in this video we're going to look at isaac newton's first and second laws of motion as well as the ideas of circular motion and inertia newton's first law basically says that a resultant force is required to change the motion of an object or in other words if there isn't a resultant force then the object's motion won't change and this principle works for both stationary and moving objects this means that if the resultant force on a stationary object is zero or in other words there is no resultant force then the object will remain stationary and if the resultant force on a moving object is zero then it will carry on moving at the same velocity because in both cases there's no resultant force acting on the objects and so their motion won't change in contrast newton's second law says that if a non-zero resultant force acts on an object then it will cause the object to accelerate and remember non-zero just means anything other than zero for example if we took this particle andrew drew some unbalanced force arrows on it we can see that there would be a resultant force to the right because the right arrow was bigger than the left arrow so this particle will accelerate to the right now the confusing bit here is that depending on the initial motion of the particle this acceleration to the right could result in five different things happening for example if the object started off stationary then the acceleration would cause it to start moving to the right and if the object was already moving to the right then it would cause it to speed up however if the object had been traveling to the left then the right woods acceleration would cause it to slow down because it's acting in the opposite direction and if the object was traveling to the left more slowly it could cause it to stop moving entirely because it was slowed down so much the trickiest one to understand is that the acceleration could just result in a change in direction without any change in speed this is because acceleration is defined as the change in velocity divided by the change in time and as velocity is determined by speed and direction any change in the direction of the object also changes the velocity and so is considered acceleration we can see this in practice if we look at circular motion like the orbit of the moon around the earth even though the speed of the moon remains constant we still say that it's accelerating because its direction is always changing slightly this happens because the earth's mass exerts a gravitational pull on the moon which acts perpendicular to the moon's motion so overall the moon ends up spinning around the earth with a constantly changing velocity because the direction is changing but a constant speed the other part of newton's second law is that the size of the resultant force is directly proportional to the acceleration that it causes for example if you made the resultant force twice as big then the acceleration that it causes would also be twice as big we can see this in the equation f equals m a which says that the resultant force is equal to the mass of the object multiplied by the acceleration that it causes so if our particle from earlier had a mass of 0.25 kilos and was acted on by a force to the left of 30 newtons and a force to the right of 42 newtons then we could use this information to work out his acceleration first we're to need to find the resultant force which would be 42 minus 30. so 12 newtons to the right then we'd rearrange our equation to get force over mass equals acceleration and plug in our values to give 12 newtons over 0.25 kilos to get an acceleration of 48 meters per second squared the last thing we need to cover is the idea of inertia which is the tendency for the motion of an object to remain unchanged it's basically just newton's first law namely that unless acted on by a resultant force objects at rest will stay at rest and objects in motion will stay in motion meanwhile an object's inertial mass measures how difficult it is to change an object's velocity and we find it by dividing force by acceleration which is just the equation for newton's second law again so basically a large mass like the moon will have a lot of inertia so will require a really big force to change its velocity and even then the acceleration will probably be quite small anyway that's everything for this video so if you enjoyed it then try checking out some of our other physics videos and we'll see you next time