What makes something have more friction

One of the most obvious effects of friction for students is that it can lead to the gradual wearing away of things; this can be seen in examples like the uneven soles of our shoes, balding bicycle tyres and the deliberate action of rubbing sandpaper on a surface.

Friction is a confusing influence in our everyday experiences of motion because people are often not aware that friction involves forces. For those who think that things move because they contain a force that keeps them moving, friction can be seen as responsible for gradually using up this force. The alternative conception of forces being contained in objects is discussed in the focus idea Pushes and pulls.

Scientists are not completely sure what causes friction; however it is thought to be caused by the interactions between the tiny bumps on surfaces as they rub against each other. The bumps on each surface bend and exert a force on each other making it hard for the surfaces to slip over each other. The force on the moving surface is in the opposite direction and resists the motion. As an example, the surface of a car tyre the tread pattern is designed to maximise the friction between the tyre and road surface over a wide range of weather conditions.

When the friction is substantially reduced because of snow or ice on the road, the tyre slips and is unable to steer, drive or break the car.

Explore the relationships between ideas about forces and friction in the Concept Development Maps - Laws of Motion. Friction can slow things down and stop stationary things from moving. In a frictionless world, more objects would be sliding about, clothes and shoes would be difficult to keep on and it would be very difficult for people or cars to get moving or change direction. Students should be encouraged to consider how dependant their world is on the beneficial action of friction.

However, the point is that contact area does indeed matter. I am talking about contact area, not surface area. Suppose you put a rubber ball on a glass plate. As you push down on the rubber ball, it will deform such that more of the ball will come in "contact" with the glass. Here is a diagram of this. Greater contact area means greater frictional force.

If the contact area is proportional to the normal force, then this looks just like Amontons' Law with the frictional force proportional to the normal force. Of course this model "breaks" when the contact area can no longer increase. As I add more and more mass onto the friction box, there is less and less available contact area to expand into. In a sense, the contact area becomes saturated. I suppose that if I kept piling on the weight, the friction force would eventually level out and stop increasing.

This really isn't a big deal. The Amontons' Law isn't a law at all ok - it depends on your definition of Law. It's just a model. Let me give an example. Gravitational Model. Near the surface of the Earth, we can calculate the gravitational force on an object using the following model. The g vector is the local gravitational field. On Earth, it points "down" and has a magnitude around 9. We often call this gravitational force the weight and it's a very useful model.

Even though this model is useful, we still know it's wrong. The above gravitational model says that it doesn't matter how high above the surface of the Earth you are, the weight is the same. Of course that's not true, but it's approximately true when close to the surface. This says that the gravitational force decreases as the two interacting objects get further away from each other. If you put in the mass of the Earth and the radius of the Earth you get a weight that looks just like the mg version.

So, at some point the two versions of gravity agree. The same is true for friction. The introductory physics version of friction works for some stuff and a more complicated version of friction works for other cases. However this is a good thing for ice skating and sledging. When you rub your hands together quickly the friction warms them up. Slippery substances such as oil reduce the friction between two surfaces.

This is known as lubrication. Have you ever felt as though you were walking in place when you were trying to walk into a strong wind? The air resistance is working against the force applied by your legs opposing motion and reducing acceleration.

Start your child on a tailored learning programme Weekly resources sent direct to your inbox Keep your child's learning on track. Trial it for FREE today. There are three different types of friction: Dry Friction - Dry friction occurs when two solid objects touch each other. If they are not moving, it is called static friction. If they are moving, it is called kinetic or sliding friction. Fluid Friction - Fluid friction involves a fluid or air. The air resistance on an airplane or water resistance on a boat is fluid friction.

Although liquids offer resistance to objects moving through them, they also smooth surfaces and reduce friction. Rolling Friction - Rolling friction occurs when a round surface rolls over a surface, like a ball or wheel. There are 3 main factors that will influence the total amount of friction: The roughness of the surfaces The weight of the object The surface area how much is touching. Friction only happens with solid objects , but you do get resistance to motion in both liquids and gases.

This doesn't involve sliding surfaces like friction does, but is instead the kind of resistance you get if you try to push your way through a crowd. It's a colliding situation, not a sliding one. If the gas is air, this is referred to as air resistance. If you were in a space shuttle and re-entering the atmosphere, the bottom of the shuttle would be getting very hot.

The collisions that occur between the molecules of the air being compressed by the shuttle, heat up the air AND the shuttle itself. The temperature on the top of the shuttle is also warm, but nowhere near the temperatures found on the bottom. Nobody completely understands what causes friction.

Partly, friction happens when the rough edges of one object snag on the rough edges of another object, and some of the objects' energy has to be used to break off those rough edges so the objects can keep moving. And when you rub two soft things together, like your hands, sometimes they squish into each other and get in each other's way. But even completely smooth, hard things have some friction. This friction is the result of the molecules in both objects being attracted to each other.

Friction and resistance gallery: Car tyres are engineered to use friction when braking Friction around us We need friction to light a match!

How will different terrain, like gravel, affect friction? Air resistance is a form of friction The heat generated by the ice skates' blades makes some of the ice right under the blade melt; the water reduces friction under the skate and helps it slide Rubber soles on running shoes create friction, stopping us from sliding when we run!

The water on this slide reduces the friction, making it easier to slide Less friction can be dangerous! How air resistance affects a plane. In some cases we want to prevent friction so it's easier to move.

A good example of this is a ball or wheel. They roll to help reduce friction. Another way to reduce friction is with a lubricant like grease or oil.