What if there were no friction?

Slide a book across a table. It slows down and stops.

What makes it stop? And what if nothing ever did?

Slide a book across a table and it soon stops. Push a box across the floor, let go, and it comes to rest. It can seem as if moving things just naturally run out of go.

But something is actually slowing them: friction, the grip between surfaces that rub together. In this lab, a block slides down a ramp and across the floor. What happens if you take friction away completely?

Experience suggests that moving objects naturally slow down and stop unless something keeps pushing them. Newton's first law, an idea Galileo first developed and Newton clarified, says the opposite: motion continues at constant velocity unless a force changes it.

Friction is the force that hides this from us in everyday life. This lab measures a block's slide down a ramp and along a floor with real friction coefficients, and then with friction set to zero.

A wooden block slides down a steep wooden ramp.

The ramp grips the block a little. On the floor, the grip slows the block until it stops.

In the model, a wooden block sits at the top of a 4-meter wooden ramp tilted at 35 degrees. That is steep enough to beat friction's grip, so the block starts to slide. Friction still pulls back, so it speeds up more gently than it would on ice.

It reaches the bottom at about 5 meters per second. Then, on the flat wooden floor, friction slowly takes its speed away, and it stops after sliding about 4.4 meters.

Wood on wood has μ_s ≈ 0.5 and μ_k ≈ 0.3 (OpenStax). A block stays put while tan θ ≤ μ_s, which here means up to about 26.6°. At 35° it slides with a = g(sin θ − μ_k cos θ) ≈ 3.22 m/s².

After 4 m it reaches v = √(2aL) ≈ 5.07 m/s. On the floor the only horizontal force is kinetic friction, giving a deceleration μ_k·g ≈ 2.94 m/s² and a stopping distance d = v²/(2μ_k·g) ≈ 4.37 m.

Olly makes the ramp and floor perfectly slippery. There is no grip at all.

The block zooms down. On the floor it keeps sliding and never stops!

Now pick the imaginary no-friction surface. With nothing gripping it, the block speeds up faster down the ramp and reaches the bottom at about 6.7 meters per second.

On the floor, nothing slows it at all. It keeps sliding at exactly the same speed, on and on. Try a gentle slope too: with no friction, even a tiny tilt makes the block slide, because there is no grip to hold it still.

With μ = 0, the ramp acceleration is simply g·sin θ ≈ 5.63 m/s², so v_b = √(2gL sin θ) ≈ 6.71 m/s, the same as a frictionless drop through the ramp's 2.3 m height. On the floor no horizontal force acts, so the velocity stays constant and the stopping distance is infinite.

With no static friction either, the block can no longer rest on any slope: tan θ ≤ 0 holds only for a flat surface. At 10° it still slides, reaching about 3.7 m/s.

Things keep moving unless something slows them down.

Friction is a grip that slows them. No grip means nothing slows them.

Isaac Newton's first law says that a moving thing keeps moving at the same speed in a straight line unless a force changes its motion. On the floor, friction is the only force pushing back on the sliding block, so it is the only reason the block stops.

Take friction away and nothing is left to slow it, so it slides forever. Our model ignores air resistance and uses typical friction values; real surfaces vary with wetness, wear and temperature.

Coulomb's friction law treats static friction as adjustable up to μ_s·N and kinetic friction as μ_k·N, independent of speed and contact area. On the floor N = mg, so mass cancels from the deceleration μ_k·g; that is why the block's mass does not change the result.

The model's limits: the coefficients are textbook values for clean material pairs, it ignores air resistance, rolling and tipping, assumes a smooth corner at the foot of the ramp, and offers no carpet because no textbook coefficient is available. A truly frictionless surface does not exist.

Ice is very slippery. It has only a little friction.

Friction helps us too. It lets us walk without slipping.

Ice sliding on ice has very little friction. In our model, a block on an icy ramp slides about 73 meters across the floor before it stops, more than fifteen times farther than on wood.

Friction slows things down, but it also lets us move. If you have ever tried to walk on ice, you know how hard it is without it. Shoes on ice have only about a ninth of the grip of shoes on a wooden floor.

Ice on ice has μ_k ≈ 0.03 (OpenStax), giving the model's block a 73 m floor slide, compared with 4.4 m for wood and none at all for rubber on dry concrete (μ_s ≈ 1.0), which holds the block still at 35°.

Friction both resists motion and makes it possible, as OpenStax puts it: try walking on ice. Its table gives shoes on wood μ_s ≈ 0.9 but shoes on ice only 0.1. Lubrication lowers both coefficients considerably: dry steel on steel has μ_k ≈ 0.3, oiled steel about 0.03.