A river's water speed varies with depth and distance from the banks; can an object floating freely on the surface ever travel downstream faster than the river's fastest-flowing water?
Problems by archive
Post a problemIn the classic Millikan oil-drop experiment, tiny charged droplets are also jostled by random molecular collisions (Brownian motion): how much does this fundamentally limit the precision of measuring the electron's charge this way?
When a whip is snapped, why does its tip produce a sharp crack loud enough to sound like a gunshot, and what makes the tip move fast enough to cause it?
If you hold a stretched Slinky vertically and let go of the top, why does the bottom of the Slinky stay completely still, floating in place, until the collapsing coils above reach it?
Why does a properly thrown boomerang travel out in a curve and return to the person who threw it, instead of flying in a straight line like any other thrown object?
Why does distant thunder sound like a long rolling rumble rather than a single sharp bang, even though the lightning stroke that causes it happens almost instantaneously?
Why does liquid poured slowly from a teapot or measuring cup sometimes cling to and run down the outside of the spout instead of falling cleanly away as a free-falling stream (the 'teapot effect')?
How can water striders and similar insects stand and skate across the surface of a pond without breaking through, and how do they push themselves forward without solid ground to push against?
How can a cat dropped upside-down with zero initial spin still rotate itself in mid-air to land on its feet, given that it starts with zero angular momentum and nothing to push against?
When a spinning coin or disk settles flat on a table, why does the frequency of its wobbling/rattling sound rise dramatically just before it comes to rest, instead of dying away smoothly?
Why do you sometimes see one or more faint, closely-spaced extra bands of color (supernumerary arcs) just inside the primary rainbow, in addition to the usual bow?
When you bend a dry piece of spaghetti until it snaps, why does it almost always shatter into three or more pieces instead of cleanly breaking in half?
When skipping a flat stone across a pond, what throwing angle between the stone and the water surface gives the largest number of bounces, and why does that particular angle work best?
Why does a pile of dry sand (or any granular material) poured from a point always come to rest at roughly the same characteristic slope, the 'angle of repose,' regardless of how much sand is poured?
Why does a Superball thrown with backspin straight down sometimes bounce back toward the thrower instead of continuing forward, and why does its spin direction reverse after the bounce?
Is it true that the Earth's rotation (the Coriolis effect) makes bathtubs and sinks drain counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, and if so, why don't ordinary home bathtubs reliably show this?
What physically produces the sharp popping sound when someone cracks a knuckle by pulling or bending the finger joint, and why can't you crack the same knuckle again right away?
Why does a brief flash of green light sometimes appear at the very top edge of the sun in the instant it disappears below a clear, distant horizon at sunset (or reappears at sunrise)?
Why does a ring of light (a halo), or a pair of bright spots on either side of the sun (sun dogs), sometimes appear around the sun or moon when thin high clouds are present?
If you strike a free rigid rod with a sharp blow at one point, why does a particular other point on the rod feel almost no jolt at all, while striking closer to that point makes your hand sting badly, as if the rod 'chooses' where to react?