I heard that if a car accelerates from rest, a balloon would move forwards instead of backward. Why is this? Science says it’s because of density, but I don’t get their explanation. It also seems like it’s violating newton’s first law.
In: 8
Specifically a balloon full of Helium will move in the opposite direction that what is expected when a car accelerates fast.
Normally you would expect an object to move backwards in a car because of momentum, but a Helium balloon will move forward.
This is because the Helium is much lighter than air, and it’s the air in the car that moves backwards pushing the balloon forward.
Imagine the car was 1/2 filled with water, and there was a rubber duck floating on the surface.
When you accelerate, the water sloshes into the “back half” of the car, making the rubber duck get pushed towards the front of the car.
Same thing with a helium balloon and air. Something less dense (helium/duck) gets displaced by something more dense (air/water). Just a little less intuitive.
The air in the car “sloshes” back, just as your intuition tells you things in a car ought to do as the car accelerates. The balloon winds up going forward since that’s “easier.”
To replace the situation with one that perhaps feels much more intuitive, consider a tank half full of water. As the car accelerates the heavy water sloshes to the back of the tank, which means all of the air in the tank actually got pushed forward.
The balloon trick trips people up because we see the balloon and account for it as something that’s a real thing, while ignoring the air as another thing that’s in the car.
When you hit the gas, everything tries to move backwards in the car. You do, the coffee in your mug does, the dice hanging from the mirror do. The air in the car, too.
Since the car is full of air, it can’t really move backwards at all – there’s just more air back there blocking it. What if there was *not* more air back there? What if instead there was something even lighter, which the air could push out of the way as it moved?
Well, a helium balloon is lighter than air, so as the air rushes back it forces the balloon forward.
This effect is exactly the same reason that the balloon floats up in the first place. The energy of the air falling to take its place is greater than the energy required to lift it.
It actually isn’t violating the first law! The helium balloon is currently suspended in the air in the car. Because the balloon is less dense than the air, Archimedes’ principal states that the air exerts a bouyant force on the balloon equal to the displaced mass of the air. This bouyant force will naturally push it to the top, or least dense, portion of the air.
So there is already an outside force acting on the balloon.
Now, when the car accelerates the air inside the car tries to stay in place due to Newton’s first law. This creates a less dense part of the air towards the front of the car. Archimedes’ pricipal delivers an ourside force on the balloon, pushing it towards the front. When the air returns to its normal state the balloon returns to a resting point.
Smarter everyday has a great video on it, it’s due to air pressure in the back of the car increasing when the car moves forward, this makes the balloon float to the front where there’s less pressure as the car movement has left the air in its place, to be collected by the rear hatch.
If a car accelerates at 1g, it’s like the force of gravity is pulling the air and balloon towards the back of the car.
What happens normally when actual gravity pulls air and balloon towards the earth?
The balloon moves in the opposite direction.
another good thing to think about is what would happen if the car had no back hatch or rear window, just open space, but all the other windows were in place and shut?
and what if the car had nothing on any of the sides?
When you accelerate your car the air in it moves along with the car because it has nowhere to go. This is why when you drive your car you don’t feel a rush of air (other than the air conditioner).
The air in you car is about 80% nitrogen, 20% oxygen and maybe a few other trace gases thrown in. The mass of a helium balloon including that teensy bit of rubber is probably near indistinguishable from the mass of the air inside your car and thus will probably behave like it’s just ‘part of the gang’.
The helium balloon moves forward for the same reason that it floats! Acceleration and gravity act very similarly (we can fake gravity with acceleration and acceleration can feel like gravity).
Helium balloons float because as much as they want to be lower (because of gravity pulling them down), the air around them wants to be lower more.
For a helium balloon to fall down a bunch of air has to move *up* out of the way (effectively swapping place with the balloon). But the air that would have to move weighs more than the balloon does, so the force due to gravity pulling the air down is more than the force pulling the helium balloon down [this is where density comes in; same volume of stuff, but the air is more dense so has a higher mass]. So the air wins; the air goes down, the balloon goes up!
Same thing happens in the car. Everything in the car wants to move to the back. But the air (with more mass per volume, so more inertia) wants to move to the back more than the balloon does. So the balloon ends up “floating” towards the front. *Something* has to fill up that space at the front of the car, and the balloon is the lightest thing.
————
On Newton 1, there are a few ways of stating it, but a convenient way of stating it here is:
> An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line **unless acted on by an unbalanced force**.
This gets applied slightly differently depending on our perspective.
If we look at things from inside the car (taking the car as our “reference frame”), we’re in an accelerating reference frame (accelerating forwards), so everything in the car experiences a pseudo-force pulling it backwards (this is what you feel; being pushed back into your seat). Newton 1 doesn’t really apply then as the balloon is being acted on by an unbalanced force (pseudo-force of the acceleration pulling it backwards, reaction force of the air pushing it forwards, air force > acceleration force, so balloon accelerates forwards).
If we look at things from outside the car, as the car accelerates it pushes the air inside the car forwards, and that air pushes on the balloon (pushing the balloon forwards); so again, unbalanced forces, the balloon can accelerate without Newton 1 causing us problems.
Latest Answers