What Makes Surfaces Slippery? A Look at the Physics

For the most part, human beings intuitively know how to keep our balance. We tread cautiously on ice, oil slicks and wet floors. Usually, we only slip when we lose awareness of our surroundings.
But what’s the basic physics at work? What causes certain surfaces to be “slippery” to us?

What Makes Surfaces Slippery?

We’ll save the heavy math for another time. Conceptually speaking, there’s just one big idea to understand:

Friction

Scientific American Describes Friction as “the force that opposes motion between two surfaces that are touching one another.”
At the molecular level, friction happens when the molecules of one object resist being moved by the action of another object, making it more difficult for the objects to rub against each other.
Friction is the property that enables us to walk because of the resistance between our foot (shoe) and the ground. Friction also allows the rubber tires on our cars and bicycles to grip the road.
Without friction, we’d literally be slipping and sliding everywhere we went. Worse, we’d never stop because there would be no resistance to bring us to a halt.
Every day, nearly every action you take is guided by friction, and yet, you don’t notice it until you need it—and it’s not there. Hence, the danger of slip and fall accidents. When we expect friction but don’t find it, we’re especially vulnerable.

Measuring Friction

Experts use a metric known as the coefficient of friction (COF) to measure friction in a given location.
The COF represents the exact level of resistance your foot would encounter if it made contact with the floor. Low numbers translate to slippery surfaces. For example, the COF of a recently swept concrete sidewalk is 0.8. If, however, that sidewalk is covered in a layer of ice, the COF plummets to just 0.3. Linoleum floors fall somewhere in between.
Experts set 0.5 as a limit of safety; any surface with a COF of under 0.5 is unsafe for pedestrians.

How Friction Affects Balance

The less friction two objects have between each other, the easier it is for them to move (or slip) against each other.
Thus, the less friction our shoes have with the floor, ground or sidewalk, the more likely we are to slip when we walk. Likewise, if something like water or oil gets between our shoes and the sidewalk, those molecules reduce the resistance between our feet and the ground, making the surface slippery.
Examples to Illustrate

• Oil or grease: Oil molecules tend to have more friction with each other than with other objects around them (a property called viscosity). This dynamic causes oil molecules to move easily with almost anything that touches them; this is why oil on any surface makes the surface more slippery.
• Ice: As LiveScience explains, it’s not the smoothness of ice that makes it slippery, but the thin layer of water that forms on the surface as the ice melts. Liquid water has less friction than the solid ice beneath it, making icy surfaces naturally slippery.
• Water (especially on smooth surfaces): Puddles seem like a recipe for slipping, but a lot depends on the surface beneath the water. No matter how wet, we can typically avoid slipping if the type of surface is rough enough to overcome the reduced friction resulting from water accumulation.
Hence, your ability to safely walk on wet sidewalks, as long as they don’t freeze. However, on smooth floors such as tile or marble, water can negate any natural friction the floor provides. Soapy water can enhance this effect because of the natural oils in the soap. Incidentally, when rainwater makes roads slippery, it’s usually due to the oils on the surface of the road—not the water itself.
• Wax and floor finish: A freshly-waxed surfaced looks beautiful, but it can increase the risk of slipping. This concept is not as simple as it seems, however. In select cases, wax can actually control slip. If improperly maintained, however, the potential for slipping may increase, particularly if the surface becomes wet. Likewise, other floor treatments can significantly increase slip factor. This is often true of lubricating sprays, especially if applied in excess.
• Cleaning agents: Technically, cleaning solutions should cut through grease to minimize slip. In reality, however, solvents are more complicated than they appear. High-alkaline solutions, for example, may reduce friction by leaving behind a soapy residue if not properly rinsed. Experts from the National Floor Safety Institute recommend non-alkaline cleaners that emulsify grease and rinse freely.

Ultimately, it is the premise owner’s responsibility to understand the role friction plays on their property, and how the concept can best be applied to reduce risk.
While we instinctively can keep our balance on surfaces with proper friction, we can’t be expected to remain upright if we encounter unexpected puddles or improper floor finishes. In such cases, negligence means failing to take measures that would maximize friction and keep us safe.

What do you do if you slip and fall and are injured?

Slip and fall cases can be tough. Many times there is fault with multiple parties. Reach out to the personal injury attorneys at the Smiley Law Firm to see if your slip and fall injury is one that can be compensated by the at-fault party.
Did you or a loved one suffer an injury in a slip and fall accident? You’re not alone. The compassionate team at Smiley Law Firm is happy to help.

Call 504-434-7700 today to schedule a case consultation.

Learn More

• Slip and fall / Trip and fall attorneys
• Five things you need to do if you slip and fall and are injured
• Trip and fall case action requirements