KS3 Physics - Friction

In this video we're learning about friction, so we'll cover what friction is, and then take a look at the advantages and disadvantages of friction. Let's begin with what friction is. Friction is a force that occurs when two surfaces move or try to move across each other. This makes it a contact force, which means it only occurs when the surfaces are touching. For instance, when you slide a book across a table, the movement of the book doesn't carry on forever because the friction between the book and the table slows it down and eventually stops it. It's also important to note that friction doesn't just happen between solid objects, like books and tables though. It also happens between solids and fluids, like air and water, and when this happens, it's called drag. For example, when a car drives through air, it has to push air particles aside that collide with the car's surface, creating air resistance, which is a type of drag. Similarly, when a boat moves through water, it has to push water particles aside, creating water resistance, another type of drag. And when that object moves faster, it has more collisions with the fluid particles per unit of time, and so drag increases. That's why it's harder to run through water than to walk through it. When you try to move faster, your body collides with more fluid particles that it needs to push out of the way, so there's more drag resisting your movement. Next, let's talk about the advantages and disadvantages of friction. One of the advantages of friction is that it allows movement to stop. For example, friction between the tyres of a car and the road surface allows the car to brake and to change direction. Without friction, the car would keep moving even when you tried to stop. A similar advantage is that friction prevents slipping. For instance, there's friction between the wheels of a bike and the ground that prevents tyres from slipping, and there's also friction between your hands and the handlebars that keeps your grip firm and stops your hands from slipping off. But friction isn't always helpful. One of the disadvantages of friction is that it wastes energy. When friction occurs, for instance, between a car's tyres and the ground, it converts energy into thermal energy that's usually wasted as heat, rather than being used to move the car forward. Friction also limits the top speed of an object, because remember, high speed causes large air resistance or water resistance, depending on what the object is moving through. This is especially true when objects are not streamlined, and is why vehicles like cars and aeroplanes are designed to be more streamlined, so that they are more aerodynamic. This minimises the force of friction by allowing air or water to flow smoothly over the vehicle's surface, reducing the number of particle collisions that lead to drag. Because there's then a smaller amount of air resistance, the object can move faster and more efficiently. If you haven't heard yet, you can find all of our videos on our website, cognito.org. You'll also find questions, flashcards, exam-style questions, and past papers. And we track all of your progress, so that you always know what to study next. So sign up for free by clicking here, or browse our playlist here on YouTube.