Three blocks

1. May 9, 2010

farrah003

1. The problem statement, all variables and given/known data

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.

2. Relevant 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?

3. The attempt at a solution

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

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Last edited: May 9, 2010
2. May 9, 2010

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.

3. May 9, 2010

farrah003

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

but it said thats not the right answer

4. May 9, 2010

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

5. May 9, 2010

tiny-tim

I'm confused

show your equation for part b.

6. May 9, 2010

farrah003

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

7. May 9, 2010

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.

8. May 9, 2010

farrah003

so then what formula do i use ?

9. May 9, 2010

tiny-tim

Ftotal = ma

10. May 9, 2010

farrah003

so then i do 14=5a
then the a is 2.8 n if i multiply that im gonna get 14 again ... im confused !!

11. May 9, 2010

tiny-tim

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.

12. May 9, 2010

farrah003

lol i still dunno what numbers to use in what formula

13. May 10, 2010

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.