Friction Question (1 Viewer)

[mandog97]

I keep getting C and the answer is A. Can somebody show me where I'm going wrong? Thanks

 

[Drongoski]

(23 x 10 - P sin(30)) x 0.4 = P cos(30)

The Normal Reaction 'N' is not 23 x 10, but reduced by P sin 30


.: 23 x 10 x 0.4 - 0.4 x P x sin 30 = P x cos 30

.: P = (0.4 x 230)/(0.4 sin30 + cos 30) = 86.30188 ... Newtons

The basic idea is: Friction = Coefficient of Friction x Normal Reaction

 

[D94]

View attachment 32404

I keep getting C and the answer is A. Can somebody show me where I'm going wrong? Thanks

How can someone show you where you went wrong if you don't show us what you did?

 

[mandog97]

(23 x 10 - P sin(30)) x 0.4 = P cos(30)

The Normal Reaction 'N' is not 23 x 10, but reduced by P sin 30

I'm still confused. Could you show your working to me if that's possible?

 

[keepLooking]

Not mine ->

 

[mandog97]

Thank you so much!!!!

 

[Drongoski]

See working out.

 

[D94]

Not mine ->

Just want to say that this is an absolutely disgusting way of solving a basic friction question. It's not intuitive and doesn't assist struggling students in demonstrating what is going on.

I can provide another method if needed (basically what Drongoski has above with a FBD).

 

[mandog97]

Just want to say that this is an absolutely disgusting way of solving a basic friction question. It's not intuitive and doesn't assist struggling students in demonstrating what is going on.

I can provide another method if needed (basically what Drongoski has above with a FBD).

Yeah I never do that. Could you show me your way of doing it? Thanks

 

[keepLooking]

Just want to say that this is an absolutely disgusting way of solving a basic friction question. It's not intuitive and doesn't assist struggling students in demonstrating what is going on.

I can provide another method if needed (basically what Drongoski has above with a FBD).

It isn't my method and I surely haven't found a method that is considered more intuitive, I would appreciate if you could provide another method!

 

[mandog97]

It isn't my method and I surely haven't found a method that is considered more intuitive, I would appreciate if you could provide another method!

I got it. Do you have a copy of Copeland (pg. 94/95)? It goes through friction questions really well, particularly this type.

 

[Drongoski]

There is nothing to it:

The block moves when the horizontal component of P (i.e. P x cos 30) exceeds the maximum friction (= the normal reaction x coeff of friction}; about to slide when the two are equal. This latter was what my original post expressed.

 

[D94]

Yeah I never do that. Could you show me your way of doing it? Thanks

It isn't my method and I surely haven't found a method that is considered more intuitive, I would appreciate if you could provide another method!

I know it's not your method, I even quoted the part where you said that.

It's how you set up the solution that makes it more intuitive, not the actual solution itself. A basic Free Body Diagram will make all the difference: http://puu.sh/kj2Us/afec99c63e.png