Why when you suddenly stop a car people and packages fly forward but a helium balloon inside the car goes backwards?

However, there is a second effect. Because the air in the car tends to stay in motion it rushes foward. It will be compressed at the front of the car and made less dense at the back. This means the air nearer the front of the car will be at higher pressure than the air at the back. Looking at the small bit of air around the baloon this means there will be more pressure on the front of the baloon than the back. This inbalance of pressure means there is a backwards force on the baloon.

Of course both effects are present on the people, the packages and the balloon. The inbalance in air pressure is pushing the people and packages backwards as well. It is just that people are quite dense (have a lot of mass per cubic inch) whilst helium balloons are very low density (have very little mass per cubic inch.) This means the first effect will be much bigger than the second for people and packages. The second effect will be a bit bigger than the first for a helium baloon. The net result is that people and packages are "pushed foward" whilst the baloon is "pushed backwards."

In fact we can predict which object will go fowards and which will go backwards. It depends if they have higher density than air. (Are lighter than air.) When a car is braking, anything goes forward in the car if the density of the object is greater than air. Helium is one of the few gases which is lighter than air, so it always floats in the air, and it will "float" backward.

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