← Back to course
Sports Physics 6-8 / Week 05 / Game Day
6/6
Week 05 · Friction and Air

Game Day

Game Day: The filter drop
// The forces that slow things down
⏱ about 20 min

Game Day: The Filter Drop

Saturday morning, Rocket sets a stack of paper coffee filters on the patio table and a timer beside them.

"One filter, then two nested together, then three," he says. "Same shape every time. Only the weight changes."

Raven holds a single filter at shoulder height, open side up. "Predict first. Heavier falls faster, or the same?"

"Faster," Rocket says. "More weight needs more drag to balance it, and more drag needs more speed."

The filter drifts down like a tiny parachute. Raven calls the time. Nova hovers level with it on the way down.

"What do you notice?" Raven asks. "It stopped speeding up almost right away."

Nova's light glows. "Then you just watched terminal velocity."

  1. Friction is a force that opposes motion between surfaces that touch, always pointing against the motion.
  2. Static friction holds a resting object still and is usually greater than kinetic friction on a sliding one.
  3. Friction depends on both materials. Rubber on dry concrete grips, and steel on ice slides.
  4. Drag is the force of air or water on a moving object. It grows with speed and with the area facing the air.
  5. A falling object speeds up until drag equals the pull of gravity. Then it falls at a steady terminal velocity.
◇ FAMILY FILTER DROP
Gather five paper coffee filters, a timer and a tape measure. Pick a drop spot about two metres high that you can reach standing.
Mark the drop height with tape on a wall or door frame. Each person predicts: will two nested filters fall faster than one?
Drop one filter, open side up, and time its fall. Repeat three times and write the times.
Nest two filters together and repeat. Then three, four and five. The shape stays the same, only the weight changes.
Talk it over. Did more filters fall faster? Why must a heavier object fall faster to balance its weight with drag?
For a grown-up
The drop height is reached standing on the floor. No one climbs a chair or stair rail to drop anything.
A doorway with no door swing, or a patio, keeps the filters out of the way of younger children.
This activity follows a take-home experiment in an OpenStax college physics text, which you can read at openstax.org.
Safety first
Drop from standing height only. Never climb to get higher.
Keep the drop lane clear of people and pets while a filter is falling.
Pick up every filter when you finish.
THINK IT THROUGH
  • Read the question.
  • Tap your answer.
Five nested filters have the same shape as one filter. Why do they fall faster?
Why do you drop each stack three times?
Game Day checkTrue or false?
A falling filter reaches a steady speed when drag equals the pull of gravity.?
Nesting filters changes their shape a lot.?
Friction and drag both push against motion.?
WHY THIS EXERCISEBoth of this week's forces oppose motion. One comes from a surface and one from the air.
Draw a falling filter with an arrow down for gravity and an arrow up for drag. Draw a stack of three with a longer gravity arrow.
← Friday