Maybe I’m just too dense to figure this out….
In: Physics
Helium is light than air. Thats what make balloon float.
WEIGHT is Mass X Graity, however, because helium is lighter than air, it counteracts some weight by providing a upward force thus making the ballon lighter despite a bigger mass.
As helium runs out of the balloon, air doesn’t enter, thus the lift force gets smaller and smaller, and the actual weight of the rubber part starts to return to normal.
It you let some helium out, it will get lighter because helium has mass. However, it will also become smaller, which will cause it to displace less air. It’s displacement of air with a lighter gas that allows the helium balloon to float.
The balloon gets lighter. But I think you need to compare this to other things based on the same principle: boats and hot air balloons.
Keep in mind that a hot air balloon used to lift people off the ground is also VERY heavy. Adding hot air to it doesn’t change that. What matters is that the weight of the air it displaces is more than the balloon’s (plus the basket and riders) weight, so it floats.
When the weight of a boat is less than the weight of the water it’s displaced, the boat floats. If the boat shrinks a LOT but weight ”slightly” less, it will have a net sinking effect because it’s displaced less water, but not enough to make up for the weight reduction.
A small helium balloon shrinks and gets lighter, but now the ratio of weight to the balloon’s rubber to helium goes in favour of the balloon rubber which is much heavier than air. The balloon will begin to sink, or at least not float as easily.
The real answer has to do with density. Density is mass divided by volume. Helium is less dense than air. As in, a tub of air is heavier than the same tub of helium. Obviously helium is also much less dense than the rubber balloon.
When you inflate the balloon you are slightly increasing its mass and weight by adding helium, but at the same time you are decreasing its density, because it’s getting way bigger.
Now we can think about gravity. Gravity is pulling everything down, but more dense things get pulled down more strongly. So, the air around the inflated balloon gets pulled down more strongly than the balloon itself. So, the air goes down and pushes the balloon up.
This is what the other commenters are talking about when they say ‘displacement’.
Just to be extra clear, letting helium out of a balloon would make it lighter, in terms of both weight and mass.
Weight is a measure of the gravitational force. Assuming the balloon is near Earth’s surface, the weight depends only on the mass of the balloon (rubber and helium). Letting helium out decreases the total mass, and reduces the weight.
The other force at play here is the buoyant force. Assuming the balloon is in air, the buoyant force is equal to the weight of the displaced air. So buoyant force depends on the volume of the balloon. Letting helium out reduces the volume and so reduces the buoyant force.
At some point you will let out enough helium that the gravitational force exceeds the buoyant force and the balloon will no longer float.
It’s like deflating a ball. It would be lighter (less weight) but also A LOT less buoyant (it would sink in water). Balloons “float” in air the same way that a ball floats in water.
The actual balloon itself (the piece of rubber that we call a balloon) doesn’t change mass at all in the problem – it simply changes shape.
However, the “system” of the ballon plus the gas inside it does change mass. When it is inflated, the balloon contains helium atoms. Those atoms have mass. When the helium atoms are let out of the balloon, the system loses that mass. So the balloon *system* gets lighter.
That may seem counter intuitive because the inflated ballon *feels* so much “lighter” than the deflated balloon – and it even floats! This is because of a different property called density. Density takes into account the object’s mass AND volume, or how much space it takes up.
The inflated balloon takes up wasaaaaaay more space then the deflated balloon, so though it is heavier by a little, it is still a lot less dense – that is, its mass is spread out over a much bigger space. This makes it feel “lighter” and makes it float in the air (because it’s less dense than the air).
Less mass, more weight. So depending on your definition of lighter, either answer can be correct.
The balloon will lose it’s buoyancy. That is the relationship between an objects density and the medium’s density (in this case air). It may seem like because it sinks it is heavier, but technically, it has less mass.
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