Why do your shoes grip the floor but slip on ice?
Explored it? Here's the recap
Why do your shoes grip the floor but slip on ice?
The short answer
How slippery something is depends on the pair of surfaces touching, not on one object alone. Rough, rubbery surfaces grip each other hard and resist sliding; smooth, icy surfaces barely grip, so even a gentle slope or a small push sends them sliding. This grip between surfaces is called friction.
How it works
Two things fight whenever something might slide. Gravity pulls the object down, and on a slope part of that pull points downhill and tries to start a slide. Friction between the two touching surfaces pushes back and tries to hold it still. The object stays put as long as friction can match the downhill pull. The grippier the pair of surfaces, the bigger that downhill pull has to get (a steeper slope) before the object finally lets go and slides.
What people get wrong
A common belief is that whether something slides depends only on how steep the slope is, or on how heavy the object is. Neither is the whole story. Two ramps at the exact same angle behave differently if they are made of different stuff: an icy ramp lets a block slide while a rubbery ramp at the same angle holds it still. And weight mostly cancels out, because a heavier object both pulls harder downhill and grips harder, so it is the surface pair that decides.
The catch
Grip is both a helper and a tax. High grip keeps you safe so you do not slip and can stop and steer, but it costs energy: heavy boxes are hard to push, tires wear out, and rubbing surfaces heat up. Low grip glides smoothly and for free, which is great for ice skates, oiled gears, and water slides, but it is dangerous exactly when you need to grab the ground, because you cannot stop or steer. Neither extreme wins everything.
Questions kids ask
Why is ice so slippery?
Ice has a very low grip with most surfaces, so there is almost no friction to fight gravity. A common reason is a thin slick layer at the surface of the ice, and because the grip is so small, even a gentle slope or a light push is enough to start something sliding.
Does a heavier object slide more easily?
Usually not much. A heavier object is pulled harder downhill, but it also presses down harder and so grips harder, and these two effects mostly cancel. What decides whether it slides is the slope and the pair of surfaces touching, far more than the weight.
Why does water or oil make floors slippery?
A layer of water or oil keeps the two solid surfaces from touching directly and grabbing each other. The smooth liquid film slides easily, so the grip between the surfaces drops and things slip much sooner than on a dry floor.
If grip is helpful, why do we make some things slippery on purpose?
Because low grip means smooth, easy, energy-saving motion. Oiled gears, sled runners, and water slides are made slippery so they glide instead of grabbing. The trade is that you give up the ability to stop and steer, so we choose high or low grip to fit the job.
For grown-ups
A block on an incline begins to slide when the gravity component along the slope exceeds the maximum static friction. That threshold is the angle of repose, where tan(theta) equals the coefficient of static friction (mu_s). Importantly, mu_s is a property of the two materials together, not of the object's weight, which cancels from the inequality. So a high-friction pair like rubber on dry concrete (mu_s near 1) needs a slope close to 45 degrees before it slips, while a low-friction pair like rubber on ice (mu_s near 0.1) lets go on a slope of only a few degrees.