• Support PF! Buy your school textbooks, materials and every day products Here!

Friction between two boxes

  • Thread starter x86
  • Start date
  • Tags
    dynamics
  • #1

Homework Statement


Selection_017.png


Homework Equations


F=ma
Ff = N u

The Attempt at a Solution


Pretty much, I did the same thing in the solution.

For box A:
N = mg
Ffs = mg*uk = 9.81*20*.4=78.48N

I note that Ffs > 60 N, so the boxes will move with the same acceleration

Now there are two ways I can approach this problem, each gives me a different answer??

1) Take two boxes as the same system, then Fx = 70*a = 60 ==> a = 0.857 m/s^2
2) Look at the FBD of box 2, note that Fx = 50*a = 60 ==> a = 1.2 m/s^2

However, both of these answers are different than the solution.

Where did I mess up (and why do 1 &2 give different solutions?)
 

Answers and Replies

  • #2
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,156
502
Solution messed up the math, although equation is correct. . When you look at the lower Block , you did not properly address the force acting on the lower block. It isn't 60. Draw an FBD.
 
  • Like
Likes x86
  • #3
Check for errata. Looks like they did 60 = 70a --> a = 70/60, which is wrong.
 
  • Like
Likes x86
  • #4
Solution messed up the math, although equation is correct. . When you look at the lower Block , you did not properly address the force acting on the lower block. It isn't 60. Draw an FBD.
Isn't this the FBD? Ffs on the top block is -60N, and since its an action-reaction pair it should be +60N on the 50kg block


EDIT: Actually, now that I think about it, the friction on the bottom/top block should be different, because its making the top block accelerate.

So the I have more equations to solve:

20a = -Ff + 60
50a = Ff

20a = -50a + 60
70a = 60
a = 60/70

Thank you everyone for your help
 
Last edited:

Related Threads on Friction between two boxes

Replies
13
Views
8K
Replies
4
Views
15K
  • Last Post
Replies
1
Views
4K
Replies
4
Views
798
  • Last Post
Replies
6
Views
578
  • Last Post
Replies
2
Views
6K
  • Last Post
Replies
5
Views
4K
Replies
11
Views
9K
Replies
6
Views
2K
Replies
1
Views
3K
Top