Quick math shortcut or no?

[SaintJude]

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A balloon with mass M is released into the air. Neglecting air resistance what will be the balloon's initial upward acceleration when released from rest (of air = Y)

A. 4
B. 5
C. 13
D. 19

Before doing the math on this problem, would it be correct or incorrect to assume that the answer has to be greater than 9.8 m/s^2?

 

[chiddler]

is there enough info to answer this question?

Don't we need the density of air, density of fluid in baloon, volume of baloon?

To answer your question, no I don't think it's not ok to assume that. Draw a FBD:

Up is buoyant force, down is gravity.

So buoyant force may be just slightly more than gravity making acceleration very small. And vice versa.

 

[SaintJude]

is there enough info to answer this question?

Don't we need the density of air, density of fluid in baloon, volume of baloon?

To answer your question, no I don't think it's not ok to assume that. Draw a FBD:

Up is buoyant force, down is gravity.

So buoyant force may be just slightly more than gravity making acceleration very small. And vice versa.

Mhm, makes sense. thanks. i should have realized that

 

[gettheleadout]

Just to clarify, there's no way to answer this question as written in the OP, right? We definitely need some given value to get a numeric answer.

In any case I agree with chiddler's response. For the balloon to be just hovering in air it would have to be experiencing an upward force equal to the downward force of gravity. This would make it's acceleration zero, since it's not moving up or down. Increase that upward force just a bit and its acceleration is now slightly above zero upward (even though the magnitude of the upward force specifically is necessarily greater than that of gravity.)

 

[SaintJude]

Just to clarify, there's no way to answer this question as written in the OP, right? We definitely need some given value to get a numeric answer.

correct

 

[MedPR]

What would we have to know to give it a numerical answer? The density of the substance the balloon is filled with and the area of the balloon?

 

[TXKnight]

What would we have to know to give it a numerical answer? The density of the substance the balloon is filled with and the area of the balloon?

I'd think density and volume?

 

[TXKnight]

A balloon with mass M is released into the air. Neglecting air resistance what will be the balloon's initial upward acceleration when released from rest (of air = Y)

A. 4
B. 5
C. 13
D. 19

Before doing the math on this problem, would it be correct or incorrect to assume that the answer has to be greater than 9.8 m/s^2?

I think that assumption is incorrect because what matters is the buoyant force, acceleration is only a component of it.