Welcome students, I am Mr. Buskerini and for a unit on forces and motion, today's lesson will be about friction and breaking. Let's imagine your driving along the road is suddenly a cat passes in front of you. Obviously, you start to break as soon as possible. Today we are going to see what factors will affect the probability that you hit or not hit the cat. That will include your reaction time, speed, that will include also how in shape your car is. The first factor you have to take into account is what we call thinking distance. Now, as soon as you see the cat, you start thinking, okay, I need to stop. But there is an amount of time between when you actually see the object and you start doing something namely pressing the brake pedal, during which your car is still going at exactly the same speed. Now, that amount of time is called reaction or thinking time. And the distance that is covered by your car as it travels at the same speed while your mind and your body are reacting to that visual input is called thinking distance. So that is how far the car travels before you actually start breaking. And that, by using the magic triangle that makes distance speed and time, we can easily see that distance is equal to speed times the time where V is the speed of your car. And T is what I call previously the thinking or reaction time. Now, reaction time can be affected by a lot of factors. The most common ones are how awake you are. So if you are tired, your reaction time will become bigger. Your reactions are slow. Your reaction time increases also when you are distracted. For instance, if you are texting someone or your cell phone or you are talking with someone or looking around, so every time you are distracted by something, your reaction time increases. And finally, your reaction time increases if you are under influence of drugs, alcohol, etc. All these factors will increase by bigger small amount of your reaction time. Therefore, it will increase your thinking distance. Okay, at this point, you have reacted. You are starting to apply the brakes on your car. So the car is slowing down and eventually will stop. Now, the distance covered in the second part of the movement of your car is called the braking distance. So that is the distance how far your car travels as soon as you start braking until the car stops. And the obvious factors that affect your braking distance, the most important one is your speed. The higher your initial speed, the longer it will take your car to slow down. And that is again why we have speed limits. Also, it is affected by the state of the road. If the road is wet or even icy, the braking distance will be bigger. Finally, it will depend on the state of your car. And namely, how well you shape your tires and how good your brakes are. So these are all things that are going to affect your braking distance longer or shorter. So my life, let's put these two distances together. If we are going to graph the speed time of a car which is first, you have a car, let's say the cat in front of you. So you are reacting to the fact that there is a cat in front of you. So you start braking. If you represent that in a graph, this is how well it will look like. At the beginning, when you are still reacting, the car is going at exactly the same speed. So this is what happens to your reaction or thinking time. Then you start to apply your brakes and let's assume that that will affect with a constant deceleration of your car. Until it stops. And this will be the braking time. Together, we call this stopping time. And related to that, we have a stopping distance. If you have a graph like this, it's very easy to find the stopping distance. Because again, remember, the distance can be derived from a speed time graph by looking at the area under the graph. So this area will be the thinking distance. This area will be the braking distance. And these two sum together will be the stopping distance. So what was the learning goal of this lesson? By the end of this lesson, you should be able to identify the factors that affect the thinking distance and the braking distance. Our next and last lesson, and actually we have a new unit about forces and pressure, will be about turning forces. So what happens, where a force is not pushing or pulling, but it's making an object go around a point.