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Tomislav Sen}anski

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Scientific editor Prof. Tomislav Senćanski Illustrated by Miroljub Milutinović

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INTRODUCTION This is our second book of experiments you can do at home. It contains new topics and new materials. Some topics are new and some are carried over from Book 1. If you manage to simplify some experiments or update them, you will give the book new value. Experiments are essential supplements to the knowledge of young people who have an interest in science. Thinking in a scientific spirit means understanding the events and phenomena which happen around us all the time. The illustrations in this book both provide an incentive for the work and also simplify it.

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CONTENTS MOVEMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Are you moving? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 One movement, two paths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 How far to the sea? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

INERTIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Coins on your elbow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Bottle on a bottle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Lazy clothes-peg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

THE FORCES OF ACTION AND REACTION . . . . . . . . . . . . . . . 12 Train propelled by air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Flying apart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Water merry-go-round . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

GRAVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Which is quicker, gravity or the eye? . . . . . . . . . . . . . . . . . . . . . . . . 14 Humpty Dumpty can never fall . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Centre of gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Balancing sculptures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Your body just won't listen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

LIQUIDS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Household fountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Self-watering flowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Alcohol or water? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Jet-propelled boat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Water roses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Water mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Metal which swims on water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

AIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Glasses stuck together without glue . . . . . . . . . . . . . . . . . . . . . . . . 26 Lift the coin without spilling the water . . . . . . . . . . . . . . . . . . . . . . . 27 Blow down the coin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Blow the paper ball into the bottle . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Are you stronger than air? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Crumple the can without touching it . . . . . . . . . . . . . . . . . . . . . . . . 31 Air lifts weights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Aerodynamic paradox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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LIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Can you see the candle? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Measuring the tree's height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 The shadow game . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Tin can camera. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Is it burning or not? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 A candle burning in water? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Now you see it, now you don't . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Make eyeglasses with your fingers . . . . . . . . . . . . . . . . . . . . . . . . . 38 Convex mirrors and convex lenses . . . . . . . . . . . . . . . . . . . . . . . . . 39 The busy blacksmith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

SOUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 How sound travels and is reflected . . . . . . . . . . . . . . . . . . . . . . . . . 41 Vary the tone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 The rubber string band . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Patterns formed by sound. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Pictures formed by sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 The squeaking box. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Cracking paper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 The Panpipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Singing bottles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Tolling bells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

LOOKING AT CHANGES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Plastic milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Stalactites at home. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 How strong is an egg? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 The Mรถbius strip. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

ELECTRICITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Electroscopes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 The indecisive ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 How to make an electrical spark . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

ELECTRIC CURRENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Make a lemon battery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Electric coins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 The frightened earthworm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Volta's battery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

MAGNETS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Knitting-needle compass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 A compass in a plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Spinning and jumping top . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 A hovering paper clip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Electricity and magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

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MOVEMENT lants grow, birds fly, water flows downstream. All these things are movement. But a plant can also wilt, a bird can alight on a tree, and water can reach a lake or the sea. We can then say that they have come to a state of rest. Can there be a state of absolute rest, or immobility? What does physics say about this? The bird might be at rest relative to the Earth, but it is still moving relative to the Sun. This means that it is moving and resting at the same time. We can speak about movement only when we observe it relative to another body, which we call a body of reference, and movement itself is thus relative.

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Are you moving? This experiment will show you the significance of the body of reference.

Necessary materials: a blindfold

How the experiment is performed 1. One of you should be blindfolded, and then two others should carry that person as shown. 2. The bearers should alternate making forward steps with stepping up and down while standing still.

What will happen? The blindfolded person will never know whether she is moving or not.

Why? When we are unable to see our surroundings, we cannot observe the change in our position relative to other things. We are therefore not sure if we are moving or not.

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One movement, two paths We can never speak about movement if there is no body of reference, which we usually view together with a co-ordinate system, and we then speak of a reference system. The path (trajectory) of an object depends on the system of reference. The next experiment will show how.

Necessary materials: two pieces of cardboard as shown, paper and a pencil

How the experiment is performed 1. Cut a vertical slit on cardboard piece No. 2. 2. Draw "observers" on both pieces. 3. Place the pieces on a blank sheet of paper as shown. 4. Hold down piece No. 1 and slide No. 2 towards the right, while moving the pencil tip up and down the slit.

What will happen? The "observer" standing on piece No. 2 will see the pencil moving up and down in a straight line, while the "observer" on piece No. 1 will see it moving in a zigzag line, which you will be able to check if you look at its trace on the paper.

Why? In order to be able to describe the trajectory of an object, we must know relative to what its movement is being observed - we must define a system of reference.

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How far to the sea? We know what speed is from our every-day life. If objects traverse different paths in the same period of time, we say that their speeds differ. How can we determine the speed at which water flows in a river?

Necessary materials: two wooden pegs, a piece of wood, a stopwatch or watch with a second hand

How the experiment is performed 1. Fix the pegs on the river bank. They will be bodies of reference. 2. Measure the distance between them. 3. Throw the piece of wood into the river and time how long it takes to pass from one body of reference to the other. 4. Divide the distance by the time elapsed. You will get a figure showing the speed, to which you will add the unit of measurement (metres per second, or m/s).

Additional idea You can measure the speed of an electric train in the same manner, if you know the distance between the poles carrying the wire feed.

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INERTIA he property of objects of remaining in a state of rest or uniform movement in a straight line until a force acts on them to alter that state is called inertia. How can you test this physical property?

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Coins on your elbow Necessary materials: several metal coins

How the experiment is performed 1. Place a coin on your elbow as shown. 2. Lower your elbow sharply.

What will happen? Before it starts to fall, the coin will stay in its position for an instant due to its inertia.

Additional idea Organise a coin-catching competition. See how often you can catch a coin, thanks to its inertia, before it hits the ground.

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Bottle on a bottle Necessary materials: two bottles, a piece of thin cardboard

How the experiment is performed 1. Place the bottles on top of each other with the cardboard between them, as shown. 2. Pull the cardboard sharply.

What will happen? The upper bottle will retain its state of rest. Warning! Quite a lot of skill is needed for performing this trick. You can practice for it by surrounding the bottom bottle with pillows so the top one doesn't break if it falls.

Lazy clothes-peg Necessary materials: a glass, a piece of cardboard, a clothes-peg

How the experiment is performed 1. Put the cardboard on the glass and the clothes-peg on it as shown. 2. Hit the cardboard sharply with your finger.

What will happen? The clothes-peg will not follow the cardboard, but will make a 180-degree somersault and fall into the glass.

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THE FORCES OF ACTION AND REACTION hen an object exerts a force on another (action), the second objects exerts on the first an equal force in the opposite direction (reaction). These forces will be shown in the following group of experiments.

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Train propelled by air Necessary materials: a toy train track, a train car, a plastic tube bent at an angle, a balloon, some string

How the experiment is performed 1. Fix the balloon to the tube and tie the tube to the railway wagon as shown. 2. Inflate the balloon through the tube and quickly put it on the track.

What will happen? The air will be expelled in one direction and the car will move along the track in the other.

Additional idea Fix a balloon to a plastic tube. Thread a piece of string through the tube and tie its ends to two chairs, stretching it tight. Let the air out of the balloon. As it expels the air, the balloon and the tube will travel down the string.

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Flying apart Necessary materials: an empty matchbox, matches, a razor blade, thread

How the experiment is performed 1. Use the thread to tie the razor blade around the box as shown. 2. Hang the box with another piece of thread as shown. 3. Break the thread holding the razor blade by burning through it.

What will happen? The razor blade will fly to one side (action) and the box to the other (reaction).

Water merry-go-round Necessary materials: a small plastic bottle, two plastic tubes, some string.

How the experiment is performed 1. Bend the tubes, pierce the bottle and insert the tubes, and then hang the bottle, as shown in the picture. 2. Fill the bottle with water.

What will happen? Water will flow through the tubes and the bottle will begin to turn in a direction opposite to that of the flow of water.

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GRAVITY ravity is a mysterious force: everyone knows that it exists yet it is very difficult to understand it. It is gravity that is responsible for the planet Earth attracting all other objects. When you see images of astronauts seemingly floating in space in a state of complete weightlessness, that does not mean that the Earth is not exerting a gravitational force on them.

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Which is quicker, gravity or the eye? Here is a way to use the force of gravity for a game.

Necessary materials: white cardboard, a ruler, a pen, scissors

How the experiment is performed 1. Cut out a piece of white cardboard 30 centimetres long and five centimetres wide. 2. Mark five-centimetre divisions on the cardboard, making six segments in all. 3. Ask a friend to hold the cardboard hanging vertically just above your hand. 4. When your friend drops the cardboard, try and catch it as fast as you can.

What will happen? However easy it looks, you will never be able to catch the bottom end of the cardboard.

Why? It is a race between gravity and your body. By the time the message from your brain has reached the muscles of your hand, gravity has already pulled the cardboard down several centimetres.

Additional idea Have your friend drop a ping pong ball down a tube held vertically. Try and smack the ball with a ruler as it leaves the tube and before it hits the ground.

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Humpty Dumpty can never fall Objects always tend to assume the most stable position possible. The lower their centre of gravity, the easier it is for them to return to a state of equilibrium. In this experiment, you can make a Humpty Dumpty who will show you this rule.

Necessary materials: a plastic box from the chocolate egg surprise, a metal ball, plasticine, a felt-tipped pen

How the experiment is performed 1. Fix the metal ball with the plasticine in one end of the box. 2. Close the box and draw a face on it.

What will happen? Whatever you do with the Humpty Dumpty, it will always bounce back to the same vertical position.

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Centre of gravity How can you determine the exact position of a body's centre of gravity? Here are a few simple experiments.

Necessary materials: a book, a broom, a ruler and other objects

How the experiment is performed 1. Balance a book on your index finger, helping yourself with your other hand until you find the spot where you can easily balance the book horizontally. 2. Hold the broomstick horizontally on both your outstretched index fingers. Slowly move the fingers towards each other, one after the other, until they meet at the broom's centre of gravity.

3. Try and balance the broom vertically on your forehead. 4. Try various combinations of any two bodies and finding their state of equilibrium (an example is shown in the picture).

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Balancing sculptures

Necessary materials: an unopened bottle, a cork stopper, a needle, two forks

How the experiment is performed Carefully stick the needle into the cork, then stick the two forks into opposite sides of the cork as shown. Balance the other end of the needle on the cap carefully until the whole assembly stands in a state of equilibrium.

Additional idea Balance a ruler on a pencil held horizontally. Place various objects on the ruler's ends (erasers, sharpeners etc.). To preserve the balance, the position of the pencil will always have to be nearer the heavier of the two objects.

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Your body just won't listen If you want to keep your body in a state of balance, its centre of gravity and your footing - your contact with the ground - must be on the same vertical line. This rule can be tested in the following ways.

How the experiment is performed 1. Stand up straight with your back against a wall. Without bending your knees, try to pick up a pencil from the floor. Can you do it? 2. Stand up straight with your left shoulder and left leg touching a wall. Try and lift your right leg. Can you do it? 3. Sit on a chair with your back straight and your legs bent at the knees at a right angle. Now try to get up without leaning forward or tucking your fleet under the chair. Can you do it?

Why? Here is why you are not able to perform the desired movements. As you sit in the chair, the vertical line passing through your centre of gravity passes through the floor somewhere between the chair legs. When you try to get up, the support is shifted to your feet, and your centre of gravity isn't above them, so you lose your balance and fall back in the chair. The explanation for the first two situations is similar.

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LIQUIDS iquids are highly mobile substances. Water can even "climb" through very narrow tubes called capillaries. The free, or upper, surface of a liquid is always horizontal.

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Household fountains A liquid's weight produces hydrostatic pressure, which acts in all directions, as we shall show in this experiment.

Necessary materials: two plastic bags, a pin, water

How the experiment is performed 1. Fill the bags with water and seal them. 2. Holding in the positions shown in the picture, pierce with the pin.

What will happen? The water will flow out in different directions, as the hydrostatic pressure acts on it. Note: To help your experiment succeed, you should also pierce the bags at the top, so atmospheric pressure can also act on the water.

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Self-watering flowers This experiment will allow you to keep your flowers properly watered while you're on holiday.

Necessary materials: a vessel for water, a base for it, a thin rubber or plastic tube

How the experiment is performed 1. Fill the vessel with water and place on a base higher than the level of the flower pot. 2. Link the two vessels with the tube as shown.

What will happen? The flowers will always have a sufficient quantity of water thanks to the principle according to which the levels of liquid in two vessels connected with a tube will always be the same. Note: When you want the watering to begin, draw air out of the end of the tube which goes into the flower pot. The water will begin to flow towards the pot, but at a slow rate because of the reduced atmospheric pressure acting on the end implanted into the earth.

Additional idea You can check the level of plots of land by using a long hose filled with water. However twisted the hose may be, the level of the water that you can see at its ends will always be the same.

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Alcohol or water? Is alcohol lighter than water? We can check this without weighing the two liquids.

Necessary materials: two identical small glasses, an ordinary postcard, some water and some brandy

How the experiment is performed 1. Fill both glasses to the brim, one with brandy and the other with water. 2. Put the postcard on top of the water, lift the glass and turn upsidedown. 3. Put the water glass on the brandy glass and pull the postcard towards you a bit.

What will happen? After a few minutes, the brandy will flow up into the top glass and the water will sink to replace it.

Why? Alcohol is lighter than water and will rise. While the liquids are flowing in opposite directions, there will be a little mixing.

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Jet-propelled boat Most powered boats and ships are driven by propellers. Propellers "cut through water", forcing it towards the rear, in reaction to which the boat moves forward. But there are also vessels which can move very fast using a water-jet, a fast stream of water expelled from the engine towards the back. In this bathroom experiment we shall make a simple water-jet.

Necessary materials: an empty plastic bottle, plasticine, a spike, a balloon

How the experiment is performed 1. Put some plasticine in the bottom of the bottle to give it weight and therefore also stability. 2. Make a tiny hole near the bottom of the bottle. 3. Insert the balloon into the bottle carefully. 4. Spread its end over the top of the bottle and fix some plasticine around the top to give it weight. 5. Fill the balloon with water to a volume about half that of the bottle. 6. With your fingers over the top of the bottle, lower it into a bathtub filled with water. 7. Let go of the balloon. Watch a jet of water spurt out and drive back your "boat".

What will happen? As water rushes out, the "boat" moves forward. Water rushing out exerts pressure on the water in the bath, and a force driving the boat in the opposite direction is created. The quicker the water comes out of the balloon, the faster the "boat" moves.

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Water roses Make paper roses which will open their petals by themselves.

Necessary materials: smooth paper, a pencil, scissors, a vessel with water

How the experiment is performed 1. Cut out flower shapes as shown. 2. Colour and fold the "petals" inwards. 3. Put on the water surface.

What will happen? The petals will open slowly.

Why? Paper is made mainly of plant fibres, which contain tiny tubes (capillaries). As the water enters the capillaries, the paper slowly swells, and, just like what happens with dried flowers, its petals will slowly open.

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Simple Science Experiments 2  

s imple Tomislav Senćanski Tomislav Sen}anski Scientific editor Prof. Tomislav Senćanski Illustrated by Miroljub Milutinović INTRODUCTION

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