Are you kidding me? That remark might come across your head soon after you read the title above. How come you read a fresh daily twice a day? Is it really a daily or perhaps I mistake it for a semidaily?? No, if I said a daily it is really a daily which means it is published once a day. However there is a way for you to read a fresh daily twice a day! Okay, before you insist on your denial you should heed my explanation below first before you make your own conclusion whether my explanation makes sense or not. 😉
Suppose, the future longest suspension bridge in the world that spans the Sunda strait connecting the island of Sumatra and the island of Java is already completed. And suppose the bridge also carries the double railway tracks which are parts of Medan-Surabaya railway tracks. Here it is, daily at noon, a train pulls out from Surabaya for Medan. Also daily, again at noon, another train leaves Medan for Surabaya. Suppose the entire journey takes a week (seven days). The simple question is: How many trains will you meet on your way from Surabaya to Medan? You probably rush to give the answer: seven. That’s wrong though. Besides meeting on the way the seven trains that will leave Medan after you depart from Surabaya, you will also meet the trains in your direction already on the way at the time you left! Consequently, the right answer is not seven, but fourteen instead!
Let’s now proceed further. Each Medan train (a train which leaves Medan for Surabaya) has on board fresh daily Medan newspapers. If you are interested in Medan news, you will naturally buy fresh papers at each stop (suppose each time two trains about to pass each other they make a brief stop). How many fresh newspapers then will you buy on your a week journey? I guess that by now you will give the correct answer of fourteen. After all, each train you meet brings a fresh paper, and because you meet fourteen trains, it means you read also fourteen fresh papers! But since you will be travelling only a week, you will be reading a fresh Medan daily twice a week! How’s that??
This seems a surprising paradox, doesn’t it? For a short time, you might think “Yeah… you are right. But how come it goes like that?” But actually the logic behind this case is readily understood. The first Medan train you meet will bring a fresh (ehm…. I mean ‘a stale’) Medan newspapers from seven days ago!*) Since you head in their direction while they head in your direction it means your speed is doubled in reference to their perception and vice versa. Thus it means that you will meet the fourteen Medan trains one after another at the rate of two trains a day instead of one train. Aha! Now after the brief explanation I hope everything is now crystal clear for you to understand why is it possible for us to read a fresh daily twice a day! 😛
The last question is why am I telling you this? This case might help you understand to some extent the properties of Doppler effect in particular or some properties of the sound in general. That is if you are interested in very basic physics in entertaining ways! 😉
*) The first Medan train you meet is the one you meet at the initial point of your journey in the Surabaya station at the Medan train’s arrival meanwhile the last Medan train you meet is the one that you meet at the last midnight (suppose all trains travel at the same and constant speed) before you arrive at the Medan station. The Medan train you meet at the Medan station when you arrive is counted out since the Medan daily is ubiquitous in Medan and you don’t need to buy it aboard.