## 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 AReality
haha.. was eating nasi lemak... didn't really think & clicked move backward...

Originally Posted by AReality
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.

In exam, we only accept the 1st answer. So no matter how hard you try, the 1st answer counts .

2. Originally Posted by AReality
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 balloon should be treated as an extra-large air molecule, it moves backwards when the pressure changes during acceleration. The scenario of the balloon being pushed forward occurs only when the acceleration stops, and the air pressure at the back attempts to equalize - pushing the balloon forward.

As long as acceleration (not velocity) is maintained, the balloon will tilt backwards. In fact, the angle of tilt is directly proportional to the acceleration. Under increasing acceleration, the tilt increases; under constant acc, the angle of tilt remains constant - backwards.

***********************

Can we go back to discussing photography, pleeeeaaazzz?

/me reminds myself not to open this thread again.

3. Quiz closed

Why?

As the closed car accelerates forward, inertial forces send the air in the car backward. This compresses the air behind the balloon, pushing it forward.

The winners are bearycute , shyamdsundar. Here are their explanation.

Originally Posted by bearycute
However, since we are talking abt helium filled balloon here, the inertia of the the surrounding air is higher as compared to that of helium since air is more massive. Thus, the air surrounding the balloon tends to move back relative to the motion of the vehicle. this pushes the helium balloon forward.... my 0.0002 cts worth...
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).
OK, so wat is the gift??

Merry Christmas to bearycute, shyamdsundar .

Yes, my greeting is the gift. A moderator greet you with your name wor, shouldn't you be proud ?

4. i had intended to keep quiet, but i think it might be detrimental to all those taking their O and A level physics. So here's an 'alternative viewpoint' for these people to consider...

The air pressure at sea level is higher than the air pressure in the mountains. Why is there not a constant updraft?

Bcoz an acceleration force (aka gravity) maintains that pressure differential.

Similarly with this balloon case. A difference in pressure alone does not move anything. It is only when air moves, that it is able to exert a force on another object. In other words, only when pressure is being created or equalized, not in static equlibrium.

The balloon can only be moved forwards or backwards by air molecules hitting it when they move forwards or backwards.

It is true that the air pressure at the back is higher than the air pressure in front when the car accelerates. However, when the car is accelerating, the pressure differential is being created - the air molecules are moving backwards, not forwards. The balloon can only move backwards with the air.

When the acceleration ceases - then the air pressure equalizes. *Then* the balloon moves forward. BUT NOT WHEN THE CAR IS ACCELERATING.

Helium has nothing to do with it - the complication of helium is in the vertical component only.

Ok, i'm not here to start a war. If ninelives or anyone has a better explanation on why the balloon goes forward *during acceleration*, then i'm all ears.

/sorry for digging this up again.

5. Frankly, in the A level n O level, most probably we would use the concept of inertia to explain...

Let's try another approach...
There are 2 factors involved (a) Archimedes' principle and (b) "pseudo-forces". A helium filled balloon is less dense than air and the buoyant force it experience is greater than its weight. It therefore floats upwards against gravity.

Now when the car accelerates say in the forward direction, a backward force is generated on massive bodies inside the car because of their inertia. Such forces which occurs in the accelerated frames of reference (such as an accelerated car) are called "pseudo-forces" to distinguished them from "impressed forces". Unlike "impressed forces", "pseudo forces" do not arise from the action of other physical bodies. They act on object in the accelerated frame, proportional to their mass and acceleration in the accelerated frame. Though the term "pseudo" is conventionally used, there is nothing "unreal" about these forces.

The horizontal backwards pseudo force inside the car generates the equivalent of a horizontal gravitational field (this equivalence is one of the most profound principles of physics, discovered by Einstein and popularised w the help of his famous "thought experiment" of a freely falling lift inside which gravity disappears). The helium filled balloon being lighter than air will move against this "pseudo gravitational field (Archimedes' principle).

Hence... in conclusion...the balloon moves FORWARD!!!...Phew! never type such a long reply in CS before...haha

My 0.000002 cents worth

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