coz my friend told me that the tension n normal reaction force were the same :-[*....wow...can u post ur working please?

yeah of course

Resolving forces perpendicular to plane:

$N+F\sin{30}=20\cos{30}$

$\implies N=10\sqrt{3}-\frac{1}{2}F$ (1)

Resolving forces horizontal to plane:

$F\cos{30}=20\sin{30}$

$\implies F=20\tan{30}$

$\implies F=\frac{20\sqrt{3}}{3}$

Substitute F into equation (1):

$N=10\sqrt{3}-\frac{1}{2}\cdot\frac{20\sqrt{3}}{3}$

$\implies N=10\sqrt{3}-\frac{10\sqrt{3}}{3}$

$\implies N=\frac{20\sqrt{3}}{3}$
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: pHysiX on November 12, 2008, 05:54:34 pm: ok...so the normal reaction force is a downwards force on the plane? That is y we have to take away the vertical component of the force F to find the normal reaction force?

the tension in the string is simple to calculate. thanks for the speedy reply btw =]. awesome
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: ell on November 12, 2008, 06:25:26 pm: Normal reaction force acts away from the plane:

(http://dl-client.getdropbox.com/u/57727/forces.gif)

Forces acting perpendicular to plane: $F_{net}=\mbox{Normal + Tension} - \mbox{Weight Force = 0}$
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: pHysiX on November 12, 2008, 07:54:36 pm: alright thanks mate. i understand the theory to that but i guess this is the way i'm going to handle those questions:

Normal reaction force + all vertical components on TOP of the plane = Object reaction force (weight * cos(theta)) + all vertical components on UNDERSIDE of the plane...

right now, it's working for all the questions i'm doing so i hope it's correct. thanks for the help!
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: ell on November 12, 2008, 07:58:04 pm: If you're doing Specialist next year (I'm assuming you're in year 11) you'll do a lot more of this stuff, except with the added fun of friction/rough surfaces :P
Title: Re: Inclined plane + Inclined force. Normal reaction force = ?
Post by: pHysiX on November 12, 2008, 07:58:45 pm: rofl...i actually prefer next year's one coz i love pulleys...makes so much more sense =]