the classical block problem

azoroth
Posts: 4
Joined: Thu Jun 05, 2014 8:31 am

the classical block problem

Postby azoroth » Thu Jun 05, 2014 8:52 am

hi , i have been preparing for the physics GRE and i m following the Yoni khan book of conquering the physics GRE, and i m kind of stuck in the very first problem..

A block opf mass 5 kg is positioned on an elclined plane at an angle 45 degree. A force of 10 N is applied to the block parallel to the ground, if the coefficient of kinetic friction is 0.5, what is the acceleration of the block?

i solved out the problem first for motion without kinetic friction. so
mg*sinx-F sinx = 20*sqrt 2............(1) net downward motion


the considering friction,
mu*mg*cos x= 25sqrt 2........(2) net upward motion
so the accelearion should be sqrt2 upward, but the solution says it should be sqrt up, where am i getting it wrong ?
please help

bfollinprm
Posts: 1197
Joined: Sat Nov 07, 2009 11:44 am

Re: the classical block problem

Postby bfollinprm » Thu Jun 05, 2014 12:38 pm

....friction can never cause acceleration, bro.

YF17A
Posts: 94
Joined: Sat Aug 30, 2008 4:42 am

Re: the classical block problem

Postby YF17A » Thu Jun 05, 2014 2:25 pm

Hi, one of the authors here. Remember that the frictional force is always proportional to the normal force, which receives an additional contribution from the applied force in this problem. Also it looks like you forgot a factor of 1/2 in your calculation of the frictional force: cos x = sqrt(2)/2, not sqrt(2). The combination of those two should get you the correct answer.

(And of course friction can cause acceleration! That's how a train works.)

bfollinprm
Posts: 1197
Joined: Sat Nov 07, 2009 11:44 am

Re: the classical block problem

Postby bfollinprm » Thu Jun 05, 2014 7:11 pm

sorry, yes.

azoroth
Posts: 4
Joined: Thu Jun 05, 2014 8:31 am

Re: the classical block problem

Postby azoroth » Fri Jun 06, 2014 1:05 am

that helps, thanks

kangaroo
Posts: 130
Joined: Fri Jan 13, 2012 5:31 am

Re: the classical block problem

Postby kangaroo » Fri Jun 06, 2014 6:19 pm

I must say it's alarming that you are struggling with this. Classical mechanics is a basic tenet when studying physics, and if this is still not ingrained by now...well it's time to go through undergrad again.

SSM
Posts: 87
Joined: Mon Nov 01, 2010 10:57 pm

Re: the classical block problem

Postby SSM » Fri Jun 06, 2014 10:20 pm

I must say it's alarming you're still as arrogant and egotistical as when you started graduate school several years ago, kangaroo.




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