32 May/June 2022

Physics

8

// B Y C O U R T N E Y L I N D E R //

PHOTOGRAPHY BY LAKOTA GAMBILL

D

ESPITE WHAT THAT LONG-FORGOTTEN

stain on your white shirt might have

you believe, humans are pretty good at

walking with a cup of coffee and avoid-

ing spills, even if our success rate isn’t

quite 100 percent. Every time you man-

age to get your cup of joe from one point

to another spill-free, you’re intuitively completing

a little-understood feat of physics: manipulating a

complex object such as liquid.

That’s according to a group of researchers at

Arizona State University (ASU) who have been

modeling the coffee-carrying phenomenon in an

attempt to imbue robots with the same finesse.

In a world of increasing automation, machines

are expected to perform more dexterous motions,

explains Brent Wallace, a Ph.D. student at ASU’s

School of Electrical, Computer, and Energy Engi-

neering who was involved in the work.

“But even for simple tasks, like carrying a cup of

water or a cup of coffee, the robot struggles. Every

day, you and I make a cup of coffee, and 99 out of

100 days, we don’t spill it on ourselves,” Wallace

says. “So how do we get leverage on tackling those

kinds of problems? Well, let’s study how humans

behave in those situations.”

Building on prior work at Northeastern Uni-

versity, which found that humans have two main

approaches in manipulating a complex object like

a f luid, the ASU team simulated those responses,

lasering in on the transition phase between the

two to understand why humans exhibit a binary

response—and to see how robots could learn to do

the same in the future. The findings were published

in the journal Physical Review Applied in late 2021.

Approach No. 1 is called a low-frequency strat-

egy, and it involves human participants exerting a

steady, slow-changing back-and-forth force on the

coffee mug. As a result, if you swing your mug to the

left, the java inside follows suit, like a pendulum.

This is called in-phase synchronization. Alter-

nately, approach No. 2 is a high-frequency strategy

wherein people exert a jerky, rapidly changing force

on the mug. As a result of this approach, if you

swing your mug to the left, the java inside moves to

the right side of the cup. This is known as antiphase

synchronization.

Since both strategies worked, albeit on opposite

ends of the spectrum, Wallace assumed that some

participants in the Northeastern study switched

How the

Complicated

Physics Inside

Your Coffee Mug

Could Lead to

Better Robots

The natural human

gait walking along

a straight path (x)

affects the trajectory

of the coffee (z) in

the cup based on the

velocity of the gait.