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Have you ever thrown a frisbee around with friends at your local park or on the beach? Or have you played sports like Ultimate Frisbee or Disc Golf?
If you have, chances are you’ve honed your technique to see how far or how accurately you could throw.
But did you ever stop to wonder exactly what forces of physics were at work in keeping these plastic discs in the air?
In this article we’ll ask that very question: how do frisbees work?
How does their design help them to stay in the air? And how can certain changes to this design change how a disc flies?
How do Frisbees work? Forces at play
When you throw a frisbee you do two things: you give the disc spin and forward thrust.
The combination of these two forces is what allows the frisbee to fly, and without one or the other the disc would not be able to travel very far at all.
In the following paragraphs we’ll take a look at exactly how each of these forces acts on the frisbee to keep it in the air.
The first question you’ll probably want to know is what keeps a frisbee up.
We know that the force of gravity is constantly pulling the disc towards the ground, so if the disc doesn’t fall there must be an equal force pushing the disc upwards.
In fact there is: this force is called “lift.”
As the disc flies horizontally, it acts like an airplane wing, slicing through the air.
Since the frisbee is a solid disc, the air has to be pushed out of the way for the frisbee to pass through.
Frisbees are designed with a rounded top (as are airplane wings), and this forces the air passing over the top of the disc to travel faster than the air passing underneath it, which can move in nearly a straight line.
As air moves faster its pressure decreases, and so the increased speed of the air above the disc creates a pressure difference, resulting in an overall upward force on the frisbee.
If that explanation is enough for you then feel free to skip ahead to the next section.
But in case you’re looking for more science, we should talk about why exactly the air pressure above the frisbee drops when the frisbee moves past it.
The answer depends on something called Bernoulli’s Principle.
This principle applies to fluids like water and air, so we can first imagine a tube with a cross-sectional area A1.
If water is moving through this section at a rate of V1, that means the total volume of water passing this point in the tube is A1V1.
Now let’s imagine that the cross-sectional area of the tube suddenly decreases by half to A2.
Assuming there are no leaks in the tube, we know that the same volume of water must be passing through A2 as at A1 (i.e. A1V1=A2V2).
Therefore, we know that the velocity of the water V2 must increase for the same amount of water to pass through a smaller space in the same amount of time.
You can see this principle in action when you put your thumb over the tip of a hose, causing the water to shoot out through a smaller space but at a greater velocity than before.
Bernoulli’s equation depends on the principle of conservation of energy.
If the fluid suddenly moved faster at A2, where did this new energy come from? Energy can’t be created from nowhere.
The answer is that it was stored as potential energy in the pressure of the water.
Hence when the velocity of a fluid increases its pressure decreases correspondingly.
You can test this principle with the example of the garden hose as well.
As you cover the tip of the hose, you can feel the pressure of the water building up against your thumb.
Air works the same way as water, so that when the layer of air above a wing or a frisbee is forced to accelerate by the shape of the object, this air also drops in pressure.
The air pressure above the frisbee becomes lower than the air pressure below the disc, creating a net upward force called lift.
So if lift is caused by the forward movement of a frisbee through the air, where does spin come into play?
Spin is important because it keeps the disc stable.
If you simply push a frisbee forward it may glide for a moment, but it will quickly become unstable and tip over.
This is because the aerodynamic forces acting on the frisbee are not balanced, the lift being greater on one side than the other.
But when you spin a frisbee you give it angular momentum, which works to resist the unequal forces caused by the air.
To understand the stabilizing effect of angular momentum, consider how difficult it is to balance on a bike that is standing still, and how much easier this becomes when the bike is moving.
This is due to the angular momentum of the moving wheels, which stabilizes your movements along the axis of the rotation.
Frisbee makers take advantage of this principle by distributing much of a frisbee’s weight close to the outside of the disc, giving the disc higher angular momentum and therefore greater stability.
The final force that acts on a frisbee is drag. This is the force of the air passing over the frisbee and slowing it down.
Frisbees are streamlined to minimize drag, and some disc golf discs even have a sharp edge to further increase aerodynamics, but nevertheless drag is what slows all frisbees and eventually brings them back to the ground.
A frisbee’s flight is a product of five forces.
Aerodynamic lift pushes a frisbee up at the same time as gravity pulls it down, allowing the frisbee to glide horizontally.
Forward momentum causes the frisbee to fly in the direction of the throw while drag causes it to slow and eventually stop.
And finally angular momentum keeps the frisbee level, offsetting any uncentered forces that would otherwise cause the disc to flip over.
Featured image credit: DepositPhotos.com