## View Poll Results: the ballon will

Voters
50. You may not vote on this poll
• stay where it is

21 42.00%
• move forward

6 12.00%
• move backward

19 38.00%
• burst

4 8.00%

# Thread: Quiz of the day (15)

1. Originally Posted by sweat100
I think it should move backward relative to a the child in the car. this is because when u resolve the forces acting on the balloon, there will be only the tension, weight and uptrust(if u consider upthrust). u will find that there is a component of the tension force which balances the weight and unthrust(vertical component) of the balloon. then there must be a compnent of force to move the balloon foward(since car accelerating forward), and this can only be achieved when the balloon is slanting back. Therefore i think it should move backward.
Any unsecured item on the dashboard will move backwards, but a Balloon is light, it may not have enough force to push away the air around it and move backwards.

2. If goes by ur reasoing, then a 1 cent coin should not move if it is placed in the car? I think no matter how it will still move backward cos there the balloon still have weight. And F=ma, so there will still be a force acting on the balloon, making the string slant "\" this way, \ is the string.
People correct me if i am wrong.

3. It's going to move all over the place since a kid can never keep still.

4. take note that the questions asks for the moment of acceleration, not when the car is cruising at steady speed.

5. According to Newton's Law the air should have a higher density than helium so the air will push the baloon(with helium) forward.

6. ## Moves forward

I think many neglected the word "Helium".
We know that helium is lighter than normal air. Hence, it tries to float, or move in a directoin opposite to the gravity (or density difference).
In an accelerating car, a similar condition arises, as, due to inertia, the air tends to move backwards, causing a density rise. Hence, Helium, which is lighter, will be pushed towards the less denser area (front of the car).

This can also be simulated with a water bottle pushed forward. We can see the motion of air bubbles.

7. Originally Posted by shyamdsundar
I think many neglected the word "Helium".
We know that helium is lighter than normal air. Hence, it tries to float, or move in a directoin opposite to the gravity (or density difference).
In an accelerating car, a similar condition arises, as, due to inertia, the air tends to move backwards, causing a density rise. Hence, Helium, which is lighter, will be pushed towards the less denser area (front of the car).

This can also be simulated with a water bottle pushed forward. We can see the motion of air bubbles.

way too far fetched. you are taking an insignificant phenomenon and using it to define your guess. sure there will be a pressure differential in the car at the initial moment of acceleration, but compared to the forces in play due to inertia it can safely be neglected.

8. Originally Posted by shyamdsundar
I think many neglected the word "Helium".
We know that helium is lighter than normal air. Hence, it tries to float, or move in a directoin opposite to the gravity (or density difference).
In an accelerating car, a similar condition arises, as, due to inertia, the air tends to move backwards, causing a density rise. Hence, Helium, which is lighter, will be pushed towards the less denser area (front of the car).

This can also be simulated with a water bottle pushed forward. We can see the motion of air bubbles.

i think u got it... i would tend to stick to inertia as my explanation though...

9. Originally Posted by wacko
way too far fetched. you are taking an insignificant phenomenon and using it to define your guess. sure there will be a pressure differential in the car at the initial moment of acceleration, but compared to the forces in play due to inertia it can safely be neglected.
Most of the time, to simplify the situation, lots of things are neglected or treated as insignificant. However though, i think in this case, u might just have to "take air into play" as it is in fact more massive than helium...

Just my 0.002 cts worth though

It is just like asking whether a balancing beam will tip to one side when comparing 1 kg of iron in one side and 1 kg of cotton wool... most of us will say the same.... but that isn't true!...ooops...that might be another question altogehter... maybe i would post it after this balloon qn is resolved...

10. From the perspective of a stationary observer outside the car, the ballloon should appear to stay where it is at the point of acceleration--that is, until the string attached to the balloon exerts a tension on the balloon, causing it to move along with the direction of the car.

Newton's first law of motion: "an object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force."

11. Move forward..!!

12. Originally Posted by ninelives
A family is out for a drive in a car with all vents and windows closed. The child in the back seat is holding a string attached to a floating helium-filled ballon. When the car accelerates forward, does the ballon :

Poll will be closed in 3 days. Let see how high your IQ is.

If car move forward , ballon follow forward. if backward then back ward... lah... ballon will follow where the car goes as long as it is in the car

13. Originally Posted by bigmouse
If car move forward , ballon follow forward. if backward then back ward... lah... ballon will follow where the car goes as long as it is in the car

why leh?? if it was not helium-filled ballon leh ?

14. I should think the balloon will remain in more or less the same position
in relative to the inside of the car since the air is pretty stationery.
Unless you count the air-con.

move forward if you counting from the road.

15. This is the question I've always wanted to ask, but in my scenario, I have a fly/insect in place of the balloon. Will the fly/insect behave the same way as the balloon (assuming it is flying very slowly)? Answers/explanation appreciated.

16. But I thought we came to a conclusion that the baloon will move forward relative to the car?

17. Originally Posted by Terence
It's going to move all over the place since a kid can never keep still.
This is probably the correct answer, but just for discussion's sake...

The air inside the vehicle is NOT still. Under acceleration, the entire sealed volume of air must move forward together. It is propelled forward by its container, ie the car interior. However, bcoz it is a fluid, the rear end of the volume pushes forward on the volume in front. Thus, under acc conditions, the air at the rear is denser and the air in front is less dense. This would imply a movement of air mass to the rear (under acc only); this movement of air mass would cause the balloon to move backwards. The pressure will only stabilise when acceleration conditions are no longer present.

18. Take the unit mass of the air w.r.t vacuum space at positive.
Car accelerates.
The mass of air inside does not want to move. But,
the interior-back of the car pushes the air forward.
Since the car is accelerating, the force will always be there.
The air at the back is compressed, the air at the front is vacuumed.
Thus, there will always be a difference in pressure between the front and back of the balloon.
balloon moves forward due to difference in presure.

The unit mass of the balloon(with helium) w.r.t vacuum space is less than the mass of the air. Thus, the unit mass of the balloon w.r.t air is negative.

Heh... It's been a long time since I did physics...
Am I correct? Correct me if i'm wrong.

19. clap clap clap

wow! we can even learn science here in CS!
thanks for the explanation man.

20. Originally Posted by AReality
But I thought we came to a conclusion that the baloon will move forward relative to the car?
Tthe quiz/poll will be closed in 3 days later. Which is tonite. Never read instruction eh?

BTW, it is balloon not baloon or baboon.

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