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

Answer 1

When applying the brakes, the car is decelerating. Everything in the car, the people in the car, packages in the car, the balloon in the car and the air in the car, will tend to stay in motion. This is because an object in motion tends to stay in motion, according to Newton. However, there is a second effect. Because the air in the car tends to stay in motion, it will continue to move forward 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 balloon this means there will be more pressure on the front of the balloon than the back. This imbalance of pressure means there is a net backwards force on the balloon. Both effects are present on the people, the packages and the balloon, too. The differential 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), and helium balloons are very low in 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 balloon The net result is that people and packages will be "moving forward" because of their inertia, and the balloon will be "pushed backwards" because of the higher air pressure in the front of the car. In fact, we can predict which object will go forwards 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 a helium balloon always floats in the air, and it will "float" backward.