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

Can the normal force between a blocks accelerating together be zero?

  • #1

Homework Statement


Three rectangular blocks of equal mass m slide to the right along a frictionless horizontal surface accelerated by a force F applied to the left side of the left-most block. (a) Draw a free-body diagram for each block. (b) Determine the magnitudes and directions of all of the forces on each block in terms of m, F and/or g.


Homework Equations





The Attempt at a Solution


As for vertical forces, each block has the force of gravity downwards and the normal force from the surface pointing upwards, which sum to 0. That part is fine.

As for the horizontal forces, on block 1 I have the applied force to the left, and the normal for from block 2 to the right. For block 2 I have the normal force from block 1 to the right and the normal force from block 3 to the left. And then for block 3 there I have the normal force from block 2 to the right. Using Newton's Law of equal and opp. pairs, I know that the normal force of 2 on 1=-(force on 1 on 2) and normal forces of 2 on 3=-(force 3 on 2).

By equating these forces and using Newton's Law, I get that the force of 2 on 3 is just F (applied force) and 3 on 2 is -F. And 2 on 1/1 on 2 is zero. Is that possible? Am i missing something?

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
454
0
By equating these forces and using Newton's Law, I get that the force of 2 on 3 is just F (applied force) and 3 on 2 is -F. And 2 on 1/1 on 2 is zero. Is that possible? Am i missing something?
you made a mistake somewhere doing this.

The easiest way to do it is to start by calculating the acceleration of all the blocks by using F = m*a for all of them together
 
  • #3
tiny-tim
Science Advisor
Homework Helper
25,832
249
Hi mistymoon_38! :smile:

Hint: Call the acceleration A.

So the net force on the three blocks as a whole is F = 3mA.

Then what is the net force on the first block, to produce an acceleration of A? :smile:
 
  • #4
Hey! We have the same problem for my Physics Recitation.

I disagree with your block 1 which might lead to your error. I said that The Force Applied is going to the right and The Force (2 on 1) is going to the left.

This means that F(1 on 2) = -F(2 on 1) so it should not equal zero but be the same.
 

Related Threads for: Can the normal force between a blocks accelerating together be zero?

Replies
8
Views
5K
Replies
1
Views
4K
Replies
15
Views
673
Replies
13
Views
3K
Replies
5
Views
1K
  • Last Post
Replies
6
Views
408
Replies
8
Views
7K
Replies
10
Views
1K
Top