ICSE Class 6 Physics Chapter 3 Friction

3 Friction

When we roll a ball on the ground, it goes up to a certain distance with a decreasing speed and then it finally stops. Similarly, if we stop pedalling our bicycle on a plain road, its speed starts reducing and ultimately it stops. Why does the ball or bicycle stop after moving for sometime? It happens because the ground renders an opposing force to both, the ball and the bicycle (acting between the two surfaces in contact of the two bodies). This opposing force is known as the frictional force and is responsible for stopping the ball or the bicycle.

Therefore, friction is that force which opposes the relative motion between the two surfaces that are in contact with each other.

The word friction is derived from a Latin word 'fricare' which means 'to rub'.

For example, if we place a box on a table top and then move it towards right, friction will act between the table top and the box towards left and if we move the box towards left, friction will act towards right. It means that the force of friction opposes the motion of the box. This resisting force between the two bodies in contact which prevents either body to move smoothly and continuously is called friction or frictional force. This is the force that slows down the motion of two surfaces in contact when they move across each other. Friction always opposes the relative motion between the two bodies in contact, irrespective of the direction of motion.

For example, when a block is pulled, by force F, towards left the force of friction acts towards the right and when the block is pulled towards the right the force of friction acts towards the left.

Factors which Affect the Force of Friction

1. Friction depends on the smoothness of the surface

Friction occurs because most surfaces are not perfectly smooth. Even a table top which may appear smooth to you has little bumps on it. These bumps can be seen with a fine microscope. When two surfaces with little bumps move on each other, the bumps interlock and slow down the motion of the surfaces. That is why, friction occurs. Rougher the surface, more will be the friction.

2. Solids, liquids and gases all tend to stop things moving in or over them

They all exert frictional force.

When we play cricket on the ground, grass obstructs the movement of the ball. Moreover, when there is dampness or moisture on the ground, the movement of the ball slows down. This is because the presence of any type of medium - solid, liquid or gas exerts frictional force and thus slows down the movement.

The force of friction between a solid and a liquid surface is less than that between two solids.

The force of friction between a solid surface and a gas is less than that between a solid and a liquid surface.

3. Friction depends on normal reaction of the weight of the object

Greater the weight, greater is the normal reaction. For example, if we apply force on a heavy object, the force of friction is large and if we reduce the weight of the object and apply the same amount of force on it, the force of friction will be less. So, we conclude that more the weight of the object, more the normal reaction and hence more will be the force of friction.

4. Friction does not depend on the area of the surfaces of contact

If two objects with similar weights move in a small area or a bigger area, the force of friction will remain the same. Hence, we can say that friction does not vary with size, provided the weight of the object remains the same.

Properties of Friction

1. Friction is a force that always opposes relative motion, i.e., it slows down and ultimately stops a moving object.

2. Friction always produces heat. The tyres of vehicles become hot after a long drive due to friction between the tyres and the road. Similarly, the moving parts of a machine become hot due to friction.

3. Friction causes wear and tear. Our shoes, tyres of vehicles, parts of machines, etc., wear out due to friction.

Types of Friction

Friction is a self adjusting force up to a maximum limit. When the applied force is gradually increased, the force of friction also increases at the same rate upto its maximum limit and the body remains stationary. This force of friction is called force of static friction or just static friction. If the applied force is increased further, a stage comes when the body begins to just move. At this stage, the force of static friction is maximum.

When an object rolls on a smooth surface, like a ball rolling on a cemented surface or a bicycle, bus, car, etc., moving on the wheels, the friction produced between the rolling object and the surface is called the rolling friction.

When we slide an object on any surface, the friction produced between the object and the surface is called the sliding friction. A glass slab placed on a table top, when moved produces more friction than a rolling object. The sliding object as compared to the rolling object is obstructed by more force due to which sliding object faces more friction than the rolling object. The force required to keep a body in motion is less than the force required to start the motion. Therefore,

Static friction - Sliding friction - Rolling friction.

It is our common experience that when heavy objects are to be moved, we use rollers to make the movement easier. This is because the rolling friction is always less than the sliding friction.

It is easier to roll than to slide a body over another. That is the reason it is convenient to pull luggage fitted with rollers.

Sliding is replaced in most machines by rolling by the use of ball bearings. Common examples are the use of ball bearings between hubs and axles of ceiling fans and bicycles.

Vehicles like cars, trucks, bicycles, etc., are provided with wheels to make their movement easier.

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