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## Main Question or Discussion Point

so lets say you have two masses attached by a spring in the middle. Also, there is a drag force proportional to the speed, that is F= -bv.

I have this problem of writing the equations right.

I envision, that I get the masses far apart, so I know there will be force exerted by the spring to the right, trying to return the the equilibrium position.

Thus, one equation will be

k(x2-x1) - bv = ma1

x1 and x2 represent the distance.

So here is my problem. For the second equation, its natural to think that by Newton's third law there will be an opposite force exerted by the spring, on the second mass, and so I assume the second mass wants to move to the left, because of that spring. The equation I get is:

-k(x2-x1) + bv =ma2.

This is the problem. In the second equation, in answers I am seeing, bv should be -bv, and I have no idea why!! since I take moving right as positive, the second mass is moving left so it should turn positive.

PLZ, what am I missing?

I have this problem of writing the equations right.

I envision, that I get the masses far apart, so I know there will be force exerted by the spring to the right, trying to return the the equilibrium position.

Thus, one equation will be

k(x2-x1) - bv = ma1

x1 and x2 represent the distance.

So here is my problem. For the second equation, its natural to think that by Newton's third law there will be an opposite force exerted by the spring, on the second mass, and so I assume the second mass wants to move to the left, because of that spring. The equation I get is:

-k(x2-x1) + bv =ma2.

This is the problem. In the second equation, in answers I am seeing, bv should be -bv, and I have no idea why!! since I take moving right as positive, the second mass is moving left so it should turn positive.

PLZ, what am I missing?