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Notes from the field ·

What Pushes Back on a Moving Thing? Water Resistance and Friction Experiments

By Ahmed, founder

Every moving thing meets resistance. A swimmer pushes through water, a cyclist pushes through air, a sled slides over snow. Two experiments make that push-back visible.

Experiment 1: which shape sinks fastest?

You need: modelling clay, a tall clear cylinder or jar of water, a stopwatch and a ruler.

  1. Make several clay shapes with the same mass: a ball, a flat disc, a pointed cone, a long rod.
  2. Drop each into the water from the same height, and time how long it takes to reach the bottom. Do 3 trials each.
  3. Compare the averages.

What happens: compact, streamlined shapes (the ball or pointed cone) sink fastest. The flat disc takes longest, because it pushes against more water. Water holds back some shapes less than others.

Experiment 2: dragging a block

You need: a wooden block with a hook, a force meter (or a rubber band and a ruler), and four surfaces: smooth table, sandpaper, carpet and a towel.

  1. Attach the force meter or rubber band to the block.
  2. Pull the block steadily across each surface, and read the force.
  3. Record and compare.

What happens: rougher surfaces need more force to pull the block at a steady speed. That shows friction: a contact force that opposes motion.

What’s happening

  • Friction acts between surfaces in contact, and depends on how rough they are and how hard they press together.
  • Fluid resistance (drag) acts when something moves through water or air, and depends on shape, size and speed.

Engineers use these ideas all the time: smooth, pointed shapes for boats, planes and cars; textured soles for grip.

Streamlining in nature

Dolphins, sharks and penguins have smooth, tapered bodies that cut through water with less resistance. Many birds fold their wings tight when they dive.

Common mix-ups

  • “Friction is always bad.” Without it, you couldn’t walk or hold a pencil.
  • “Only solids have resistance.” Air and water resist motion, too.
  • “Heavier things have more drag.” Shape and size matter more than mass for drag.

The full lesson is What pushes back on a moving thing? in Forces at Work. Younger students begin with Why does a ball stop rolling? Sign up for three lessons free.