# Mechanics problemWatch

Thread starter 13 years ago
#1
Any help would be greatly appreciated.

A small box of mass is sitting on a board of mass and length L. The board rests on a frictionless horizontal surface. The coefficient of static friction between the board and the box is and the coefficient of kinetic friction between the board and the box is, as usual, less than
Find , the constant force with the least magnitude that must be applied to the board in order to pull the board out from under the box.

And a minor unrelated check, am I right in thinking that the sum of the angles of a geodesic n-gon on the sphere of radius R is (Area of n-gon)/RÂ² + (n-2)π?

Thank you.
0
quote
reply
13 years ago
#2
For the first question, do you mean the smallest force that can be applied without causing the box to slip?

And for the second question, I'm not entirely sure, but I think you're right.
0
quote
reply
Thread starter 12 years ago
#3
(Original post by dvs)
For the first question, do you mean the smallest force that can be applied without causing the box to slip?
No - such a force would be 0 and so not very useful.

And for the second question, I'm not entirely sure, but I think you're right.
Thanks

As usual, reputation is on offer.
0
quote
reply
12 years ago
#4
I meant the force required to pull the board out without causing the box to slip. Anyway, I think I know what you mean now.

Try to think of it as if the box is moving while the board remains at rest. Then we find that the acceleration of the board is a = (u_s m1 g)/m1 = u_s g. This is the acceleration required for the box to slide off. So now we apply N2L, and note that kinetic friction is equal to static friction for the minimum force:
F_{min} - f = u_s m2 g
F_{min} = (m2 + m1)g u_s
0
quote
reply
Thread starter 12 years ago
#5
Thanks for that, I think I understand now.
0
quote
reply
X

Write a reply...
Reply
new posts

Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• University of Lincoln
Mini Open Day at the Brayford Campus Undergraduate
Wed, 19 Dec '18
• University of East Anglia
UEA Mini Open Day Undergraduate
Fri, 4 Jan '19
• Bournemouth University
Undergraduate Mini Open Day Undergraduate
Wed, 9 Jan '19

### Poll

Join the discussion

Yes (281)
27.6%
No (737)
72.4%

View All
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.