ingridscience

Students add a drop of methylene blue (or iodine to a slide).
Students scrape cheek cells from the inside of their cheeks with a toothpick, then wriggle them off in the drop on the glass slide.
Drop on a cover slip.
Look at the cheek cells under low power to start to find the cells.
Then increase the power, keeping a good-looking cell in the centre when moving up to the highest power.
Look for the nucleus in the centre of the cell.
With methylene blue, other cell organelles are also visible, and sometimes bacteria (dark blue spots outside the cheek cells).

Add some dried rice to the bottle and cap.
Shake.

Change the objects in the bottle to see how they change the sound.
With older students, discuss why the sound might change.

Show students how to secure the string to a chosen object so that it can hang freely, feeding the free ends of the string through the loop to hold it tightly if necessary.

Then wrap the free end of the string three turns around each index finger and push the finger against the flap of each ear (the tragus).
he ears are blocked so do not hear any sounds through the air, and the sounds coming up the string will be heard through the bones of the finger and the jaw.
(Practice with the string only at the carpet to check that students are pushing against the right part of their ear.)

Lean over and swing the object so that it bangs against a table or chair, which starts it vibrating. You only hear sounds coming up the string, through your finger and then through your jaw bone into your ear... and not through the air.

Students can anecdotally share their observations at the end, or use a worksheet (see attached below).

Questions to prompt students with experimentation:
What was the difference between objects made of different materials (e.g. plastic and metal)?
Try making the ringing sound, then touching the object or string to stop the ring.

After students have successfully heard some sounds through the string, ask them to compare with the sounds through the air. Swing and bang an object, but do not cover your ears.

What is happening?
As you bang the object it vibrates. The vibrations travel up the string to the bones in your ear, where you hear them as a sound that has passed only through solids. Different materials transmit vibrations differently and so the sound changes.

Objects that have a molecular structure that can vibrate and resonate more (i.e. metal) make a longer, ringing sound than plastic or metal.
Sounds through the string sound deeper and more resonant than sounds through the air, because the solids can transmit the lower frequencies (lower notes) than air.

To hear sound through another solid, lay your ear flat on a desk. Knock on the other end of the desk with your knuckles.

Animals hearing through different materials
Just as the sound is quite different through the string compared to through the air, animals that hear sounds through water or the ground hear sounds quite differently from us.

Sound travels faster and further through liquids (e.g. the ocean) and solids (e.g. the ground), than the air. But more energy is needed to transmit sounds through liquids and solids, so very quiet sounds do not transmit well.

Animals that hear sounds through the water:
Fish and marine mammals such as whales, dolphins. Lower frequencies travel well under water, and a long way.

Animals that hear sounds through the ground:
Snakes lack external ears and internal eardrums. They hear through their jaws (two jaws - can hear in stereo), the sound going directly to the cochlea. They bury themselves in sand to make their hearing more precise.
Elephants detect these seismic waves with the skin of their feet and trunk. Communicate danger from miles away.
Blind mole rat knocks its head against the walls of its tunnels to signal to its neighbors.
Termites in danger will bang their heads on the ground, which spreads like a chain reaction through the colony.
Kangaroo rats drum their foot on the ground with danger.
Frogs and spiders also hear through the ground.
https://blog.nationalgeographic.org/2013/11/14/good-vibrations-7-animal…

This models how sound travels by moving vibrations.

Pairs of students stretch the slinky (or space phone) between them.
Flick the slinky forwards to make a wave.
See how the vibrations move along as a wave, as one part of the slinky pushes the next part.
This is how sound waves move: molecules bump the next molecules along, forming a wave of vibrations.
These are called longitudinal waves, and is how sound moves through solids, liquids and gases.

With the space phone the added cones mean that the sound of the coils vibrating are amplified, to make a strange, spacey sound.

Make a transverse wave by flicking the slinky sideways. Sound waves can only move like this through solids.

Look for the wave coming back along the slinky, which models an echo. An echo in a big room, or across a valley, is the sound waves bouncing back.

When these vibrating molecules reach our ear, they make our ear drum vibrate which transfers the energy vibrations to our inner ear where they stimulate neurons. The nerve fires and sends a signal to our brain, that we perceive as sound.

Activity video on YouTube.

Stick, clay and styrofoam sculpture
For older students, hand out the materials and challenge them to balance the toothpick on their finger. Show them that it is impossible with just a toothpick, but tell them that they have other materials to add to the toothpick to help them. Give them time to try. If necessary, give them clues that there needs to be most of the mass of their sculpture under the point of the toothpick, and then that the skewers can help to add the mass of the clay low down.
Encourage students to borrow each other's ideas, and that no idea is a bad idea. Encourage students to experiment with various configurations of the materials and see how their new models balance, or not (e.g. what happens if you remove one skewer, what happens if you change the position of the skewers, does it work without the clay at the end of the skewers?...) Tell them to try their sculptures on other things in the classroom.
For younger students show them how to make a simple sculpture: push the toothpick into the styrofoam, then add a skewer with a clay ball, as shown in the second photo. Ask them to try, then modify the sculpture if they like. Encourage students to balance their sculpture on other things in the classroom.

If pushed to one side, the sculpture can right itself.
The sculpture can be rocked back and forth.

Fruit sculpture
Quick to set up.
Try and balance the small end of a grape on your finger. It's pretty much impossible.
Add some mass below the grape, using forks, or other heavy objects, stuck into the grape at an angle.
If the forks are heavy enough an apple will work too.
A banana with a curve can be balanced on your finger if it curves downwards, as the curve puts some mass below the balance point, making it more stable.

Explanation in terms of force and centre of mass
The sculpture balances because the average position of all the mass ("the centre of mass") is underneath the balance point (the point of the toothpick). When gravity exerts a force on this centre of mass, it is pulling underneath the point of the toothpick (not above or to one side of it), therefore the toothpick is stable.
If the sculpture is tipped to one side, gravity pulls the weights back down to the lowest position, bringing the centre of mass below the toothpick once again. As long as the centre of gravity is below the balance point, any sculpture is stable.

Applications of this concept:
Engineers try to design a sports car's centre of gravity as low as possible to make the car handle better.
When high jumpers perform a "Fosbury Flop", they bend their body in such a way that it is possible for the jumper to clear the bar while his or her centre of mass does not.
When balancing on a beam, we stretch out our arms and move them around, to keep our centre of mass over our feet - not as stable as having the centre of mass below our feet, but better than having our arms by our side. Tightrope walkers have a long pole with weights on the end, which lowers their centre of mass, making them more stable.
Astronomical objects in motion are balanced around a centre of mass, forming stable coupled orbits (until a collision or other event moving one of them will make them realign around a new centre of mass).

Sit in the chair.
Hold the cans in outstretched arms, and ask someone to start the chair spinning (or spin it with your feet). It does not need to be going super fast.
Bring your arms inwards, and feel yourself speed up.
Play around to experiment with the forces you are experiencing.

Explanation for younger students:
When the heavy cans change place, the speed of the chair spinning must also change. This is because the amount of force stays the same in the whole system (the chair, you and the cans). With arms outstretched a lot of force is needed to move the mass at the end of your arms, so there is less force to turn the chair. When your arms are pulled in it takes less force to move the weights, so more force can go into spinning the chair and it spins faster.

Explanation for older students:
A rotating object has a constant angular momentum (unless it is acted upon by an outside twisting force).
Angular momentum is the product of angular velocity and moment of inertia.
Angular velocity is velocity measured in degrees.
Moment of inertia depends on both the mass of an object and on how that mass is distributed.
The farther from the axis of rotation the mass is located, the larger the moment of inertia. (Moment of inertia is larger when your arms are extended.)
When the mass moves inwards the moment of inertia decreases, so the velocity increases.

Discussion on sports: this phenomenon is why an ice skater or a dancer doing a pirouette draw their arms and legs in to spin faster. Also used by divers or gymnasts when they go into a tuck position to flip or twist at a faster rate.