Simple Demonstrations With Water (1962)

[Music] water is something we use every day to learn more about water let's observe some simple demonstrations that show what water does I filled this glass right up to the brim what will happen if I add more water as I add water slowly with this syringe watch the water in the glass it bulges over the top the surface of the water acts as a kind of film holding the water in the glass we say water has surface tension we can test the surface tension of water using this needle I I'll put it into this wire holder and lower it to the surface very carefully the needle is held up by the surface tension scientists know that surface tension is caused by the molecules of water attracting each other if we break the surface film the needle sinks so we've seen something about the surface of water now let's experiment with water below the surface this glass tube has a sheet of rubber across one end watch what happens when I push the tube underneath the water the rubber sheet is pushed up into the tube by the water pressing against it water exerts pressure let's see more about water pressure here are two cylinders filled with water this one has holes around the bottom covered with tape I'll remove the tape the water is pushed out by the water pressure inside the container the water is being pushed out about the same distance so the pressure must be the same at each hole this one has holes from top to bottom I'll remove the tape and now at the top hole the water is pushed out the shortest distance the farther down a hole is the farther the water is pushed out the deeper the water the greater the pressure here's another way to see that the deeper the water the greater the pressure I'll use this glass tube again as I push it down slowly watch the sheet of rubber at the end the deeper it goes the more it is pushed in by the pressure what will happen if I pour water into the tube I'll use this colored water so we can see it clearly the water inside the tube exerts pressure too pushing the rubber sheet down when the water is the same level inside the tube as outside the rubber sheet is flat so water pressure pushes down as well as up does it push in every direction let's see I'll use this pressure gauge whenever anything such as my hand presses against the end of this tube the needle on the gauge shows the change in pressure notice that the pressure is increasing as I lower the tube into the water now I'll keep the end of the tube at the depth marked by this line no matter which way I turn the end of the tube as long as I keep it at the same depth the pressure is the same so water exerts equal pressure in all directions what effect does this pressure have on something in the water here are three blocks this one floats high in the water this block does not float as high this block sinks why do some objects float and others sink when I push down on this block I can feel a force pushing up on it we call this upward force of water buoyancy water can produce a buoyant force when I place this cube in the water it sinks does buoyancy have any effect on this Cube even though it doesn't float we'll find out with this scale I'll weigh the cube and mark the weight with this Arrow now watch the scale as I place the cube in the water the cube weighs less in water than it did in air while the cube does not float the buoyant force of water still acts on it is every object boed up by the same amount of buoyant force this balance and these metal cylinders will help us find out watch the water level rise as I lower the cylinder into the water any object in water pushes aside or displaces some of the water the this larger cylinder displaces much more water although the cylinders are different sizes the larger cylinder is made of a lighter metal and they weigh the same they balance each other now I'll place the balance so the cylinders are in the water in water the larger cylinder weighs less than the smaller one remember the larger cylinder displaces more water the more water and object displaces the greater the buoyant force acting on it let's measure the buoyant force acting on this plastic box with this scale we can measure the force in pounds As I push it down the Box displaces more water and the buoyant force increases now the box is being boyed up with a force of 2 lbs how much water does the Box displace for 2 lbs of upward Force I'll mark the water level on the side of the box now I'll let just that amount of water into the box I'll weigh the water with this scale it's set so it won't weigh the container but only what's in the container the water weighs 2 lb this is the same amount as the buoyant force when the Box displaced this much water so the buoyant force acting on an object is equal to the weight of water the object displaces let's see how the buoyant force affects the three blocks we saw earlier one is very light one is a little heavier and one is very heavy this block is so light that it displaces very little water a small amount equal to the small buoyant force that keeps it afloat this block weighs about the same as all the water it can displace will it sink or float it barely floats its weight is just about equal to the buoyant force acting on it this block weighs more than all the water it can displace will it sink or float it sinks so an object floats when it displaces enough water to equal its own weight when it weighs more than the most water it can displace it sinks this boat floats the water it is displacing is equal to the weight of the boat now I'll add more weight to the boat as the weight increases the boat sinks lower in the water now the boat has sunk very low in the water what will happen when the boat becomes heavier than all the water it can displace it sinks the boat is made so it can be enlarged I'll keep the same weights in the boat but make it bigger now it can displace more water than it did before the boat floats easily with all the weights in it it is displacing more water now so the buoyant force acting on it is greater we've seen three important things about water we saw that water has surface tension water exerts pressure and water can produce a buoyant force these three basic principles will help us as we learn more about water and the way it behaves