Is it harder to START a box sliding or to KEEP it sliding?
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What else makes you wonder?
Does every surface grip a still object harder than a sliding one, or are some floors the same either way?
Use the story’s model. Change one thing, predict first, then imagine a matched test.
Where else do you feel something pop loose and then go easy — a stuck jar lid, a squeaky door, a zipper?
Use the story’s model. Change one thing, predict first, then imagine a matched test.
If you could make the static grip and the kinetic grip exactly equal, would anything ever lurch?
Use the story’s model. Change one thing, predict first, then imagine a matched test.
After you watchIs it harder to START a box sliding or to KEEP it sliding?
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.
Try this next
- What if the floor were super slippery, like ice? Imagine the experiment on ice and predict first: does the box still pop loose with a lurch, or barely grip at all? Then push a real object across a smooth floor and a rough rug and feel which one snaps free harder.
- What if you push slowly instead of yanking hard? Predict whether a slow, gentle build-up still ends in a sudden lurch. Then lean on a heavy chair and add force little by little, and watch for the exact moment the static grip lets go.
- What if the box were twice as heavy? Predict before you test: does extra weight make the starting pull and the sliding pull both bigger, or only one of them? Then drag a backpack empty, then full, and compare the first heave to the easy glide.
Now you — bend it
- What if Thought experiment (no slider for this): the lab has one knob — the amber pull. There's no ramp to tilt. So picture it instead: drag the SAME box up a slope you slowly steepen until it starts sliding on its own, with no pull at all. What does 'tilting steeper' stand in for here?On a ramp, gravity does the pulling for you — a steeper tilt is just a bigger sideways pull, exactly like cranking the amber slider up. The static grip still maxes out at the same break-free amount. So slide the real pull up to the break-free line and watch it pop: that pop is the same moment the box would slip on a just-steep-enough ramp.
- What if The lab lets you ease your pull DOWN after the box breaks free and it keeps gliding. How far down can you ease it — to 40? to 35? to 34? — before the box stalls and re-sticks?Once moving, the only thing it has to beat is the gliding drag (35), not the stuck grip (60). Predict the exact pull where it slows, stops, and the strong stuck grip grabs it again.
- What if Thought experiment (no slider for this): the break-free value (60) and the gliding drag (35) are baked in — you can't drag them closer together. But you CAN steer the lurch with the one knob you have. Picture shrinking that gap toward zero: what would happen to the pop when the box breaks free?The lurch is leftover force = your pull minus the drag the instant it moves. You can't move the 60 or the 35, but you can shrink the leftover yourself: break the box free with a pull just barely over 60 instead of a big heave. Try it on the amber slider — a smaller overshoot makes a gentler pop, and a gap of zero would mean no pop at all.
Can you prove it?The size of the forward lurch when the box breaks free is set by how much your break-free pull overshoots the gliding drag — not by how heavy the box is. — Pick a break-free pull just barely over 60 and note how little the box jumps; then crank to a hard heave near 100 and break it free, and note the bigger jump. The drag stayed 35 both times, so the only thing that changed is the leftover force (pull minus 35) at the instant of release — a bigger overshoot means a bigger lurch. Reason it through: leftover force divided by mass is the acceleration, so a heavier box would lurch less for the same overshoot, but the overshoot itself, not the weight, is what you controlled.
Design your own test:Before you slide it, predict: does breaking free with a barely-over-60 pull versus a full heave change WHETHER it starts, or only how violently it lurches forward once it does?
Explain it to a 6-year-old: When something is sitting still, the floor hugs it tight — you have to win a tug-of-war to get it going, and then it lets go and slides easy.
The whole story
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.
Talk about it
- Guess first: do you think it takes more force to START the box moving or to KEEP it moving? Why?
- What's something at home that feels glued down, then suddenly pops loose when you pull hard enough?
- Once something is sliding, why can you ease off your push a little and it still keeps going?
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.