Three blocks missing one mass

1. Sep 18, 2014

So I was sick last week for 3 classes and I really missed something and don't know what im doing.

Three blocks rest on a frictionless, horizontal table (see figure below), with m1 = 6 kg and m3 = 14 kg. A horizontal force F = 111 N is applied to block 1, and the acceleration of all three blocks is found to be 3.4 m/s2

(a) Find m2.

For this I got M2=12kg It was wrong
I did F=ma for both blocks
F=6x3.4=20.4
F=14x3.4=47.6
111-20.4-47.6=42.6
42.6/3.4=12

I think I have to use gravity 9.8?

(b) What is the magnitude of the normal force between blocks 2 and 3?

2. Sep 18, 2014

tjmiller88

For part (a), because the 3 blocks are touching each other, they really just act like one mass, so the way you are splitting up the forces is not correct. Your formula should actually be F=(m1+m2+m3)*a. Make sense?

You would only have to use gravity in this type of problem if there was friction involved, but in this case there is none.

Now for part (b), you want to ask yourself what force needs to be applied to block 3 in order for it to travel 3.4 m/s2.

See if that helps.

3. Sep 18, 2014

Staff: Mentor

It is no problem to split up the forces like that.

The problem is just in the calculations. bradsmith: check your numbers and you'll find the error.

4. Sep 18, 2014

Ok F=(m1+m2+m3)xa
111=(6+m2+14)X3.4
111=(m2+20)x3.4
111=3.4m+68
111-68=3.4m
43/3.4m=12.64

is b 90.57
F=a(m2+m3)
F=3.4(12.64+14)
F=90.57
Thanks for not giving me the answer, I learn better by doing it.
I think I did something wrong again

5. Sep 18, 2014

Staff: Mentor

Why did you add masses 2 and 3 for (b)?
You didn't attach the figure, but I guess the masses are in the order "1 2 3" and the force acts at block 1. So the force between 2 and 3 just have to accelerate block 3.

That's a forum rule for exactly this reason :).

6. Sep 18, 2014

Yes it force is applied to 1 and is in order as 123. so that means add all 3
6+12.64+14+32.64
f/m=a
111/32.64=3.4
so its 32.64?

Im not sure why I did it that way. I thought because I only needed 2 and 3 that was the way to do it.

7. Sep 18, 2014

Staff: Mentor

What is 32.64 now?
And what are its units?
What do you mean with "needed"?

8. Sep 18, 2014

Let me start over.
Three blocks rest on a frictionless, horizontal table (see figure below), with m1 = 6 kg and m3 = 14 kg. A horizontal force F = 111 N is applied to block 1, and the acceleration of all three blocks is found to be 3.4 m/s2

all 3 blocks are side by side F--->123

(a) Find m2

F=(m1+m2+m3)xa
111N=(6kg+m2+14kg)X3.4m/s2
111N=(m2+20kg)x3.4m/s2
111N=3.4m+68kg
111N-68=3.4m/s2
43N/3.4m/s2=12.64kg

(b) What is the magnitude of the normal force between blocks 2 and 3?

F=ma
111=(m1+m2)a
111=26.64*3.4
111/90.57=1.22

I don't know what the other stuff is from

Last edited: Sep 18, 2014
9. Sep 18, 2014

Staff: Mentor

Is that true? Do you use a force of 111 N just to accelerate blocks 1 and 2?
3=5?
Randomly mixing numbers does not help.

Forget about blocks 1 and 2 for a while. Which forces act on block 3? What acceleration results from those forces?

10. Sep 18, 2014