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Appliance Science: Dishwashers and the physics of water

What do rockets have in common with your dishwasher? Appliance Science looks at the physics that get your dishes clean.

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richardbaguley.jpg
Richard Baguley
Richard Baguley has been writing about technology for over 20 years. He has written for publications such as Wired, Macworld, USA Today, Reviewed.com. Amiga Format and many others.
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Colin_McDonald2.jpg
Colin McDonald
Essentially born with a camera in hand, Colin West McDonald has been passionately creating video all his life. A native of Columbus, Ohio, Colin founded his own production company, Stoker Motion Pictures, and recently wrote and directed his first feature film. Colin handled photography and video production for CNET's Appliance Reviews team.
Richard Baguley
Colin McDonald
3 min read

Colin West McDonald/CNET

We tend to take things in our home for granted, casually accepting the miracles of chemistry, physics and biology that our appliances involve. Take your dishwasher, for instance: a device which cleans all manner of foods from a huge pile of dishes, quickly and efficiently. When you actually stop and think about it, the amount of work this involves is impressive, and the physics of this process are more complex than you might first think.

So how does a dishwasher use the power of water to wash dishes? Let's take a look at the physics of water and how dishwashers use these forces to scrub your dishes.

Although the specifics differ, all dishwashers have the same fundamental design: a sink at the bottom that fills with water, a pump that moves this water and spray arms, sprayers and other devices that squirt this water onto the dishes.

When you look inside any dishwasher, one of the most obvious things you'll see is a wash or spray arm, a rotating bar that sprays water onto the dishes, helping to dislodge the food. The dishwasher pumps water through this, but there is no motor to rotate the arm. Instead, the dishwasher uses the pressure of the water to spin it around.

The water jets on the spray arm are angled, so the water sprays out at an angle, usually about 45 degrees off the vertical. The force of this water pushes the arm, and it rotates. This shows the third of Newton's laws of motion. As the man himself said in Latin in his 1687 bestseller, "Actioni contrariam semper et aequalem esse reactionem." To translate: "For every action there is an equal and opposite reaction."

So, as the water sprays out of the spray arm, it pushes the spray arm back again, sending it spinning. It's the same principle that rockets use: hot gas gets pushed out of the bottom, forcing the rocket up, up and away.

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Colin McDonald/CNET

Samsung came up with a new spin for their 2014 models, with a design called Waterwall. This replaced the bottom spray arm with a motorized crossbar that deflects horizontal water jets up onto the dishes, producing, as the name ijmplies, a wall of water.