Newton's Law question, Not sure if right

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Homework Statement



[PLAIN]http://img208.imageshack.us/img208/9987/forcediagram.png [Broken]

In the figure above, two small objects (magnets for instance) of the same mass exert forces on each other of a magnitude F. When object A is subjected to an external force of 3.6x10^-2 N, A accelerates at 1.6 m/s/s and B accelerates at 2.1 m/s/s as shown. Find F. Neglect other forces, assume accelerations are parallel.

Homework Equations


F=ma
ƩF = ƩmƩa



The Attempt at a Solution



I'm not sure if the solution i got is entirely correct but this is it

ƩF(A) = 3.6x10^-12
ƩF(B) = (3.6x10^-2) + F

I got this because B is accelerating faster which means the force on B must be greater if the masses are equal.

Using this, I found the mass of A, by F = ma

3.6x10^-2 = m(1.6)
m = .0225 kg, so A and B are .0225 kg

To find F, I substitute the mass into F = ma for F(b)

So

(3.6x10^-2)+ F = .0225*2.1
F = 1.13x10^-2 N

Is that correct?
 
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Answers and Replies

  • #2
PeterO
Homework Helper
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Homework Statement



[PLAIN]http://img208.imageshack.us/img208/9987/forcediagram.png [Broken]

In the figure above, two small objects (magnets for instance) of the same mass exert forces on each other of a magnitude F. When object A is subjected to an external force of 3.6x10^-2 N, A accelerates at 1.6 m/s/s and B accelerates at 2.1 m/s/s as shown. Find F. Neglect other forces, assume accelerations are parallel.

Homework Equations


F=ma
ƩF = ƩmƩa



The Attempt at a Solution



I'm not sure if the solution i got is entirely correct but this is it

ƩF(A) = 3.6x10^-12
ƩF(B) = (3.6x10^-2) + F

I got this because B is accelerating faster which means the force on B must be greater if the masses are equal.

Using this, I found the mass of A, by F = ma

3.6x10^-2 = m(1.6)
m = .0225 kg, so A and B are .0225 kg

To find F, I substitute the mass into F = ma for F(b)

So

(3.6x10^-2)+ F = .0225*2.1
F = 1.13x10^-2 N

Is that correct?
You have not allowed for F acting on each body. being as B accelerates faster than A - moves away from it - the force F must be repulsive.

For A, 3.6 x 10-2 N acts to the right, anf F acts to the left.
For B, just F acts to the right.

From that you can get what you are after.
 
Last edited by a moderator:

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