Understanding Constant Velocity and Acceleration in Frictionless Environments

[Reisen]

Your standard first-post-pleading-for-help here :)

This doesn't make sense:

Say you've got a setup like in picture 1 (floor is frictionless)

If the F readout on the scale is 18N, and the box is 5kg and F=ma, then 18N/5kg = 3.6m/s^2 = a, right? Okay...

Now, if the velocity is constant, there is no acceleration, yes? But then F = ma, then F = 5kg * (0m/s^2) = 0. But if the boxcar were moving forward at a constant rate with a frictionless floor, the box would slide to the back, wouldn't it? The rope has to be exerting SOME force on the box to keep it in place, so that's why I'm wondering how this doesn't work.

Thanks in advance to whoever points out my mistake :)

-Reisen

 

[bd1976]

Well if everything in your picture, the box, car and scale is moving at the same constant velocity initally then nothing would change. Everything would in the same position relative to everything else.

However if the box is not moving initally but everything else is then yes, the box would move towards the back of the truck and the scale would have to apply a force on the box to accelerate it.

 

[Reisen]

Ah, yes. Thank you. I was getting the real world all confused with the perfect world of elementary physics :D

That answers my question, thank you.

-Reisen