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Activity

Air resistance

Summary: 
Show the affect of air resistance by dropping different weights and shapes from a height.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Materials: 
  • objects of similar size but different weights e.g. chopstick, pipe cleaner and straw or square of paper, cereal packet card and wooden tile;
  • a scale to measure g
  • optional: a small book
Procedure: 

Drop pairs of objects from the same height, and record which hits the ground first (do three trials for each).

In a group discussion compare the objects of the same shape (e.g. chopstick, pipe cleaner and straw e.g. square of wood, cardboard and paper of the same size) - which will likely fall fastest to slowest in that order.
Discuss why one falls faster than the other - the heavier ones fall faster. Weigh the items to confirm, comparing those of the same shape (we had chopstick 3g, pipcelaner 1g and straw 1/2g of one shape, then a wood tile 37g, cardboard square 3g and paper square 1/2g). The heavier ones fall faster because the force of gravity is greater on objects with a larger mass. Air resistance will be pushing up with about the same force for those with the same shape. So, on balance, the net force downwards is greater for heavier objects.
Then compare two objects the same weight, but that fall at different rates e.g. chopstick and square of cardboard. Discuss why they fall at different rates - their shapes: the cardboard is flatter, so hits more air coming down. The air pushes up on it and slows it down more than the chopstick with less area for the air to push up on. The paper falls more slowly than any of them, even though it is the same weight as the straw.

As an object falls , the force of gravity on the mass (called weight) and the force of air resistance pushing up (which depends on the shape) are both acting on a falling object. The rate that it falls is determined by how those forces balance out.

Discussion:
In many sports, we design equipment and shape our bodies keep air resistance to a minimum e.g. bike racers bend down to reduce air resistance and bike one behind the other to be sheltered from air resistance e.g. a bobsled is shaped to be as streamlined as possible, so reducing air resistance. e.g. skiiers and speed skaters wear suits that are very smooth, so reducing air resistance.
In some sports e.g. parachuting, we use air resistance to our benefit so that we do not land on the ground too fast.

Addditional surprising activity:
Drop the paper on top of the heavier wood tile (or a small book).
The paper will stay with the book and fall faster than on it's own.
The paper experiences no air resistance because it is on the book, so falls as fast as the book. The book falls faster than paper because it is heavier - the weight of the book overcomes air resistance.

Notes: 

Replace the wooden tile (which is loud on a floor, and breaks after a while), with a square of carpet tile.

Grades taught: 
Gr 2
Teacher: 
Ingrid
Teaching site: 
Kerrisdale Annex Elementary
ProD for Elementary teachers
Activity originally developed and delivered: 

Kerrisdale Annex