Do You Understand Buoyant Forces in Water?

[dtcool2003]

Physics Questions... Can you answer them?

Need Help Please! Can you answer them?


(1,2,3) A balloon is partially inflated with air, tied to a rock and thrown over the side of a boat into a deep fresh water lake. The rock and balloon sink toward the bottom.

1. As the rock and balloon sink toward the bottom, the magnitude of the buoyant force acting on the balloon __________.
(a) decreases (b) increases (c) remains constant (d) is impossible to predict

2. As the rock and balloon sink toard the bottom, the magnitude of the buoyant force acting on the rock ___________.
(a) remains constant (b) decreases (c) increases (d) is impossible to predict

3. As the rock and balloon sink toward the bottom, the speed of the rock _________.
(a) increases (b) remains constant (c) decreases (d) is impossible to predict


(10,11,12) A partially filled balloon is tied to a bowling ball and dropped into a deep lake. After the balloon is completely submerged and as the ball and balloon sink toward the bottom of the lake,...

10. ... the buoyant force acting on the balloon ________.
(a) remains constant (b) decreases (c) increases (d) is impossible to predict

11. ... the buoyant force acting on the bowling ball _________.
(a) increases (b) remains constant (c) decreases (d) is impossible to predict

12. ... the average force tending to crush the bowling ball ________.
(a) decreases (b) increases (c) remains constant (d) is impossible to predict


Q(13,14,15,16) A treasure hunter salvaging gold bullion from a sunken ship, puts several gold bricks into a basket and ties a large balloon onto the basket. He then inflates the balloon with air from a compressed air cylinder until it and the basket start to rise taking the precious load toward the support ship on the surface. As the stuff rises toward the surface,...

13. ... the buoyant force exerted on one of the gold bricks ________.
(a) increases (b) remains constant (c) decreases (d) is impossible to predict

14. ... the wieght of one of the gold bricks _________.
(a) decreases (b) increases (c) remains constant (d) is impossible to predict

15. ... the buoyant force acting on the balloon _________.
(a) remains constant (b) decreases (c) increases (d) is impossible to predict

16. ... the speed of the basket _________.
(a) remains constant (b) increases (c) decreases (d) is impossible to predict

 

[Dick]

Yes, I can answer them. But that's not important. Whether you can answer them is. What do you know about buoyant force? Write that down and then think about how moving up and down in water might affect it.

 

[dtcool2003]

buoyant force

wouldnt buoyant force increase the farther u take the balloon deeper into the water because the balloon wants to go up not down?

 

[Dick]

The balloon doesn't 'want' anything, it's pushed by forces. You should find in your book that the buoyant force on the balloon is equal to the weight of water it displaces. The amount of water the balloon displaces depends on it's volume. How might going shallower or deeper affect the volume of the balloon and why?

 

[dtcool2003]

yeah i got 1 and 2

i am stuck on 3 i think its C, bc as you go deeper the pressure affects the speed by slowing it down... is that right?

 

[Dick]

Try 1) first. That 'pressure affects speed by slowing it down' is really fuzzy thinking. The speed of the balloon is affected by the unbalanced forces acting on it. And pressure is one of them, but the pressure differences are contained in the concept of 'buoyant force'. Concentrate on that. Tell me more about the balloon volume.

 

[dtcool2003]

i really have no idea on that

i have no idea at all on any of that... i think i got number one wrong too... isn't it remains constant bc the density, gravity and volume doesn't change right..

 

[Dick]

You did get it wrong. The pressure inside the balloon equals the external pressure. As you go deeper, the external pressure grows. The only way the gas in the balloon can match this external pressure is to contract. The volume of the balloon shrinks. How does that affect the buoyant force?

 

[dtcool2003]

it increases the buoyant force bc the external pressure is higher then the internal pressure so it acts upon the balloon with more force

 

[Dick]

No, no. You are using fuzzy logic again. The internal pressure is always equal to the external pressure for a balloon. The buoyant force equals the weight of the displaced water. A smaller balloon displaces less water. What then happens to the buoyant force?

 

[dtcool2003]

i don't get it...

i thought the pressure increases as it goes down, so the balloon basically has to increase its pressure to meet the pressure of its exterior.. if its not that then i don't understand this

 

[Dick]

You can increase the pressure in a balloon by decreasing it's volume. Blow up a balloon and push on it. The harder you push the smaller it will get and the harder it will push back.