Re: Why does a pogo stick need more energy on a carpet than concrete surface

Dear Sir/Madam,
Good day to you.
I would like to know why a pogo stick needs more energy to perform (jump to a certain height) on grass field as compared to a concrete pavement? Does it have something to do with the elastic spring force in the spring or is it simply due to the type of collision of the base of the pogo stick with the surface of the ground?

Hello Leow,

The short answer to your question is that the softer surfaces (carpet or grass) absorb more of the energy of the pogo stick than a hard, concrete surface would.

A pogo stick is really an energy storage device. As you 'jump' on a pogo stick, your body gains height and increases the 'potential energy' of the system (of you and the stick). Now, as you come down, your weight causes the pogo stick to compress the spring. At the lowest point, the spring has "captured" most of the energy of the jump. The compressed spring now tries to expand, lifting you and the stick up to some height. A lot of the energy is 'returned' to the system by giving you the boost.

In an ideal situation, you would be able to bounce up and down forever without putting in any more energy. But in reality, you need to work hard to keep the jump going. This is because of friction and other forces that absorb your jumping energy.

A large amount of this energy may be absorbed by the surface that the stick lands on. A hard surface, like concrete, will not absorb the energy. However, a soft surface, like grass will deform and compress when the pogo stick lands on it.

Here is a question: I said that both the spring and the soft ground will compress due to your weight on the pogo stick. Why does the spring return the energy, but the ground does not? There are a lot of different reasons, but mainly, when the ground compresses, it does not recover, or “push back” on the pogo stick. It may slowly recover some amount, but not before you are long gone from the spot. The energy actually goes into heating the ground (from the friction of rubbing particles together as the spot gets compressed).

The spring works differently though. When a spring compresses, there is very little internal friction to “use up” the energy. Instead, this energy is stored by the stretching and compressing of the spring metal, and returned in the form of a force, or push as the spring stretches back.

The reason why a pogo stick is so sensitive to the type of surface on which it is bouncing is because the bottom of the stick is usually a small circular area. Your entire weight coming down (which might be double your ‘standing’ weight) will be concentrated on this small area. This is what creates the high force (or “stress”) that can push down and compress the grass. The same thing happens to a woman in high heels- the tiny little back heel must bare most of her weight, and that causes the heel to sink deep into the grass!

You can see that the concentrated stress due to the small bottom of the pogo stick will exaggerate the difference in performance of the stick between the hard and soft surfaces.

Happy Bouncing!

Jay Shapiro