Fm1m2 = Fm3m2 - 14Force on Blocks: Find Magnitudes & Directions

[farrah003]

Homework Statement

Blocks of mass 5, 10, and 25 kg are lined up from left to right in that order on a frictionless surface so each block is touching the next one. A rightward-pointing force of magnitude 14 N is applied to the left-most block.

Homework Equations

[a] What is the magnitude of the force that the middle block exerts on the rightmost one?
What is the magnitude of the force that the leftmost block exerts on the middle one?
[c] Suppose now that the left-right order of the blocks is reversed. Now find the magnitude of the force that the leftmost block exerts on the middle one?

The Attempt at a Solution

this is what i tried doing but i think I am totally off
Fm3m2 = m2a + 14 - m1a

 

[tiny-tim]

Welcome to PF!

Hi farrah003! Welcome to PF!

(try using the X₂ tag just above the Reply box )

They all have the same acceleration, so use F = ma on all-three-together to find a.

Then use F =ma (with that a) on individual blocks.

 

[farrah003]

i tried f = ma
this is what i did
m = 5+10+25 = 40
14 = 40(a)
a = .35

but it said that's not the right answer

 

[farrah003]

sorryy my mistake i got it for part a which was 8.75 but when i did it again for part b it said wrong

 

[tiny-tim]

I'm confused …

show your equation for part b.

 

[farrah003]

F = (5)(.35) = .175

 

[tiny-tim]

That F is the total force.

Apart from the right-most block (which you solved for in part a), each block has two forces on it.

 

[farrah003]

so then what formula do i use ?

 

[tiny-tim]

F_total = ma

 

[farrah003]

so then i do 14=5a
then the a is 2.8 n if i multiply that I am going to get 14 again ... I am confused !

 

[tiny-tim]

so then i do 14=5a
then the a is 2.8 n if i multiply that I am going to get 14 again ... I am confused !

No, a is the same for all three blocks.

The left block has two forces on it: 14 from the left, and the reaction force from the right.

The vector sum of those two forces is 5a.

 

[farrah003]

lol i still don't know what numbers to use in what formula

 

[tiny-tim]

(just got up :zzz: …)

The left block has two forces, 14 and R (the reaction force from the middle block), so F = ma means 14 - R = ma.