ingridscience

Stand the bike at the front of the class.

Ask students to find places on the bike where friction is increased or decreased to make the bike function.
Friction is high (brakes, pedal surfaces) where the rubbing surfaces are rough.
Friction is reduced (wheels, handle bars) with ball bearings and grease (rolling and lubricants).

Look at ball bearings taken apart to see how they work.
Look at pictures of other places that have ball bearings (handle bars, wheels, pedals).

See attachment for stand-alone version of this activity.

For indoor activity rolling materials down ramps, there are two ways of running this activity:
1. the ramp is constant, and different materials are slid down it, including rolling balls and ice cubes.
2. ramps are made of ice and other materials, and the same heavy object slid down each of them.
Test this activity before running with your materials - friction is a complex set of forces and can lead to unexpected results.

1. Comparing blocks, balls and ice cubes on a ramp
Set up students' ramps facing the walls (so the balls will not roll across the classroom every time.) Students slide a block, a ball and an ice cube down their ramps (or use one long ramp, where the speed differences will be great enough, for a demonstration). Compare the speeds, and record results.
It is important that there is no standing water on the ramp, so students should store their ice cube on a towel and dry it off before each use. The towel can also be used to keep the ramp dry.

Discussion:
The ball and the ice went faster than the block (relative speeds of ball and ice will vary depending on the slope and what the board is made of).
The block has the most friction - as the side of the block rubs against the board, the force of friction slows it down.
The marble: when something is round it rolls, and does not rub against the other surface, so the friction is reduced.
The ice cube: the thin water layer between the solid ice and the ramp makes them slip past each other easily. It reduces the friction. The water is called a lubricant.

Why does an adult tell you to be careful if there are marbles or water or oil on the floor? If you tread on them there is so little friction (from the round shape, or the lubricant) that your leg can slip sideways and you fall.

Summarize ways of reducing friction: lubrication (water, oil, melting ice) or rolling shape.

2. Comparing ramps made of ice and other materials
Raise one end of the ice tray and the other test surfaces on blocks, so that they have the same angle of slope.
Release the heavy objects from the same height on the slopes at the same time.
Record the relative speeds. Do several trials for each.
Alternatively, objects can be flicked across a flat ice surfaces, and other surfaces. Make sure to wipe up the standing water with a towel, or other forces complicate the results.

Discussion:
The ice ramp should allow objects to move the fastest, as the very thin layer of water on the surface of the ice flows between the ramp and the object, making a very slippy interface between them and reducing friction.
Smoother ramps should be faster than rough ramps, as the rough ramps catch the object as it moves (friction) and slows it down.
Note:
A complication with this activity is that friction is not the only force involved - gravity is another force pulling the object down the slope - gravity and friction balance to determine the final speed of the object. As the slope of the ramp is kept constant and the weights of the objects are as similar as possible, the force of gravity is constant in each case, and so need not be discussed unless a student brings it up.

3. Sliding on ice with and without friction
Take students to a place with large areas of ice.
Ask students to walk and slide on the ice, and discuss that there is little friction between their shoes and the ice and so they can slip and slide.
Students take turns to wear a Yaktrax on one foot, and compare how this shoe works on the ice to the shoe without.
The Yaktrax metal coils (or the spikes or other features of other ice traction gear) dig into the ice. This makes more friction between the shoe and the ice, so that you can no longer slip and slide on the ice.

Application to sports:
When we wear ice skates, a thin layer of water between the skate blade and the ice reduces the friction between them, so we can keep moving fast. (The rubbing of the skates on the ice helps to melt the surface of the ice.)
In the sport of curling, the sweeping briefly melts the ice, which reduces the friction and allows the puck to move faster in these places.
Skis and snow boards are waxed to promote the optimal thickness of a water layer between the snow and the ski.

Other applications:
Treads on bike tyres, car tyres.
Caterpillar tracks on tanks.

Best run after looking at real wind-blown seeds, with wings and parachutes.
Discuss how seeds are light and wide, to catch the wind and move further away.

Show students the foam ball, and tell them that this is a seed. The other materials can be used to make the wings, parachute or other structures around the seed.
Stick the materials into the foam with tiny pieces of tape, or use the pipe cleaners to secure them.
The goal is to make a seed that stays in the air as long as possible (so make them light and wide).

To test how long a seed stays in the air: hold your seed above your head. Blow out as you drop the seed, to mimic the wind.
Compare your seed with its structures to a plain foam ball, to see if your seed moves further. Make sure to do three or more trials, to fairly compare how the plain seed and your seed with structures.
Even better, place a blowing fan in an area with space, which students can drop the seeds in front of.
Students drop the plain foam ball and their seed model at the same time in front of the fam.

Discussion:
Best design is (1) light so the force of gravity does not pull it down too fast, so it has time to go sideways; (2) wide so that it catches the wind and gets pushed sideways.

Real seeds are not "designed" like this, but different shapes arose by chance. The best ones gave a seed a greater chance of survival, and so their characteristics were passed to the next generation. This is called natural selection. (If one feature worked well, that seed was able to make more plants. The feature (or adaptation) is selected for.)

Video on the recently understood aerodynamics of a dandelion seed and why they stay aloft for so long: https://www.youtube.com/watch?v=N2UbaDV9O9Q

This activity inspired by:
http://www.earthrangers.org/wp-content/uploads/2016/05/NeedForSeed.pdf
https://www.scientificamerican.com/article/gone-with-the-wind-plant-see…
http://www.accessexcellence.org/AE/AEC/AEF/1995/taylor_seeds.html

Download from the Exploratorium site. Their instructions follow.

1.Leave the cap on the bottle, but peel off the label.
2.Count down about three ridges (or about three inches) from the top of the bottle. Using a scissors (or a utility knife, if an adult is doing this), cut along the ridge. Make sure you cut evenly along the edges. Trim off any bumpy ridges.
3.Recycle the bottom of the bottle; you’ll be working with the top half.
4.Take your hole punch and punch a hole as far up from the bottom of this piece as you can go. Put the straw through the hole to test the size. It should be a tight fit. If the hole isn’t large enough for the diameter of the straw, repunch the hole in nearly the same spot, making it slightly larger.
5.Cut the fingers off the glove. The glove should now look like a wide tube.
6.Cut the tube open to form a sheet of pliable material, or a membrane.
7.Stretch the membrane over the opening at the bottom of the bottle, making sure the hole- punched hole on the side isn’t hidden by the excess material.
8.Attach the membrane to the bottle with a rubber band. Wrap the rubber band around the bottle several times, making sure that the membrane’s taut.
9.Twist the top off the bottle.
10. Roll a piece of construction paper into a tube on a flat surface. Make the tube as tight and as straight as possible.
11. Put the rolled tube into the large open hole on the bottle where the cap had been. Let go of the tube when it barely touches the bottom of the membrane. It should fit securely in the hole.
12. Insert the straw in the hole on the side of the bottle and blow into the straw; your water bottle saxophone should play!
13. Tape the paper tube so it stays in place.
14. To make different sounds, add finger holes. To do this, pinch the paper tube slightly and cut out a diamond shape; repeat to make more finger holes.