B Elevator Question -- What does a downward acceleration mean?

[jwviviang]

TL;DR Summary

I just have a question (I don't know how to word this summary, sorry)

If an elevator is moving upward, what does a downward acceleration mean? When applying the free body diagram, will the positive direction be upwards since the elevator is moving up?

 

[Dale]

TL;DR Summary: I just have a question (I don't know how to word this summary, sorry)

If an elevator is moving upward, what does a downward acceleration mean?

It means that it is going up, but slowing down. Like it is going up from the lobby and about to stop at some upper floor

When applying the free body diagram, will the positive direction be upwards since the elevator is moving up?

In principle you can have up be either positive or negative, whatever you prefer. However, I personally think that having up be negative is confusing, so I would recommend usually choosing up positive.

So your acceleration would be negative and your velocity would be positive.

 

[A.T.]

If an elevator is moving upward, what does a downward acceleration mean?

If acceleration has a component opposite to velocity, then the object is slowing down.

When applying the free body diagram, will the positive direction be upwards since the elevator is moving up?

You are free to choose the orientation of your coordinate system axes, as long you apply that convention consistently.

 

[jwviviang]

If acceleration has a component opposite to velocity, then the object is slowing down.You are free to choose the orientation of your coordinate system axes, as long you apply that convention consistently.

It means that it is going up, but slowing down. Like it is going up from the lobby and about to stop at some upper floor

In principle you can have up be either positive or negative, whatever you prefer. However, I personally think that having up be negative is confusing, so I would recommend usually choosing up positive.

So your acceleration would be negative and your velocity would be positive.

I see now. So if a question says (for example) "...moving upward elevator...if the elevator has a downward acceleration a = 1.4 m/s^2..." the downward acceleration means a = -1.4 m/s^2?

 

[Dale]

I see now. So if a question says (for example) "...moving upward elevator...if the elevator has a downward acceleration a = 1.4 m/s^2..." the downward acceleration means a = -1.4 m/s^2?

Yes, in the usual setup with up being positive.