Task: Question Generation
Input (English): Friction is the force that opposes motion between any surfaces that are in contact. There are four types of friction: static, sliding, rolling, and fluid friction. Static, sliding, and rolling friction occur between solid surfaces. Fluid friction occurs in liquids and gases. All four types of friction are described below. Static friction acts on objects when they are resting on a surface. For example, if you are hiking in the woods, there is static friction between your shoes and the trail each time you put down your foot (see Figure 1.1). Without this static friction, your feet would slip out from under you, making it difficult to walk. In fact, thats exactly what happens if you try to walk on ice. Thats because ice is very slippery and offers very little friction. Q: Can you think of other examples of static friction? A: One example is the friction that helps the people climb the rock wall in the opening picture above. Static friction keeps their hands and feet from slipping. Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. Thats why its easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil. The pencil lead slides easily over the paper, but theres just enough friction between the pencil and paper to leave a mark. Q: How does sliding friction help you ride a bike? A: There is sliding friction between the brake pads and bike rims each time you use your bikes brakes. This friction slows the rolling wheels so you can stop. Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why most forms of ground transportation use wheels, including bicycles, cars, 4-wheelers, roller skates, scooters, and skateboards. Ball bearings are another use of rolling friction. You can see what they look like in the Figure 1.2. They let parts of a wheel or other machine roll rather than slide over on another. The ball bearings in this wheel reduce friction between the inner and outer cylinders when they turn. Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If youve ever tried to push your open hand through the water in a tub or pool, then youve experienced fluid friction. You can feel the resistance of the water against your hand. Look at the skydiver in the Figure 1.3. Hes falling toward Earth with a parachute. Resistance of the air against the parachute slows his descent. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. Thats why there is greater air resistance against the parachute than the skydivers body.
Output (English): when you put on the brakes of your bike, the wheels stop turning. then friction between the wheels and the road slows your bike to a stop. which type of friction is this?