A LEVEL PHYSICS REVISION: WEIGHT AND GRAVITATIONAL FIELD STRENGTH EXPLAINED

hey guys and welcome back to another one of the a-level physics videos again we're still on the mechanics topic and today we're going to be talking about uh weight and gravitational field strength so it should be noted that this is kind of split in two almost because this is the mechanics part of weight and gravitational field strength so it's not going to cover actual gravitational fields okay so the equation for weight in general is just Mass times G where m is Mass in kilograms and G is equal to the gravitational field strength or gravitational acceleration or field strength so and the units for this are Newtons per kilogram so when you multiply the mass in kg by Newtons per kg you're going to get weight in Newtons so why is this important well in a level it kind of comes back again but this gravitational field strength thing here is pretty important because they can give you situations where you might not be on earth now on Earth you need to memorize gravitational acceleration as being 9.81 meters per second squared or Newtons per kilogram okay however they could easily put you for example on the moon or Mars or Venus or Jupiter which all have different gravitational field strengths and the important thing here is that they can ask you conceptually what that means for the weight and what that means for the mass now the mass is the amount of stuff an object is made of okay so this will never ever change no matter what planet or even if you're just in space whereas your weight is actually the force that you are exerting on the ground okay and it comes from how strongly the planet or the moon or whatever you're on is pulling down on you okay so for example if they asked you know an astronaut weighs 200 newtons on Earth um he goes to the moon will hit what will happen to his Mass what will happen to his weight but his weight would decrease his Mass would stay the same now a few more things about gravitational field strength is that when objects are falling so objects in free fall will fall or with the acceleration of G okay so on Earth everything that falls on Earth can be assumed to accelerate downwards with a at a rate of 9.81 meters per second squared and if it's on the moon it's about a third of that so about 3.2 um and on Mars it's something that's 30 less so this is something you need to keep in mind whenever you're doing kind of free fall or talking about a terminal velocity uh it's also important to note that this is only if we neglect air resistance okay because of course air resistance kind of messes things up now this also means that in space where G is practically zero you don't have any weight at all next I want to talk about with weight is that you always need to include it whenever you're doing any kind of mechanics calculation so it could be a situation you have a box on the ground or you could have a ball on an edge or something like that the weights will always act completely vertically down it has no horizontal component whatsoever however this doesn't mean that it doesn't cause any kind of acceleration left or right if you notice here the weight is going straight down but since it's going straight down on a slope surface it the ball or the object will still accelerate downwards and this should also be noted we're going to talk about talk about this a little bit more in the Newton's third law video but this weight is always going to create a reaction force that goes directly opposite or perpendicular to the surface it's resting on so in this case since we have a flat surface here or approximately flat R is perpendicular to that and likewise here since we have a slanted service it's going to be perpendicular in this direction in terms of weight the key thing that you need to remember is that this is the equation you need weight is equal to mg you're going to use it quite a lot but you need to make sure that you are actually on Earth if you're going to use 9.81 they could easily change this and make it quite simple there aren't really any exam questions that we can do that just to talk about weight normally they're parts of other questions so I think we will leave it there for now