[Music] hi and welcome back to three sighs lessons cold UK by the end of this video you should be able to describe Newton's second law of motion and calculate the force required to accelerate an object you should then be able to estimate speed acceleration and forces for everyday road transport and if you're a higher tier student then you should be able to describe what's meant by inertial mass in the last video we looked at Newton's first law of motion we saw that if a stationary object experiences a resultant force of zero then it will remain stationary and if a moving object experiences a resultant force is zero then it will continue moving at the same speed and in the same direction and in the words it's velocity will stay the same however a resultant force which is not zero will cause an object's velocity to change so in this video we're looking at Newton's second law of motion and here it is the acceleration of an object is proportional to the resultant force acting on the object and inversely proportional to the mass of the object I'm remember that this symbol means proportional so what does this actually mean I'm showing you here two objects with equal masses both objects were experiencing a resultant force the top objects experiencing a resultant force of 20 Newtons to the right and the bottom object is experiencing resultant force of 10 Newtons to the right now as we said before the acceleration of an object is proportional to the resultant force acting on the object what this means is that if we've got a greater force then we have a greater acceleration so this means that the top object will experience twice the acceleration of the bottom object okay this shows another two objects on both objects were experiencing a resultant force of 20 Newtons the top object has a mass of 1 kilogram on the bottom object has a mass of 2 kilograms as we saw before the acceleration is inversely proportional to the mass of the object in other words if the mass is larger than acceleration will be smaller so once again the top object will experience twice the acceleration of the bottom object now we can use this to calculate the force needed to accelerate an object we use this equation the force in Newton's equals the mass and kilograms multiplied by the acceleration in meters per second squared and you are not given this equation in the exam so you need to learn it here's a question for you calculate the force needed to accelerate an object with a mass of five kilograms by four meters per second squared so posited your now and try this yourself okay the force equals the mass multiplied by the acceleration the mass is five kilograms on the acceleration is four meters per second squared putting these into the equation it gives us a force required of 20 Newtons here's another question for you a force of 50 Newtons is applied to an object with the mass of Nohr point five kilograms calculate the acceleration of the object so again pause the video and try this yourself okay in this case we need to calculate the acceleration looking at the triangle we can see that the acceleration is the force divided by the mass in this case the force is 50 Newtons and the masses nought 25 kilograms putting these into the equation gives us an acceleration of 100 meters per second squared okay now in the exam you could be asked to estimate the speed acceleration and forces involved in road transport so I'm showing you some of those here cars traveled around 13 meters per second on a main road in the UK and around 30 meters per second on a motorway to accelerate from a main road to a motorway and was a typical acceleration of around 2 meters per second squared so a typical family car that would require a force around 2,000 Newtons now if you're a foundation tier student then you can stop watching however higher tier students need to keep watching we're going to look at the idea of inertia if we look again at Newton's first law another way of saying it is an object will stay stationary or continue moving at the same speed and direction unless a resultant force is applied in other words object will stay still or keep the same motion unless you apply a resultant force scientists call this property of objects inertia now the inertial mass is a measure of how difficult it is to change the velocity of an object and the inertial mass is defined as the ratio of the force needed to accelerate an object over the acceleration produced an object with a large inertial mass will require a larger force to produce a given acceleration but an object with a smaller inertial mass remember you'll find plenty of questions on Newton's second law in my revision world book and you can get that by clicking on the link above [Music]