[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 acceleration you should then be able to calculate the acceleration of an object and if you're a higher tier student then you should be able to calculate the distance traveled by an object from a velocity time graph in a previous video we looked at the idea of velocity the velocity of an object is its speed in a given Direction velocity is a vector quantity as it has both magnitude and Direction the acceleration of an object tells us the change in its velocity over a given time and we calculate acceleration using this equation acceleration in m/s squar equals the change in velocity in me/ second divided by the time in seconds I've also given you the triangle for this equation now you're not given this equation in the exam so you need to learn it here's a typical question a car is traveling at a velocity of 15 m/s North it accelerates to a velocity of 35 m/s North in 20 seconds calculate the acceleration of the car so pause the video now and try this yourself okay so to calculate acceleration we divide the change in velocity by the time taken the final velocity was 3 5 m/s north and the start velocity was 15 m/s North so the change in velocity is 35- 15 giving us a value of 20 m/s the time taken was 20 seconds putting these into the equation gives us an acceleration of 1 m/s squared so what that means is that the car increased its velocity by 1 m/ second every second over a 20 second period try question a cyclist is traveling at a velocity of 6 m/s East her velocity reduces to 0 in 12 seconds calculate the acceleration of the cyclist again pause the video and try this yourself okay the acceleration equals the change in velocity divided by the time taken the final velocity was 0 m/s East and the start velocity was 6 m/s East so the change in velocity is 0 - 6 m/s this gives us a change in velocity of -6 m/s this took place over 12 seconds putting these into the equation gives us an acceleration of -.5 m/s squared in this case the object slowing down and scientists call this deceleration now we can also calculate the acceleration of an object using a velocity time graph so we're going to look at those now I'm showing you a velocity time graph here and you could be asked to plot one of the these in your exam now a key fact is that the gradient of a velocity time graph tells us the acceleration of the object in the case of a horizontal line like this the object's traveling at a constant velocity an upward sloping line shows that the object's accelerating whereas a downward sloping line shows that the object is decelerating so we're going to calculate the acceleration in the first part of the graph to do that we subtract the initial velocity from the final velocity and divide by the time in this case the final velocity is 15 m/s and the initial velocity was zero and the time is 100 seconds putting these into the calculation gives us an acceleration of .15 m/s squared looking at the last part of the graph we can see that the final velocity is zero and the initial velocity was 50 m/s the time was 300 seconds putting these into the calculation gives us an acceleration of -.5 m/s squared in this case the negative number tells us that the object was decelerating okay now Foundation tier students can stop watching however higher tier students need to continue so as we seen the gradient of a velocity time graph tells us the acceleration however the total area under the graph tells us the distance traveled in a specific Direction in other words the displacement now when we see constant acceleration or deceleration then we simply divide the graph into shapes and calculate the total area so we've got a triangle with an area of 750 a rectangle with an area of 1,500 and a triangle with an area of 2,250 adding these together gives us a total distance or displacement of 4,500 M now you might see a velocity time graph like this in this case the acceleration and deceleration are not constant to calculate the total area under the graph we need to count squares in this case there are 15 complete or almost complete squares we then have to estimate the total of the parts of squares these add up to approximately five squares so the total number of squares under the graph is 20 each square has an area of 250 multiplying 20 by 250 gives us a total distance or displacement of 5 000 M remember you'll find plenty of questions on acceleration and velocity time graphs in my vision workbook and you can get that by clicking on the link above [Music]