Pushing and Pulling Boxes

  • Thread starter 21385
  • Start date
  • #1
33
0

Homework Statement


You want to apply a force on a box so that it moves with constant speed across a horizontal floor. The coefficient of kinetic friction between the box and the floor is u. Of the four following cases, the force you apply on the box will be smallest when you

a) Push on it with a force applied at an angle 0<X<90 downward from the horizontal;
b) pull on it with a force applied at the same angle X as in (a), upward from the horizontal;
c)do either (a) or (b) since the applied force is the same;
d)pull or push with a force applied horizontally


Homework Equations


Ff=Fa, to keep at a constant speed


The Attempt at a Solution


The correct answer is b but I have no idea why.
I chose c as my answer seeing that a and b would both result in the same thing, only that the weight of the box would concentrate in the front in situation (a) and that the weight would concentrate in the back in situation (b). I felt that this difference would not change the amount of force needed. Are there any other factors I have not considered? Thanks
 

Answers and Replies

  • #2
cristo
Staff Emeritus
Science Advisor
8,107
73
The friction can be calculated by F=uR, where R is the normal reaction force. Do you know how to find the normal reaction force? More specifically, how will it be different when the force applied is a pull compared to when it is a push?
 
  • #3
33
0
The friction equation is Ff=u(mg-FasinX).
I actually have no idea why the force would be different pushing from one side or pulling from the another.
 
  • #4
cristo
Staff Emeritus
Science Advisor
8,107
73
The friction equation is Ff=u(mg-FasinX).
That's for a pull. If you are pushing the box with a force downwards from the horizontal, you will be adding to the normal force, and thus the correct equation will be Ff=u(mg+FasinX).
 
  • #5
960
0
Cristo, interesting. You're adding ergonomics to the eqn. But i see the wisdom, at arms length to pull relieves N force while pushing adds to it.
Cheers.
 
  • #6
33
0
thanks, cristo
omg, i am so stupid, i thought choice a) was pushing from the angle below the horizontal
 
Last edited:

Related Threads on Pushing and Pulling Boxes

  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
7
Views
1K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
6
Views
7K
  • Last Post
Replies
1
Views
5K
  • Last Post
Replies
3
Views
6K
Replies
5
Views
6K
  • Last Post
Replies
7
Views
1K
  • Last Post
Replies
10
Views
1K
  • Last Post
Replies
1
Views
1K
Top