Types of Friction and its Concepts

Advanced Physics Topics like quantum mechanics and relativity have revolutionized our understanding of the universe.

Explanation of Different types of Friction

Friction is of three types :

1. Static friction,
2. Sliding friction, and
3. Rolling friction.

We will now describe the three types of friction in detail, one by one. Let us start with the static friction.

1. Static Friction

The maximum frictional force present between any two objects when one object just tends to move or slip over the surface of the other object, is called static friction. Static friction is a kind of starting friction because an object just tends to start moving, it does not actually move. The object remains static (or stationary) in this case. We can demonstrate the static friction between a wooden block and a table top by performing an activity as follows.

Activity 5
A wooden block (having a hook attached to its one side) is kept on the horizontal surface of a table. A spring balance is attached to the hook of wooden block (as shown in Figure). We pull the spring balance to the right side with a small force of our hand. This will exert a force on the wooden block. The wooden block, however, does not move because its motion is being opposed by the force of friction which acts in the opposite direction (to the left side). As we increase the pulling force applied to the wooden block (through spring balance), the friction also goes on increasing. But ultimately, when the applied force becomes a little more than the maximum frictional force, the wooden block just tends to move or slip on the surface of table.

This means that the frictional force acting between the wooden block and table top has a maximum value beyond which it cannot increase. The force which we are exerting in making the wooden block just tend to move or slip is equal (but opposite) to the force of friction. In Figure we can see that when the wooden block just tends to move or slip on the table top, the spring balance shows a reading of 5 N force (5 newtons force). This means that the magnitude of static friction between this wooden block and table top is of 5 newtons.

From the above discussion we conclude that the force required to overcome friction at the instant an object just tends to start moving from rest, is a measure of static friction. Static friction comes into play when we try to move a stationary object (which is at rest). Please note that in the case of static friction, the object is actually not moving or sliding over the other object, it only tends to move or slide.

2. Sliding Friction

The frictional force present when one object moves slowly (or slides) over the surface of another object, is known as sliding friction. Thus, sliding friction comes into play when an object is sliding (moving slowly but continuously) over another object.

Activity 6
We can demonstrate the sliding friction by extending the above activity further. In the above activity
we have seen that when a certain force (equal and opposite to static friction) is applied by us, the wooden block just tends to move or slip on the table.

Let us now increase the force applied to pull the wooden block a little more (by pulling the spring
balance more). We will see that the wooden block begins to slide (or move slowly) on the table top
(see Figure). The force required to keep an object moving slowly (or sliding) with the same speed is a measure of the sliding friction. It has been found that the force required to keep the wooden block sliding (once it has started sliding) is less than the static friction. In other words, when an object starts sliding, then the friction is less. This means that the sliding friction is smaller than the static friction. For example, in Figure we can see that when the wooden block starts sliding, then the spring balance shows a reading of 4 N force (4 newtons force). This means that the magnitude of sliding friction is 4 newtons (which is smaller than the static sliding friction.

Since the sliding friction is smaller than the static friction, it is easier to keep an object moving which is already in motion than to move the same object from rest (or stationary position). Let us see why sliding friction is smaller (or less) than the static friction. This can be explained as follows : When an object has already started moving (or sliding), the irregularities on its surface do not get enough time to lock into the irregularities on the surface of the other object completely. Since the interlocking of the two surfaces is less when an object has already started moving, therefore, the sliding friction is smaller than the static friction.

3. Rolling Friction

When an object (like a wheel) rolls over the surface of another object, the resistance to its motion is called rolling friction. It is always easier to ‘roll’ than to ‘slide’ an object over another object. So, rolling friction is much less than sliding friction. Thus, rolling reduces friction. We will now describe an activity to demonstrate that it is much easier to roll an object over another object than to slide it. It will also show that rolling friction is much smaller than sliding friction.

Activity 7
Let us keep a thick book on a table. Now push the book with your hand [see Figure (a)]. We will find that it is not very easy to push the thick book lying on the table and make it move (or slide). We have to apply fairly large amount of force to make the book move (or slide) on the table. This is because when the book lying directly on the surface of table moves, then sliding friction comes into play. The large sliding friction between the surface of table and bottom of thick book makes the book comparatively difficult to move. Now, take three round pencils (round pencils are cylindrical in shape and act as rollers). Place the round pencils parallel to one another on the table top. Let us place the same thick

book over the round pencils [as shown in Figure (b)]. Now push the book with your hand. When we push the book, the round pencils kept below it start rolling or turning (like wheels) and make the book move forward easily [see Figure (b)]. We have to apply a very small amount of force to make the book move when it is placed on rollers (round pencils). Thus, it is much more easy to move the book placed on rollers than to slide it directly over the surface of table. It is much easier to move an object kept on rollers than to slide it because rolling friction is much less than sliding friction.

From the above discussion we conclude that when an object rolls over another object, then rolling friction comes into play between them. Rolling friction is much smaller than sliding friction. Since rolling friction is much less (than sliding friction) it is very easy and convenient to pull heavy luggage (like heavy suitcases, etc.) fitted with rollers. Heavy machines can be easily moved from one place to another by placing round logs of wood under

them and then pushing with the force of hands. The round logs of wood act as rollers (a kind of wheels) and make it much easier to move the heavy machine kept on them. Figure shows a large block of stone being moved by the use of rollers. Please note that the rollers turn around like the wheels and take the block of stone forward. Thus, rollers are a kind of wheels. We can now understand why wheel is said to be one of the greatest inventions of mankind. This is because wheels greatly reduce friction (by rolling) and hence can be used to move even heavy objects (like cars, buses, trucks and trains, etc.) rather easily.