Experiments with Air Resistance
In our article on gravity, we discussed how all objects have the same acceleration due to gravity. Some objects fall slower than others when dropped, due to air resistance.
Air resistance is the force of air pushing up on an object as it falls, which slows an object’s fall. The larger the surface area of an object, the more air resistance it will experience.
A popular example is a feather and a bowling ball; you can see videos of this experiment online, explained by Brian Cox. If a feather and a bowling ball are dropped at the same time, we would expect the bowling ball to speed towards the ground and the feather to float down slowly. If a feather and a bowling ball are dropped at the same time in a vacuum chamber (where there is no air), they will both hit the floor at the same time.
The reason that the feather and the bowling ball hit the ground at the same time is that there is no air in the chamber. This means that the bowling ball and the feather cannot experience air resistance. When there is air in the chamber, both the feather and the bowling ball experience air resistance. As the feather has a large surface area and is light, the effect of air resistance slows the fall of the feather more than it slows the fall of the bowling ball. Another version of this experiment was performed on the moon during the Apollo missions and can also be viewed online.
Children can perform their own experiments in the classroom to explore air resistance.
Click here to see some experiments children can try
Children can perform their own version of the feather experiment using a feather and a marble. Dropping both at the same time, the children should observe that the feather hits the ground after the marble as it floats through the air. Children could try an experiment with two sheets of paper; they could scrunch one up into a ball and leave the other one as a flat sheet. The flat sheet should fall more slowly than the ball as it experiences more air resistance due to its larger surface area.
In the image below, we can see a visualisation of the paper experiment.
In the image below, we can see a visualisation of how the air pushes on the different paper shapes.
Children in a nursery carried this experiment out for themselves; the images of this are shown below.
“Ball will drop first and that (flat piece of paper) will go second. Ball go fast and this one (flat paper) slow.”
“Bcos a ball drop faster n paper drop slower”
The children, eager to experiment more tried dropping a much larger sheet of paper together, this is shown below.
“This is a bigger bit! Let’s try this!”
“THREE, TWO, ONE, DROP!”
“It fell slowly like we said!”
Another child decided to punch holes in some paper as they predicted this would reduce the air resistance and make the paper fall quicker. This experiment is shown below.
“Need a hole in it so the air can go through the hole! If a poke a hole in it, the air can go through it. Doin’ this, pushin’ it in, I’ll show ya”
“Fifteen holes in, right there! N’ now I’ll get a normal piece of paper.”
“I think the sheet with the holes will go faster cos got holes in!”
(after making bigger holes with a hole puncher)“Got bigger holes, so the air will definitely fit through them now. I think will make it go really fast now.”
“Three, two, one, go!”
“It went faster! It really did! Went really fast it did!”
Terminal velocity
When an object falls for long enough through the air, it stops speeding up. When this happens, we say that the object has reached its terminal velocity. An object reaches terminal velocity when the force of air resistance on the object is the same size as the force of gravity on the object, meaning the object can no longer speed up.
