Friction and resistance
What are friction and resistance?
Friction is a force, the resistance of motion when one object rubs against another.
Whenever two objects rub against each other, they cause friction. Friction works against the motion and acts in the opposite direction. The amount of friction depends on the materials from which the two surfaces are made. The rougher the surface, the more friction is produced. For example, you would have to push a book harder to get it moving on a carpet than you would on a wooden floor. This is because there is more friction between the carpet and the book than there is between the wood and the book.
One kind of friction or resistance is air resistance. Air resistance occurs between the surface of a falling object and the air that surrounds it and it also works to slow the rate at which the object falls. Air resistance works with surface area, so the more surface area, the more air resistance. Think about when you drop two pieces of paper: one crumpled and one flat. The crumpled one falls faster because there is less air resistance acting on the paper.
Friction can be useful. For example, friction between our shoes and the floor stop us from slipping and friction between tyres and the road stop cars from skidding.
Friction is sometimes unhelpful. For example, if you don't lubricate your bike regularly with oil, the friction in the chain and axles increases. Your bike will be noisy and difficult to pedal.
When there is a lot of friction between moving parts, energy is lost to the surroundings as heat. Think of what happens when you rub your hands together quickly. The friction warms them up.
Top 10 facts
- Although wheels are great for rolling and reducing friction, they couldn't work without friction.
- It would be really tough just to stand up without friction.
- Friction can generate static electricity.
- The harder two surfaces are pressed together, the more force it takes to overcome the friction and get them to slide.
- Fluid friction is used a lot in water parks so we can slide smoothly and fast down giant slides.
- Racing cyclists crouch down low on their bikes to reduce the air resistance on them. This helps them to cycle faster. They also wear streamlined helmets. These have special, smooth shapes that allow the air to flow over the cyclist more easily.
- Ice causes very little friction, which is why it is easy to slip over on an icy day. However this is a good thing for ice skating and sledging.
- When there is a lot of friction between moving parts, energy is lost to the surroundings as heat. 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.
Did you know?
- 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.
Look through the gallery and see if you can spot the following:
- 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
When one object is sliding on another it starts to slow down due to friction. This means it loses energy. However, the energy doesn't disappear. It changes from moving energy (also call kinetic energy) to heat energy. This is why we rub our hands together when it’s cold. By rubbing them we generate friction and, therefore, heat. Friction is the resistance of motion when one object rubs against another. Anytime two objects rub against each other, they cause friction. Friction works against the motion and acts in the opposite direction.
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. Machines and engines use grease and oil to reduce friction and wear so they can last longer.
- A third way to reduce friction is with less surface area. This is how ice skates work. A thin blade allows for little friction between the skate and the ice. Ice skates also use lubricant in that the ice melts beneath the weight of the blade using water to allow for the skates to slide.
Friction is also a great help to us. After all, we would all just be sliding around everywhere if there wasn't friction to keep us steady! Friction is also used in car brakes, when we walk or climb a hill, in sandpaper, making a fire, and more.
Friction has many uses in life. You light a match using friction. As you strike a match, friction creates enough heat to ignite a chemical compound in the match head that then burns the rest of the match head. Car brakes work because of friction. As the brake pads rub against the car’s wheels, the car slows down. Shoes designed for some sports have special soles to use friction to your advantage. Football boots have studs to increase friction by sticking to cracks in the ground. A violinist puts rosin on his bow to increase friction between the bow and the violin strings, therefore producing sound.
However, friction can also be a real nuisance. If a door hinge squeaks, the noise is caused by friction. The moving parts of a car’s engine rub against each other and can stick together, causing the engine to seize and to stop working. Using oil in a car’s engine protects the parts from friction. Cooked foods tend to stick to pans. Teflon on non-stick cookware reduces friction between the food and the pan, causing the food to slide. Competitive swimmers wear specially designed racing suits to reduce the friction between themselves and the water so that they can swim faster. Silicone aerosols, oils, grease and ball bearings are all used to reduce friction.
Air resistance force is the force of air pushing against a moving object. Air resistance (also called drag) is a type of frictional force. Like all frictional forces, the force of air resistance always opposes the motion of an object. Usually, the air resistance force is not very strong. When you walk, for example, you are affected by air resistance force, but it does not really slow you down. If you start running, the air resistance force will become more noticeable.
Words to know:
Acceleration - the rate at which something increases in velocity or speed
Atmosphere - the mixture of gases that surrounds an astronomical object such as the Earth
Collision - the action of two moving vehicles, ships, aircraft, or other objects hitting each other
Compressed - to make something smaller by applying pressure or a similar process, or become smaller in this way
Energy - a supply or source of electrical, mechanical, or other form of power
Friction - the rubbing of two objects against each other when one or both are moving
Kinetic - relating to, caused by, or producing motion
Lubricant - a substance, typically oil or grease, applied to a surface to reduce friction between moving parts
Material - the substance used to make things
Molecules - the smallest unit of a substance that can exist
Motion - the act or process of moving, or the way in which somebody or something moves
Rate - the speed at which one measured quantity happens in relation to another measured amount such as time
Resistance - a force that opposes or slows down another force.
Rosin - a hard resin ranging in colour from amber to dark brown. Used for varnishes and other products, to increase friction, e.g. between the bow and strings of some stringed instruments.
Static electricity - a stationary electric charge that builds up on an insulated object such as a thunder cloud
Streamlined - to design or build something with a smooth shape so that it moves with minimum resistance through air or water
Surface area - a solid flat area
Just for fun...
- Make your own balloon hovercraft to understand more about friction
- A BBC Bitesize friction game for KS2 children
- Friction ramp game
- Play the BBC's MI High Friction game
- Fun with friction
- Investigate friction facts
- Play Skydive from the Stratosphere to understand more about friction
- Complete the DK findout! friction quiz
Children's books about friction and resistance
Find out more:
- Clear notes about friction for KS2 children
- More about friction for older children
- A friction worksheet and investigation
- Learn more about car safety, crash test dummies and how forces act on cars
- Forces in Action from the BBC: Friction