Why is air pushed out of a balloon

Basically it comes down to this - there are two forces acting on a parcel of air in the atmosphere: 1. Gravity pulls downward. The strength of the gravity force depends on the mass of the air inside the parcel. This force is just the weight of the parcel 2. There is an upward pointing pressure difference force. This force is caused by the air outside the parcel air surrounding the parcel. Pressure decreases with increasing altitude.

The pressure of the air at the bottom of a parcel pushing upward is slightly stronger than the pressure of the air at the top of the balloon that is pushing downward. The overall effect is an upward pointing force.

When the air inside a parcel is exactly the same as the air outside, the two forces are equal in strength and cancel out. The parcel is neutrally bouyant and doesn't rise or sink.

If you replace the air inside the balloon with warm low density air, it won't weigh as much. The gravity force is weaker. The upward pressure difference force doesn't change, because it is determined by the air outside the balloon which hasn't changed, and ends up stronger than the gravity force. The balloon will rise. Conversely if the air inside is cold high density air, it weighs more.

Gravity is stronger than the upward pressure difference force and the balloon sinks. We can modify the demonstration that we did earlier to demonstrate Charles' Law. In this case we use balloons filled with helium or hydrogen. Helium is less dense than air even when the helium has the same temperature as the surrounding air. A helium-filled balloon doesn't need to warmed up in order to rise. We dunk the helium-filled balloon into some liquid nitrogen to cool it and to cause the density of the helium to increase.

When removed from the liquid nitrogen the balloon doesn't rise, the cold helium gas is denser than the surrounding air the purple and blue balloons in the figure above. As the balloon warms and expands its density of the helium decreases. The balloon at some point has the same density as the air around it green above and is neutrally bouyant. Eventually the balloon becomes less dense that the surrounding air yellow and floats up to the ceiling.

Something like this happens in the atmosphere. A gallon of water weighs about 8 pounds lbs. If you submerge a 1 gallon jug of water in a swimming pool, the jug becomes, for all intents and purposes, weightless. Now, the desk pushes back against the person with a force of the same size. This reaction force will cause the rolling chair to move backwards.

Notice that the two forces act on different objects. The action force acts on the desk. The reaction force acts on the person. The law states that for every action force, there is an equal and opposite reaction force. Imagine hitting a tennis ball. The racket exerts a force on the ball. This is the action force. The ball exerts an equal and opposite force on the racket. This is the reaction force. The more force you use to a hit a tennis ball, the more reaction force your arm receives from the racket.

Every time your feet hit the ground when you are running, the ground hits your feet with an equal and opposite force. Learn more physics! Related Questions. Still Curious? What makes a balloon fly - Ilya Denisenko age 9 P. Tie the end of the other string to the second balloon. Use tape to attach the loose end of each of the strings to the underside of the top of the door frame. Space the balloons so that there is a gap of about 15 cm between them. Make sure to keep the balloons away from significant air flow such as a vent or fan.

Procedure Step in front of the balloons and hold the paper-towel tube so that you can blow air into the space between the two balloons. What do you expect to happen to the balloons if you blow air in between them?

Make sure that the balloons are still. Then blow into the paper-towel tube very slowly. Try to produce a steady air flow. What do you notice? Are the balloons moving?

If the balloons moved, stop their movement and then blow in between them again using the paper -towel tube. This time try to blow through the tube harder than before but still try to maintain a steady air flow. What happens to the balloons this time? Can you explain your observations?

Repeat the previous step but this time blow through the tube as hard as you can, producing a steady air flow. Do your results change with increasing air flow? Why or why not? Extra: Repeat the same tests but vary the size of your paper tube. Do smaller diameter tubes, such as a straw, have the same effect?

Extra: Find out if you can still make the balloons move if they are farther apart from each other. Change the distance between the balloons, and test if it affects your results.