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.
When you going down from A to O
The reduced potential energy equals the increased kinetic energy.
mgr1(1–cosθ1)=12mv2v2=2gr1(1–cosθ1)
When you going from O to B.
Reduced kinetic energy equals increased potential energy.
12mv2=mgr2(1–cosθ2)v2=2gr2(1–cosθ2)
Change in angle while moving from A to B
The speed at ‘O’ does not changed,
2gr1(1–cosθ1)=2gr2(1–cosθ2)r1(1–cosθ1)=r2(1–cosθ2) ∴θ2=cos−1(r2–r1(1–cosθ1)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=cos−1(r2–r1(1–cosθ1)r2)=cos−1(2–3(1–cos45˚)2)≈55.9˚
In this case, the angle is increased by about 1.24 times.