HoW & WhY In ScIeNcE

You know that sound travels with a definite speed in any medium. In air the speed of sound is nearly 330 meters/sec., while in water the speed of sound is nearly 1460 meters/sec. We can produce some sound on the surface of the ocean and see how long it takes for the sound to reach the bottom. Since no one is there at the bottom of the sea, we have to measure the time taken by the sound to reach the bottom and come back to the surface again, after being bounced off from the bottom.

Ordinary sound is not convenient for this purpose. A different kind of sound called Ultrasonic sound is used. Ultrasonic sound is a sound with high frequencies more than 20000 cycles/sec and therefore cannot be heard. You might like to know that the Arabian Sea and the Bay of Bengal have an average depth of 4 kilometers, while the Pacific ocean is very deep. At some places it is over 11km deep!!!

The old method of measuring the depth of an ocean was very simple. A heavy object like an anchor was tied to one end of a rope which was released from a ship floating in the ocean. This rope was marked like the measuring tape used by a tailor. As soon as the heavy anchor touched the ocean floor, the rope will no linger be taut. The marking on the rope then gave the depth of the ocean. This method was simple. It was, however, of no use for measuring very large depths. You know that the ocean can be very deep, often deeper than 6 kilometers.

The modern method of measuring ocean depth is quite ingenious. Do u recall how often we tell distance in minutes? When u sat that the school is 10 minutes away from your house, you are describing the distance from your house to the school. Anyone who knows how fast you walk will know how much that distance is. The same principle is used to measure the distance from the surface to the bottom of the ocean. We will of course need someone to walk this distance with a steady pace. The trick is to use the sound to do the job.

Now, consider a block of ice floating in a glass tumbler completely full me water. For making the calculations easier, let up suppose that the block of ice measures 1cm x 1cm x 1cm. Since the density of ice is 0.9 gm/cm3, it will weigh 0.9 gm and will pink 9mm under the surface of water, leaving 1mm above. Therefore, the volume of water displaced by the immersed portion of this block of ice will be 1cm x 1cm x 0.9cm, that is 0.9cm3.

When the block of ice melts, it will be converted into 0.9gm, that is, 0.9cm3 of water which is equal to the volume of the immersed part of ice. The water formed from the entire ice block will, therefore, be accommodated in the space occupied by the immersed portion of the ice block. Naturally, no additional water will be displaced in this process and, therefore, not a drop will split out.

You know all substances expand when heated and contracts when cooled. However, the mass of the substance whether hot or bold, remains the same.

Let us consider melting. When a solid melts, its volume go the liquid state is usually more than its original volume in the solid state. Since the mass remains the same, the above statement means that solids are generally denser than their liquids. You must have seen hair oil freezing on a bold winter day. As a part of the oil freezes, the frozen park sinks in the oil. Water, however, is an exception to this rule. When water becomes ice, its volume increases. Since the mass does not change, ice, which occupies more volume of the same mass is lighter, that is less dense, than water. Naturally, ice floats on water. In fact, whenever you see an object floating on a liquid, you can be sure that the density of the object is less than that of the liquid.

Density is defines as mass/volume. Density me water is 1 gm/cm3, that is 1cm3 volume of water weighs 1gm. The density of ice is only 0.9 gm/cm3, that is, a block me ice measuring 1cm x 1cm x 1cm weighs 0.9 gm. Now let up recall the law of floating objects. When an object floats in water, the weight of water displaced by the immersed portion of the object is equal to the weight of the entire object.

In the train, you do not see the ball moving horizontally with you since it keeps pace with you. You only see it going up and coming down. How would the motion of the ball appear to someone outside the train? Consider for example, a man standing on the ground outside, watching your experiment. As stated above, the ball possesses two motions at the time, the horizontal motion of the train and the vertical motion given by you. Both there motions together make the ball travel along a parabolic path. An outside observer would, therefore see the ball moving along a parabolic path. An inside observer would however, see only the vertical motion of the ball.

You are surprised when the ball lands in your hands in a moving train. You are not surprised, however when the same thing happens no a playground. Remember, the earth is spinning around its axis, completing one spin every day. It is also purring through space, in its motion round the sun. If you remember that all objects on the earth are moving along with the earth, you will understand why some of our childhood dreams are not practicable.

You may be tempted to answer that the ball will fall behind the person who throws it. You may think that, after all, the ball takes some time to go up and come down. During that time, the person would be carried forward by the train. Naturally, you would think that the ball should land behind him.

Well, instead of arguing, why don't you perform this experiment the next time you ride a train. You will be surprised to find that the ball lands right in your hands, as it does on a playground. What was wrong with your earlier argument?

In a running train, all the objects in the train acquire the motion of the train. Thus, the fans, the suitcases, the passengers, yourself and the ball in your hand, all move with the speed of the train. When you throw the ball up, it does not lose the motion it had acquired from the train. It continues to move along with the train and therefore, with you; only, it acquires a vertical motion in addition to its horizontal motion. Thus the ball, while moving up and down, also travels horizontally, keeping pace with you. The ball, therefore, lands smack in your hands.

Every object has density. Water also has density. Whether the object will sink or float is decided by the densities of the object and of water. Density of an object simply means the ratio of its weight (really speaking, mass) and its volume. If a brass bob weighs 80 gm and its volume is 10 cm3, its density will be 8 gm/cm3. Density of water is 1 gm/ cm3, i.e., 10 cm3 of water weighs only 10 gm. Since density of the brass bob is more than that of water, the bob will sink.

Now imagine that a flat disc is made out of the brass bob, by hammering it. While doing so, neither the mass of the bob (80 gm) nor its volume has changed. Therefore, the disc, like the bob, will sink in water. Now imagine that the disc is fashioned into a brass cup. No brass is added or removed in this process. However the volume of the cup will be much more than the volume of the disc. The volume occupied by the brass is only a small part of the overall volume of the cup. Most of the volume of the cup is occupied by the air inside the cup. Since the mass of the cup has remained the same and its volume has increased, the 'density of the cup' is reduced, considerably; so much so that it has become less than the density of water. No wonder that the cup even though made of brass, floats on water.

Whatever is true for a brass disc and a brass cup is true of an iron sheet and an iron ship. The densities of a steel needle, an iron nail, a solid iron bob etc., are more than that of water --therefore these objects sink. A steel cup, a steel vessel, an iron ship etc., have densities less than that of water --therefore these objects float.