[Music] hi and welcome back to three sighs lessons cold UK by the end of this video you should be able to calculate the pressure due to a column of liquid you should then be able to describe the factors which influence floating and sinking and all of this is for higher tier students only I'm showing you here a container which is filled with water what would happen if we made holes in the side of the container what we find is that the water squirts out of the container like this the water from the bottom hole squirts out much further than the water from the top hole that's because the water at the bottom of the container is at a higher pressure than the water at the top now we calculate the pressure of a column of liquid using this equation the pressure in Pascal's equals the height of the column in meters multiplied by the density of the liquid and kilograms per meter cubed multiplied by the gravitational field strength and you are given this equation in the exam so you don't need to learn it you'll also be given any of the values that you need such as density and the gravitational field strength here's a sample question for you a freshwater lake has a depth of 8 meters calculate the pressure at the bottom of the lake due to the water the density of water is 1,000 kilograms per meter cubed and the gravitational field strength is 9.8 Newton's per kilogram so pause the video now and try this question yourself ok so here's the equation the pressure is the height of the column multiplied by the density of the liquid multiplied by the gravitational field strength the height of the column in other words the depth of the lake is 8 meters the density of fresh water is 1,000 kilograms per meter cubed and the gravitational field strength is 9.8 Newton's per kilogram putting these numbers into the equation gives us a pressure due to the water of 78,000 400 per skal's so as you can see the pressure of a liquid increases with depth and that's because as the depth increases there's a greater weight of liquid acting downwards the pressure of a liquid also increases with the density of the liquid again that's because liquids with a greater density how the greater weight acting downwards now we can use the idea of pressure to explain my objects float or sink to do that we're going to look at the idea of up thrust I'm showing you have an object and most in a liquid as we said before the pressure of a liquid depends on the depth the bottom of the object is at a greater depth than the top of the object this means at the bottom of the object experiences a larger pressure than the top because of this there's a larger force acting on the bottom of the object than at the top of the object in other words there's a resultant force acting upwards and scientists call this force up thrust now for an object to float the up thrust must equal the object's weight in other words that downward force due to gravity if the iewk thrust is less than the object's weight then the object sinks ok I'm showing you here an object which we're going to lower into water now you'll notice that when we lowered the object the level of the water lowers and that's because the object displaced the water now here's a key fact that you need to learn the size of the oppos'd acting on the object is the same as the weight of water displaced by the object so what that means is that if an object can displace its own weight of water then the Oakhurst will equal the object's weight and the object will float this shows an object that's less dense than water this object only has to displace a small volume of water before the weight of water displaced equals the weight of the object so now the okk thirst equals the weight of the object and the object floats in this case the object floats high in the water this shows an object that has the same density as water in this case the object has to displace its own volume of water in order for the weight of water displaced to equal the weight of the object once again the upthrust equals the weight of the object and the object floats but in this case the surface of the object is at the surface of the water finally this shows an object that's more dense than water in this case the object cannot displace a volume of water equal to its own weight therefore the weight of the object is greater than up first so the object sinks remember you'll find plenty of questions on pressure on liquids in my vision workbook and you can get that by clicking on the link above [Music]