At the focal point

Can you find the focal point of the mirror?

Place your hand in front of the mirror and test the warmth reflected from the mirror at different points on the mirror.

The heat radiation is reflected most strongly at the mirror’s focal point. The focal point can be located with the help of both light and heat.

The light and heat radiating from a heat lamp are reflected off from the concave mirror so that the rays intersect close to the focal point. They do not meet at any one exact point, because the rays hitting the mirror are not parallel. The heat from the rays of the Sun can be utilised effectively with the help of large parabolic mirrors, if a high temperature is required at a specific point. The positive aspect of this system is the cost-free energy. The negative side is that the mirrors would have to rotate at the exact same rate as the Sun in order to ensure that the rays would always be reflected at the same point.

Archimedes is told to have lit Roman ships on fire with the help of concave mirrors at the siege of Syracuse. On the basis of practical research, this story has proven to be a myth. On the other hand, solar cookers that work using mirrors do actually work and can be used, for example, at cabins. As development aid, Finland provides solar cookers to many countries in which they are reducing the use of firewood and logging. They also relieve the workload of women responsible for the collection of firewood.

Parabolic mirrors

Do the walls have ears?

Two people are needed for this task. One person speaks into the ring at the centre of one reflector and the other listens at the ring of the other reflector.

The person with his/her ear next to the ring in the other reflector can hear the voice of the speaker – even a whisper – over the noise of the room.

The reflectors are parabolic in shape. The sound waves exiting the focal point at the centre of the ring are projected off of the concave surface in a parallel fashion towards the other reflector. When they reach the other reflector, they are all projected towards that reflector’s focal point. Without the reflectors, the sound of the speaker’s voice would simply spread to the surroundings, and only a small portion of the sound waves, insufficient for the human ear, would reach the listener.

The same principle holds true for light and heat. With the help of two parabolic mirrors, we can light a match. We simply station the mirrors at a distance of tens of metres from one another, hold a match at the focal point of one of the mirrors, and place a small, hot halogen light bulb at the focal point of the other.

A satellite dish on a rooftop operates on the same principle. The micro head located at the focal point effectively catches the TV signal sent by a communications satellite and reflected off from the dish. Satellite dishes always face the same direction, because the communications satellites remain stationary, with respect to the Earth, on their orbits at a height of 36,000 km above the equator. So, you can have ears on your roof, not just on your walls!

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Cat Matikainen

Bowling ball cannon

Can you make the tennis ball reach the ceiling?

Using the cord, pull the bowling ball up and let it drop. Observe the tennis ball in the other tube.

As the bowling ball begins to fall, the air below it is forced out through the holes in the larger tube. Once it’s beyond the holes, the bowling ball pushes the air below it towards the smaller tube, thereby blasting the tennis ball in the smaller tube towards the ceiling.

Without the holes in the large tube, the bowling ball would fall very slowly and the tennis ball would not rise particularly high. The slow fall is the result of the fact that the ball in the narrow tube combined with the air itself are very efficient in resisting the fall of the bowling ball. When the air is able to escape through the holes in the larger tube, no such air column resisting the fall of the bowling ball is created. The ball manages to achieve such a great speed so that it easily pushes the air in front of it, thereby sending the air with force into the smaller tube and launching the tennis ball to great heights. The rapidly falling bowling ball produces a much greater thrust than a slowly falling bowling ball. The speed of the tennis ball is further increased by the fact that the speed of the propulsive air increases as it is forced from a larger tube into one with a smaller diameter.

Some bicycle pumps have a small hole at the upper end of the cylinder. The point of the hole is to facilitate the initial speed of the piston, thereby easing the work of the person pumping the air.

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Cat Matikainen

Slow motion with smoke

The spinning table

Can you predict what will happen to the objects placed on the spinning table?

First roll an object across the table by beginning the roll on the spinning table itself. Then try to set the object rolling onto the table from the peripheral area around it. You can use the buttons to adjust the speed of rotation.

The object will always circle in the same direction as the spinning motion of the base below it. An object that is set to roll before it enters the spinning table will roll off the table at nearly the same spot and heading in the same direction as it would have done if the table had not been spinning. The object that is set to roll directly on the spinning table will behave unpredictably.

A frictional force begins to affect the object as soon as it enters the spinning table. As the object passes over the centre of the spinning table, the direction of the friction changes to follow the direction of the spinning. Therefore, when crossing over the entire spinning table, the object is affected by an equal friction in both directions, left and right. Thus, when reaching the stationary part of the table, the object continues its path in its original direction. The friction on an object that is set to roll directly on the spinning table does not have this same symmetry. Thus, it will behave unpredictably.

The same phenomenon is illustrated, for example, when you jump off of a spinning carousel. If you want to land in one specific spot on the ground and stay there, you must resist the spin of the carousel and jump against the rotational direction. It would be, however, a great deal easier to continue in the same direction as the spinning motion and to take a few running steps upon landing on the ground, in other words, not to resist the inertia.

Blasting barrels

Can you shoot air?

Aim the barrel at the curtains. Then sharply hit the rubber drumhead of the barrel.

A sharp hit on the rubber drumhead of the barrel forces the air inside the barrel to move. A portion of the air exits the opening at the other end of the barrel and, within a few seconds, hits the curtains hanging from the ceiling.

The air that leaves the barrel does not exit the hole as a flow of air, but rather as a ring of air. The phenomenon concerns a ring-like air vortex, in which the air is moving forward as well as circling around inside the ring. The vortex is essential to maintain the shape of the ring. The pressure of the air flow is lower than the pressure of air that is still. The progressing ring of air is held together by the higher pressure of the surrounding air that is still.

The shape of this air cannon’s ammunition may be difficult to visualise, since it cannot be seen in the same way as, for example, smoke rings. In water, however, rings of air are clearly visible. Dolphins use this phenomenon to their advantage. They blow rings of air around schools of fish. The fish are trapped inside this “web” formed by the rings of air, and the dolphins can easily catch even the entire school.

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With smoke

Dolphins

Flying carpet

Low-flying carpet

Use the hose to fill the compressed-air tank. Release the hose by pressing the button and push yourself off to slide.

The carpet remains in the air for about 30 seconds, as it ‘flies’. In the end, the air in the tank runs out, the air stream weakens, the pressure under the carpet decreases and the carpet lowers to the ground.

The air leaks through the small holes in the bottom of the tank, thereby forcing the carpet upwards. The 'skirt' around the carpet prevents the leaking air from immediately escaping from under the carpet. The resulting extra pressure keeps the carpet hovering above the ground. The carpet settles at the height in which the combined lifting force of the air stream and the pressure of the air underneath the carpet are equal to the weight of the carpet with its passengers. When the carpet is off the floor, it moves nearly without friction, so it only requires the force to overcome inertia to start it off – or to stop it.

The same principle is applied to a hovercraft, which is an extremely handy means of transportation in the Finnish archipelago during the period when the sea is thawing out in spring. Hovercrafts can be used to move about on land, water and ice. The first hovercraft was designed in Sweden at the beginning of the 18th century. It was, however, never built. It is not likely that it would have worked, since it was designed to use high pressure air created by human power. As far as is known, the first functional hovercraft was built by the Finnish Engineer Toivo Kaario in 1935.

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Cat Matikainen
Flying carpet

Vacuum

Which will drop faster, a feather or a ball?

Turn the tube over and watch as the objects drop from one end to the other. Press the green button to suck air out of the tube. How do the objects fall now? To blow air back into the tube, press the red button.

In air, the feather will drop slower than the ball, whereas in the vacuum, they fall at the same rate.

The falling objects push the air out of their way, which creates drag, a force that opposes the falling motion, otherwise known as air resistance. The falling velocity of an object depends on the degree of air resistance in relation to the object’s weight. The feather will drop slower in air, because its air resistance in relation to its weight is significantly greater than that of the ball. In a vacuum, the falling velocity is only affected by the acceleration created by gravity, and this acceleration is the same for all objects. The ball and the feather will, therefore, fall at the same rate.

A falling object may achieve a velocity in which the air resistance is equal to the object’s weight. After this point, the falling velocity will no longer increase. This velocity is known as terminal velocity. Prior to the opening of the parachute, the terminal velocity of a skydiver is about 200 km/h in the normal face-down parachuting position, and as much as 300 km/h in an upright skydiving position.

The fastest momentary falling velocity measured for a human being is 1,142 km/h. The person started his jump from a helium balloon gondola that had risen 31 kilometres above the Earth's surface. The air resistance at that altitude is only 1/100 of the resistance at sea level.