Push a heavy box across the floor and you feel it: nothing happens, nothing happens, you push harder, and then all at once it gives way and slides. That sudden “give” is the difference between static and kinetic friction. Raise the pull in the lab above and watch the amber friction arrow climb to match your push, right up until the block breaks free and slides.
Why the box suddenly gives way
Friction is the force between two surfaces in contact that resists sliding. It comes in two kinds, and the whole story is that they are not equal:
- Static friction acts while the object is still. It is a reaction force: it grows to match whatever you apply, so the object stays put, up to a maximum.
- Kinetic friction acts while the object is sliding. It is a single, constant value, and it is smaller than the peak static friction.
The box “gives way” at the exact moment your push exceeds the maximum static friction. The resistance suddenly drops to the lower kinetic value, so the leftover force accelerates the box and it lurches forward.
What is static friction?
Static friction is the self-adjusting force that keeps a stationary object from sliding. Push a resting block gently and it does not move: static friction pushes back with exactly the same size, so the net force is zero. Push a little harder and static friction rises to match, still cancelling you. It is the only common force that changes its own size to keep something in place.
In the lab, while the block is STUCK, the amber friction arrow is always the same length as your blue applied arrow. That equal-and-opposite pair is why nothing moves.
Maximum static friction: the breaking point
Static friction cannot grow forever. It has a ceiling, the maximum static friction:
f_s(max) = μ_s × N
Here μ_s is the coefficient of static friction (a property of the two surfaces) and N is the normal force pressing the surfaces together. On flat ground the normal force equals the object’s weight, N = m × g, so a heavier block has a larger f_s(max) and takes more force to budge.
The block only starts to move when your applied force exceeds f_s(max). On the graph, that is the peak at the top of the diagonal, the highest point friction ever reaches.
Kinetic friction: the force while it slides
Once the block is sliding, friction switches to kinetic friction:
f_k = μ_k × N
Kinetic friction is constant: it does not keep rising as you push harder, and it does not depend on how fast the block moves. In the lab, once the block is SLIDING, the amber arrow holds a fixed, shorter length no matter how far you push the pull up, while the extra applied force becomes net force and speeds the block along.
Static vs kinetic: why starting is harder than keeping it moving
The key fact is that the kinetic coefficient is always smaller than the static one:
μ_k < μ_s so f_k < f_s(max)
At rest, the two surfaces settle into their microscopic bumps and interlock, which takes more force to break. Once sliding, the surfaces skim over the tops of those bumps and cannot re-settle, so the resistance is lower. That gap is exactly why a stuck box lurches the instant it breaks free: the resisting force drops from f_s(max) down to f_k in an instant, leaving a sudden net force.
The coefficient of friction: what it depends on
The coefficients μ_s and μ_k are dimensionless numbers (a ratio of two forces, so no units) that describe a pair of surfaces, not one object. Rubber on dry concrete is high (around 0.8); steel on ice is very low. They are found by experiment, not from a formula.
Two things they do not depend on:
- Not the contact area. A brick resting on its small end has the same friction as the same brick on its large face, because μ and the normal force are unchanged.
- Not the object’s size on its own. A heavier object has more friction only because it presses down harder (bigger N), which is why the Mass slider changes f_s(max) and f_k while μ_s and μ_k stay put.
Worked example: how hard to move the crate?
A 6 kg crate sits on a floor with μ_s = 0.6 and μ_k = 0.45.
- Normal force: N = m × g = 6 × 9.8 = 58.8 N.
- Force to break it free: f_s(max) = μ_s × N = 0.6 × 58.8 ≈ 35.3 N. You must apply more than this to start it moving.
- Force to keep it sliding: f_k = μ_k × N = 0.45 × 58.8 ≈ 26.5 N. Once moving, any push above this accelerates it.
Load the “Heavy crate” preset and raise the pull past 35 N to see it break free, then notice how much less force it takes to keep it going.
Common misconceptions about friction
- “Friction is one fixed force for a given block.” No. Static friction is variable and self-adjusting up to a maximum; only kinetic friction is a single fixed value while sliding.
- “It takes the same force to keep something sliding as to start it.” No, it takes more to start it (beat f_s(max)) than to keep it going (beat the smaller f_k). That gap is why things lurch.
- “Heavier or bigger objects have a bigger coefficient of friction.” No. μ depends only on the pair of surfaces. Weight changes the friction force through N, not the coefficient.
- “Push hard enough and friction disappears.” No. Kinetic friction keeps acting the whole time it slides; it just becomes a constant that is smaller than the static peak.
- “The coefficient of friction has units of newtons.” No. μ is a pure ratio of the friction force to the normal force, so it has no units.