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Friction Homework

Friction is the force that develops at the surfaces of contact of two bodies and impedes (opposes) their relative motion. e.g. air on an aircraft or water in a pipe.

It is not a fundamental force, as it is derived from electromagnetic forces between atoms and electrons, and so cannot be calculated from first principles, but instead must be found empirically.

When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or heat.

The force of friction between two bodies depends on the magnitude of the perpendicular forces of the surfaces in contact. This perpendicular force is known as the normal force (R) and the ratio of the frictional force to the normal force is known as the coefficient of friction.

The deformed surface pushes on the body with a normal force R that is perpendicular to the surface. This is called normal reaction. It balances mg i.e.

R = mg

Suppose a small force P is applied on the block to the right as shown. The force of friction F opposes the motion.

So long as the block does not move,

F = P

This means as we increase P, the friction F also increase, remaining equal to P always.

 Types of friction

  1. Internal friction

It arises on account of relative motion between every two layers of a liquid. Internal friction is also referred to as viscosity of the liquid.

      2.    External friction

It arises when two bodies in contact with each other try to move or there is an actual relative motion between the two. The external friction is also called contact friction.

External friction is of three types:

     1. Static friction

The opposing force that comes in to play when our body tends to move over the surface of another, but the actual motion has yet not started.As seen in the above figure, the magnitude of the static friction is not constant. It always adjusts itself so as to be equal to the applied force.

      2. Limiting friction

As we increase the applied force, a stage comes when the body is just at the verge of the moving over the other. The static friction at this stage is obviously maximum. This maximum value of static friction is called Limiting force.

      3. Kinetic friction

When we increase the applied force slightly beyond limiting friction, the actual motion starts. This does not mean that friction has disappeared. It only means that the applied force is now greater than the force of limiting force. The force of friction is stage is called kinetic friction or dynamic friction.

Kinetic friction is always slightly less than the limiting friction. This is because once, the motion starts actually; inertia of rest has been overcome. Also, when motion has actually started, irregularities of one surface have little time to get locked again into the irregularities of other surface.

Further kinetic or dynamic friction may be of types:

i.  Sliding friction – The opposing force that comes into play when one body is actually sliding over the surface of the other body is called sliding friction. For example, when a flat block is moved over that flat surface of a table , the opposing force is called sliding friction.

ii.  Rolling friction – The opposing force that comes into play when one body is actually rolling over the surface of the other body is called rolling friction. For example, when a wheel , a circular disc or a ring or a sphere or a cylinder rolls over a surface , the force opposing it is the rolling friction.


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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 force 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