The principle of riding a swing




Kids have the know-how, but they can’t tell the principal.

“How can you go so high?”
Some principles are difficult to describe in words in the world, even if everyone knows.
I think one of them is how to ride a swing.
Can you explain how to ride a swing with a physics equation?

Standing or sitting at the midpoint of a moving swing changes the length of the pendulum.
In other words, a swing can be thought of as a ‘pendulum that can change its length‘.
Consider the pendulum that moves, as shown in the picture below.

The principle of riding a swing

When you going down from A to O

The reduced potential energy equals the increased kinetic energy.
mgr1(1cosθ1)=12mv2v2=2gr1(1cosθ1)

When you going from O to B.

Reduced kinetic energy equals increased potential energy.
12mv2=mgr2(1cosθ2)v2=2gr2(1cosθ2)

Change in angle while moving from A to B

The speed at ‘O’ does not changed,
2gr1(1cosθ1)=2gr2(1cosθ2)r1(1cosθ1)=r2(1cosθ2) θ2=cos1(r2r1(1cosθ1)r2)

How to ride a swing well?

When going down, we need to increase the length of r. (= sit down.)
Conversely, when you go up, you have to stand up.
For example, let’s calculate the increase in angle by substituting the following condition.

  • r1=3m
  • r2=2m
  • θ1=45˚ (Angle of starting point)

θ2=cos1(r2r1(1cosθ1)r2)=cos1(23(1cos45˚)2)55.9˚

In this case, the angle is increased by about 1.24 times.

Loading...