# Quiz of the day (15)

## the ballon will

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#### ninelives

##### Senior Member
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 balloon. When the car accelerates forward, does the ballon :

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Move forward...

#### mjjphoto

##### New Member
None of the above. It will move in random directions.

stay where it is

#### ST1100

##### Senior Member
Move backwards.

Air pressure will increase at the back of the vehicle when it acc forwards, and stabilise only when constant speed is reached. You can feel this by standing at different positions in an MRT. 'Wind' is strongest in the centre due to changes in air pressure, and least at the two ends.

Assumes that balloon is not touching roof of car. If it is, it will stay where it is, since the change in air pressure will probably not overcome the friction btw the two surfaces.

#### bearycute

##### New Member
For some who have been observant, you might notice that when the car accelerate forward, you tend to move backwards relative to the motion of the car due to your inertia (newton's 1st law- frankly, in the strict sense, you stay put while the car move forward giving u the sensation that you r moving back). 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.... :sweat: my 0.0002 cts worth... :embrass:

#### SNAG

##### Senior Member
I've been asked this question before.... And if I am not wrong, the answer is "stay where it is"....

Don't understand the reasoning behind the answer as well... :dunno:

#### AReality

##### Senior Member
haha.. was eating nasi lemak... didn't really think & clicked move backward...
:embrass:

#### Prismatic

##### Senior Member
ST1100 said:
Move backwards.

Air pressure will increase at the back of the vehicle when it acc forwards, and stabilise only when constant speed is reached. You can feel this by standing at different positions in an MRT. 'Wind' is strongest in the centre due to changes in air pressure, and least at the two ends.

Assumes that balloon is not touching roof of car. If it is, it will stay where it is, since the change in air pressure will probably not overcome the friction btw the two surfaces.
That's not exactly true. The 'wind' you are feeling is actually from the increase in air-conditioning pressure when the air intake of the MRT increases due to the movement of the MRT That is why the effect is especially strong when the MRT is underground.

#### Xpose

##### New Member
Does this got something to do with Newton's law of motion and the what interia energy thingy? If so, I would say the balloon would look as if it is staying still in the car but it should be in forward motion as with the car and its passengers

#### wacko

##### New Member
nothing to do with air pressure, it's all about inertia, Newton's first law. none of the answers are correct, because the choices are detailed enough. when we say the baloon will remain where it is, does it mean relative to the car or relative to the outside? there is a big difference.

the answer therefore is that the baloon will seeming move towards the back of the car, but in relation to the road surface, it will be stationary.

#### Knighthunter

##### Senior Member
Agree with wacko.....
The stationary item will tend to stay stationary, if not air movement involved and the car accelerate foward the ballon will move to opposite direction of the car (relative movement to car's floor) since the baloon still have mass even relatively small.

#### HelmetBox

##### New Member
is this a trick question?

#### ST1100

##### Senior Member
Aiyah, someone get a balloon, hop into a taxi and settle it once for all.

#### Poon

##### New Member
ninelives said:
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.
move forward. the whole vehicle is moving forward.

#### mylau

##### New Member
no answer. forward? backward? relative to what? if relative to the road, then it is forward. if relative to the movement of the earth, then i dun know, cos there is not enough information given. if relative to the car, then it is backward. so many answers, can u be more specific in your question?

#### wacko

##### New Member
mylau said:
no answer. forward? backward? relative to what? if relative to the road, then it is forward. if relative to the movement of the earth, then i dun know, cos there is not enough information given. if relative to the car, then it is backward. so many answers, can u be more specific in your question?
relative to road it's forward and relative to car it's backwards? wah, champs.

#### ninelives

##### Senior Member
ST1100 said:
Aiyah, someone get a balloon, hop into a taxi and settle it once for all.
hint : you should try it.

#### sweat100

##### Senior Member
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.

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