hey guys welcome to learn today IGCSE this video is about physics unit 2 motion speed can be defined by distance travel per unit time and velocity speed in a given Direction the difference of speed and velocity is that speed is a scalar quantity and velocity is a vector quantity both speed and velocity can be calculated by the formula distance Over time however in some cases the speed of a moving object is not constant for example the object might be moving faster or slower therefore the equation for calculating the average speed of an object is the total distance traveled over the total time taken next acceleration acceleration can be defined as the rate of change of velocity in other words it describes how much an object's velocity changes every second the equation to calculate average acceleration of an an object is the final speed minus initial speed over time let's look into a worked example here says that a motorcyclist traveling at speed 30 m/s makes a turn compare the speed and velocity between point A and B you can pause this video and try it by yourself first so the speed of the motorcyclist here is constant because the magnitude is the same however the velocity is different as the direction changes and this is because velocity is a vector quantity there are two types of graphs that you should master in this chapter a distance time graph and speed time graph let's look into distance time graph first distance time graph like this one here tells you how far something traveled in a certain period of time for instance the graph here tells us that an object traveled 60 m in 12 seconds the gradient of the graph tells you the speed of the object traveling a straight line represents constant speed a flat horizontal line means the object is stationary since the gradient here is the change of speed over change of time which is the formula of the speed we can calculate the speed for each section A Very steep slope means that the object is traveling at a larger speed and a shallow slope means that the object is moving at a smaller speed however sometimes you don't always see a straight line this is an example of another trend for a distance time graph and a curved line here tells you the speed is changing if the slope is increasing this means that the speed is increasing and if the slope is decreasing this means that the speed is decreasing and to calculate the speed at a certain time you have to first draw a tangent like this take two points and now you will obtain your speed a speed time graph tells us how an object speed changes over time the gradient of the graph tells you the acceleration of the object traveling a straight line represents constant acceleration or constant deceleration and a flat horizontal line means that the object is moving at a constant speed since the gradient is the change of speed over change of time which is the formula of the acceleration we can calculate the acceleration for each section by calculating the gradient however again sometimes you don't always get a straight line and if you see a curved line it tells you that the speed is changing an increasing slope means that the acceleration is increasing and a decreasing slope means that the acceleration is decreasing and to calculate the gradient you have to first draw a tangent like this now take two points and you will obtain your acceleration an extra information that can be found in speed time graph is is the distance traveled the distance traveled by an object can be found by determining the area beneath the graph for example the distance traveled for the first 3 seconds is the area of this triangle and you will obtain 75 M and for the second part is the shape of a rectangle and you can obtain the dimension of the rectangle to get the area and repeat the step for the last part and the total value here will give you the total distance traveled in in the period of 11 seconds the last part that you should know is to describe the motion of objects falling in a uniform gravitational field with and without air or liquid resistance this is the graph representing the motion of an object falling in a uniform gravitational field with air resistance a sky diver jumping from a plane will experience a downward acting Force which is the weight and an upward acting Force which is the air resistance initi ially when the sky diver jumps off the plane the upwards air resistance is very small because the sky diver isn't falling very quickly therefore there are unbalanced forces on the sky diver initially and he accelerates as the sky diver speeds up the air resistance increases eventually growing large enough to balance the downwards weight Force once the air resistance equals to weight the fossils are balanced this means there is no longer any resultant force and the acceleration is zero they now travel at a constant speed this speed is called as terminal velocity when the sky diver opens the parachute the air resistance increases this is due to the increased surface area of the parachute opening the upward force of air resistance on the sky diver increases slowing the acceleration of the skydivers fall so the sky diver now decelerates eventually the forces balance out again and a new slower terminal velocity is reached and finally the speed decreases until zero when the skyd lands on the ground remember that object fall towards the Earth due to the acceleration of freef fall which equals to 9.8 m/s squared okay let's look at an example question astronauts and skydivers both experience freefall but their motions differ to various factors part one describe the motion of an astronaut jumping out of a space station tree marks so over here you need to know the motion of an object falling without air resistance you can pause this video and try this question so for the first Mark you could say that there is no air resistance in space it's mentioned previously that during freef fall there are two type of forces since there is no air resistance the only force acting on the astronaut is the weight hence the astronaut will fall with an increasing speed part two sketch a speed time graph for the motion of the astronaut for the first Mark label your access with units correctly and for the second Mark you should have a steady rate of the speed getting larger the longer it falls for from your specification cost you can check that all the subtopics in motion is covered that's all for this video thank you for watching till the end I hope it was worth your time for the next video I will be discussing on unit three mass and weight thank you bye-bye