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Uhh, kind of an odd question.

B

broly8877

Senior member

Alright, before I start, this is NOT HW.
I overheard this today and wonder how this is possible.

You have a box, on top of an inclined surface. At what angle will it JUST begin to slide? No weight is given
http://img.photobucket.com/alb...broly8877/untitled.bmp
^ Example

Is it even possible? I remember back in my physics days we needed more information than that.


Again, simple question for those more knowledgable.
 
Originally posted by: broly8877

So it is possible even though mass is not provided?
I always thought mass was esential to these.
Click to expand...
It will start sliding when the downward force (gravity/weight) outstrips the friction force holding the box back. Basically, since mass is in both components (gravity = m*g and I believe friction = m*u?), you arrive at a balance between the gravity constant and the friction coefficient. The effect of gravity is a function of the angle - in this case, gravity in the direction of the plane=g*sin(?).

If my friction force equation is wrong (i.e. friction is not a function of mass), then you could only find the angle as a function of mass. I know that the friction coefficient has some mass component, but I can't recall exactly what it is. Too used to dealing with fluids now. 😛
 
Originally posted by: CycloWizard
Originally posted by: broly8877

So it is possible even though mass is not provided?
I always thought mass was esential to these.
Click to expand...
It will start sliding when the downward force (gravity/weight) outstrips the friction force holding the box back. Basically, since mass is in both components (gravity = m*g and I believe friction = m*u?), you arrive at a balance between the gravity constant and the friction coefficient. The effect of gravity is a function of the angle - in this case, gravity in the direction of the plane=g*sin(?).

If my friction force equation is wrong (i.e. friction is not a function of mass), then you could only find the angle as a function of mass. I know that the friction coefficient has some mass component, but I can't recall exactly what it is. Too used to dealing with fluids now. 😛
Click to expand...

Force normal to the incline = m*g*cos(angle)
Force parallel to the incline = m*g*sin(angle)

Static Frictional Force = 0.45*Force normal to the incline.

To just start sliding, frictional force = force parallel to incline

0.45*m*g*cos(angle)=m*g*sin(angle)
0.45*cos(angle)=sin(angle)

See... no m or g left in this equation.
 
so to be annoying and finish the great explaination from TuxDave,
0.45 = sin(angle)/cos(angle) = tan(angle)
angle = arctan(coefficient of friciton) = atan(.45) = 24.23 degrees
 
Originally posted by: skene
so to be annoying and finish the great explaination from TuxDave,
0.45 = sin(angle)/cos(angle) = tan(angle)
angle = arctan(coefficient of friciton) = atan(.45) = 24.23 degrees
Click to expand...

lol... :thumbsup:
 
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