Outreach: The Physics of Frisbees

When the weather is nice, the laboratory can get rather warm. Doing an experiment outside with Frisbees can be both fun and informative, as well as a good introduction to the scientific method.

How a Frisbee works

Image from http://web.wellington.org/miller/frisbee.gif

A Frisbee creates a difference in pressure due to it's curvature. As shown in the diagram above, air travels faster over the curved surface and thus a higher pressure is created underneath the Frisbee, giving it lift.

As the Frisbee is travelling in air, a drag force is exerted on it in the opposite direction to that of it's motion.

Another physical effect is that of gyroscopic motion. As the lift force acting upon the Frisbee is centred near the front of the Frisbee, it will experience a torque which tends to flip the Frisbee over if it is not spinning. When it is spinning, the gyroscopic phenomena of precession occurs. In this case the spinning means that the torque enacts over many points on the disc, and it's net force is negligible.

Normally a Frisbee has small ridges on the top of the disc, these cause a small amount of turbulence, this allows the Frisbee to travel further.

Simple free body diagram

Finally we consider the angle of the disc. In the above diagram the Frisbee is flat, in reality it is usually tilted at some angle to the horizontal. The launch angle will affect both the lift and drag force exerted upon the Frisbee, as this will determine how much air is deflected downwards and upwards by the edge of the disc. The disc is more likely to curve towards the direction of spin if it is released at a larger angle because of this.

Gravity will eventually cause the disc to hit the ground.

Modern Frisbees are hollow discs rather than the more conventional solid discs. The modern design has a few advantages. Firstly increasing the curvature on the outer edge and inner edge means that air will flow over two surfaces rather than just the one. There is usually a small groove on the underside of the disc which increases it's lift, and the disc can be somewhat flexible, which should reduce it's drag slightly as it can adapt to changes in wind direction etc.

Equipment

You will require a reasonably long and straight patch of grass, as well as the following.

• Long piece of string (~40m)
• Wooden skewers or similar (to stick into the ground)
• Tape measure
• 2 Frisbees, one solid disc and one hollow disc
• Pen & paper

Method

Stick a wooden skewer into the ground and tie one end of the string to it. Lay the string in a straight line, using the skewers to mark every few metres, I recommend every 5 metres.

Get the participants to throw each Frisbee a number of times (using the below points of investigation) and observe the behaviour when different tilt angles / amounts of spin etc are used.

Try to get the participants to investigate the below points and think about why this behaviour is observed. Adding competition is usually a good way to get participants interested in the optimum throwing conditions. For example a competition for longest distance travelled, straightest flight etc.

Points of Investigation

• Due to the shape of the Frisbee, its aerodynamic benefits are not utilised if the disc is thrown upside down.
• Throwing the Frisbee perfectly horizontally should maximise the distance it will travel.
• If released at a large tilt angle but thrown slightly upwards, the disc should fly in a projectile trajectory, but fly to the left/right of the intended direction.
• Increasing the amount of spin should increase it's stability, decreasing or throwing with no spin will result it a very short travel distance due to the torque pitching the disc over.
• Using both a regular solid disc and a modern hollow disc Frisbee may result in the hollow disc travelling further.

If all else fails, you can always stand in a circle throwing a Frisbee around whilst discussing Physics and answering any questions that the participants have.