Why does sliding a box get easier the instant it starts moving?
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Why does sliding a box get easier the instant it starts moving?
The short answer
Starting a box moving is harder than keeping it sliding because a still object grips the floor more strongly than a moving one. The stronger grip on a still object is called static friction, and the weaker grip on a sliding object is called kinetic friction. You have to out-pull the strong static grip to break the box free, and then a smaller pull is enough to keep it gliding.
How it works
Friction is the floor resisting an object dragged across it. For the same two surfaces, the most the floor can grip a still object (maximum static friction) is usually larger than the steady grip on a moving object (kinetic friction). While the box is still, the floor pushes back exactly as hard as you pull, so it stays put until your pull beats the static limit. The instant it breaks free, the resistance drops to the smaller kinetic amount, so the box lurches and then needs less force to keep sliding.
What people get wrong
Many people assume friction is one fixed amount, so pushing feels equally hard the whole time. In reality the grip changes the moment the object moves: the static grip on a still object is generally stronger than the kinetic grip on a moving one. That is why things feel glued down, pop loose, and then slide more easily, and it is why you can ease off your push a little once something is moving and it keeps going.
The catch
The strong static grip is useful because it holds things in place, so a parked car stays put and a book rests on a tilted desk without sliding off. The cost is that the first shove is the hardest part of moving anything. The weaker kinetic grip saves effort once something is moving, but it also means a sliding object grips the floor less, so a skidding car or a slipping shoe is harder to control than one that still has its static grip.
Questions kids ask
What are static friction and kinetic friction?
Static friction is the floor's grip on an object that is not moving yet, and it can grow to match your push up to a maximum. Kinetic friction is the floor's grip on an object that is already sliding. For the same surfaces, the maximum static grip is usually stronger than the kinetic grip, which is why starting takes more force than keeping going.
Why does a heavy box suddenly lurch when it finally moves?
While the box is still you build your pull up to beat the strong static grip. The instant it breaks free, the grip drops to the weaker kinetic amount, so your pull is now bigger than the resistance and the leftover force jerks the box forward before you can ease off.
If the grip is weaker while sliding, why does a moving thing still stop?
Kinetic friction is weaker than static friction, but it is not zero, so it keeps pulling backward on a sliding object and gradually slows it down. If you stop pushing, that drag wins and the object stops, and then the stronger static grip holds it still again.
Does this only happen because the box is heavy?
No. Weight makes both grips larger, but the key fact is that the static grip is stronger than the kinetic grip whatever the weight. Even a light object is usually a bit harder to start than to keep sliding for the same reason.
For grown-ups
For a given pair of surfaces the coefficient of static friction is typically greater than the coefficient of kinetic friction, so the force to initiate sliding (up to the static maximum, μ_s·N) exceeds the force to sustain it (μ_k·N). When motion begins the resisting force drops, so for a steady applied force the net force becomes positive and the object accelerates with a lurch. This static-over-kinetic gap drives stick-slip phenomena such as squeaky hinges, bowed violin strings, and even some earthquake mechanics, and it is why anti-lock braking keeps tires near the peak static grip rather than letting them fully skid into the lower kinetic regime.