Why can coins float on the water?
While studying the ups and downs of objects, a classmate came across an interesting phenomenon: when putting a plastic ruler vertically (or horizontally) on the water, he found that the ruler sank rapidly; When he laid the ruler flat on the water, he found the ruler floating on the water.
When the ruler is placed vertically (or sideways) on the water surface, the ruler sinks because the buoyancy of the ruler is less than its own gravity; Then why does the ruler float on the water when it is flat? According to the ups and downs of an object, the buoyancy of the object is equal to gravity when it floats. Aren't the two contradictory? What is the crux of the problem?
In the chapter "Properties of Solids and Liquids" in the first volume of senior high school physics textbook, there is a small experiment: students are required to put sewing needles on the water with cotton paper. When cotton paper sinks in water, observe the phenomenon and explain the reasons. Many students think that the sewing needle floats on the water because of the surface tension of liquid, and the needle can float on the water because of the balance of gravity, buoyancy and surface tension of liquid.
So let's first understand the surface tension of liquid.
What is the surface tension of liquid?
When the molecules near the liquid surface move outward from the equilibrium position, the external air and vapor molecules have little repulsive force on it. Did not play an important role. It is only attracted by internal molecules, so the force to restore it to its equilibrium position is not as great as that in liquid, which makes the vibration amplitude of surface molecules larger than that in liquid, and some molecules with large kinetic energy may rush out of the suction range and become vapor molecules, resulting in the sparse distribution of molecules on the surface than in liquid, and the distance between molecules is relatively large (r >: R0), precisely because of the sparse distribution of molecules inside the liquid surface and the molecular spacing r >; R0, intermolecular attraction prevails, leading to surface tension of liquid. Therefore, the essence of liquid surface tension is the resultant force of intermolecular interaction, which points to the inside of the liquid. It can be seen that the force that holds up the coin cannot be the surface tension of the liquid. Then let's look at buoyancy again.
Let's do an experiment first: put a filter paper on the surface of a beaker filled with water, and put a dime on the filter paper. After the filter paper was soaked and sank, it was found that the coin was still floating on the water. Pay attention to the coins and you will find that the water surface is sunken downwards, and the coins are not immersed in the water, but floating on the water surface. It can be seen that the coin is not affected by buoyancy at this time. What is the balance between the force and gravity that makes coins float on the water?
As can be seen from the previous experimental phenomena, the water surface is depressed and deformed downward, thus producing an elastic force-supporting force opposite to the deformation direction, which is like putting an object on an inflated balloon, and the balloon is deformed (depressed downward) due to the gravity of the object, resulting in a vertical supporting force. It is this supporting force, which is equal to and opposite to gravity, that makes coins float on the water. In the above experiment, the plastic ruler floating on the water is the same as the sewing needle. )
Why do rulers, sewing needles and coins sink horizontally or vertically? Let's take coins as an example. When placed horizontally or vertically, the stress area of the liquid surface is very small, the pressure is very high, and the pressure effect is remarkable; This is the same as wearing high-heeled shoes and stepping on the sand. The heel will sink into the sand and the coin will sink into the water, thus breaking the elastic deformation of the water surface and making the support of the water meter disappear. At this time, the coin is only affected by buoyancy and gravity, and because gravity is greater than buoyancy, the coin will soon sink to the bottom of the water. When the coin is laid flat, the pressure on the water surface is very small, so the above phenomenon will not occur, and the coin will float on the water surface to maintain balance. In nature, there are many similar phenomena. Some small insects can run around or stay on the water without going into the water, and the same is true.