[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to describe what's meant by weight you should then be able to calculate the weight of an object in the last video we saw that gravity is a force of attraction between all objects gravity is a non-con force because the two objects do not need to be touching like all forces gravity is a vector quantity that's because it's got a magnitude and a Direction so we're going to start by looking at the idea of mass the mass of an object tells us how much matter the object has in it and the unit of mass is the kilogram mass is a scalar quantity because it's got no Direction so for example this elephant has a mass of 5,000 kg now the key fact is that the mass of an object does not depend on where the object is so the mass of this elephant is 5,000 Kg on the earth if the elephant could travel to the Moon it would still have a mass of 5,000 kg and if the elephant was floating in deep space it would still have a mass of 5,000 kg now the weight of an object is the force acting on it due to gravity and the unit of weight is the Newton the weight of an object does depend on where it is I'm shown you here an object with a mass of 1 kg if we place this object on the surface of the Earth then it experiences a gravitational force towards the center of the earth at the surface of the Earth the gravitational force is 9.8 Newtons per kilogram scientists call this the gravitational field strength and this has the symbol lowercase G the gravitational field strength is a measure of the force of gravity in a particular location objects on the surface of the Earth experience a force of 9.8 Newtons for every kilogram of their mass now we can calculate the weight of an object using this equation the weight in Newtons equals the mass in kilog multipli by the gravitational field strength in Newtons per kilogram now you're not given this equation in the exam so you need to learn it however you will be given any necessary value for the gravitational field strength so going back to our 1 kilogram Mass we can now calculate it weight the weight is the mass multiplied by the gravitational field strength in this case the mass is 1 kgam and the gravitational field strength for the surface of the is 9.8 Newtons per kilog multiplying these together gives us a weight of 9.8 Newtons now the weight of an object is directly proportional to the mass of the object and we can show that using this symbol what this means is that if we double the mass then the weight also doubles we can determine an object's weight by using a calibrated spring balance and I'm showing you one of these here this is also called a Newton meter and you may have used one of these at school now as we said before the gravitational field strength is a measure of the force of gravity in a particular location a key idea that you need to understand is that the gravitational field strength depends on where you are this shows an object with a mass of 100 kg on the earth its weight is 980 Newtons because the gravitational field strength on the earth's surface is 9.8 Newtons per kilogram this shows the same object on the moon the gravitational field strength of the moon is 1 .6 Newtons per kilog so the object still has a mass of 100 kg but now its weight is 160 Newtons okay now there is one final idea that you need to understand about weight the weight of an object in other words the force due to gravity can be considered to act at a single point And scientists call this point the center of mass it's worth learning the definition for Center of mass as it could come up in your exam remember you found plenty of questions on gravity and weight in my vision work book and you can get that by clicking on the link above [Music]