# Fres = ma for atwood fall machine

1. Jun 21, 2012

### aaaa202

Consider two stationary blocks of same mass suspended by a massless string over some kind of pulley. The net force on both of them is clearly 0. Now suppose we somehow gave on of them a downwards velocity by some kind of energy transfer.
Then intuitively the block would start to fall down. But is that in agreement with Fres = ma? Because the net force on both blocks would be zero and thus allow no acceleration. Though of course Fres = ma does allow for a constant velocity. Therefore: Would the block just fall down with same velocity forever?

2. Jun 21, 2012

### tiny-tim

yup!

(well, until the other one hits the pulley! )

3. Jun 21, 2012

### PhanthomJay

Energy cannot be transfered into the system unless work is done on the system, such as by applying a force to one of the blocks, hanging another mass from it, or whacking it with a sledge hammer. As long as that force is maintained, the system will accelerate. When the force is released such as in the sledge hammer case, then there no longer is energy being transferred into the system, no more work is being done on it, no more unbalanced force exists, and yes, per Newton's first law, in the absence of friction, it will continue to move at constant speed well not forever, but at least until the rising block gets caught in the pulley. Edit: as more concisely stated by tiny-Tim, yepp!