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The laws of Physics Topics are used to explain everything from the smallest subatomic particles to the largest galaxies.
Frictional force always acts in which direction?
When we push a box lying on the floor with a small amount of force, it does not move at all (see Figure). It means that the surface of floor, on which the box is resting, exerts some force on the box which acts in a direction opposite to the force of our push. In other words, some force is acting on the bottom of the stationary box which opposes Figure. When we push the box with a small its motion (due to which the box force, the friction between floor and bottom of box does not move). prevents it from moving.
This natural force between the floor and the bottom of the box which opposes the motion of box on the floor is friction (see Figure). Let us take another example. A ball moving on the ground slows down gradually and stops after covering some distance. We know that a force is required to stop a moving ball. This means that a force is exerted by the ground on the moving ball which opposes its motion and brings it to a stop.
This force which opposes the motion of ball on the ground is also friction. We can now define friction as follows: The force which always opposes the motion of one object over another object in contact with it, is called friction. Friction occurs between the two surfaces which are in contact with each other. For example, when we push a heavy box kept on floor, the force of friction occurs between the surface of floor and bottom of the box. And in the case of a ball rolling on ground, friction occurs between the surface of ground and the surface of ball. In fact, friction acts on both the surfaces in contact with each other. Friction is a force which occurs when the two objects tend to slide over each other and even when they are actually sliding (moving) over each other.
Direction of Force of Friction
The force of friction always opposes the motion of one object over another object. So, the force of friction acts in a direction opposite to the direction in which an object moves (or tends to move). This will become more clear from the following activity.
Activity 1
Place a book on the table. Give a push to this book towards the right side [as shown in Figure (a)]. We will find that the book moves through some distance to the right side and then stops. Since the book moving towards right side stops on its own, this means that the force of friction is acting on it in the opposite direction (towards left) which is opposing its motion and making it stop. Thus, when the force is applied by our push to move the book towards right side, then the force of friction acts towards left side (in opposite direction to motion of book) [see Figure].
Let us now give a push to the book towards the left side [as shown in Figure (b)], We will find that the book moves some distance towards the left side and then stops. Since the book moving towards left side stops on its own, this means that the force of friction is acting on it in the opposite direction (towards right) which is opposing its motion and making it stop. Thus, when the force is applied by our push to move the book towards left side, then the force of friction acts towards the right side (in opposite direction to the motion of book) [see Figure (b)].
In the above activity we observe that when the motion of the book is towards right side, then the force of friction acts on it towards the left side. And when the motion of book is towards left side, the force of friction acts on it towards the right side. So, from this activity we conclude that the force of friction acts in a direction opposite to the direction of motion of an object. This is why the force of friction always opposes the motion of an object/In both the cases described above, the force of friction opposes the motion of book on the surface of the table. This force of friction occurs between the surface of table and the surface of book in touch with each other.
In the above examples, we have applied the force of push of our hand to move a book lying on a horizontal table top. The book can also be moved by the force of gravity of earth provided the table is tilted a little. So, if we tilt the table somewhat, then the book kept on it will start sliding down slowly. In this case, the force of gravity is acting in the downward direction (see Figure). Since the book is moving in the downward direction, the force of friction must act on it in the opposite direction—upward direction (as shown in Figure).
Cause of Friction
Every object has a rough surface, though the surface may appear to be smooth to the naked eye. When we see through a microscope, it is found that the surfaces of all the objects have rough edges. Some of the particles on the surface of objects are in the form of tiny hills while others form grooves (see Figure). The tiny hills and grooves on the surfaces of objects are called ‘irregularities of surfaces’. When we try to move one object over another object, the ‘irregularities’ present on their surfaces get entangled (or locked) with one another (see Figure).
The interlocking of irregularities of the two surfaces opposes the motion of one object over the other and gives rise to force of friction. Thus, friction is caused by the interlocking of irregularities in the surfaces of the two objects which are in contact with each other. When we attempt to move one object over the other, we have to apply a force to overcome interlocking of the irregularities in their surfaces. More the roughness of a surface, larger is the number of irregularities on its surface and hence greater will be the friction. Thus, the force of friction is greater if very rough surfaces are involved.
In Figure we have shown a highly magnified diagram of the surfaces of two wooden blocks A and B kept one over the other. Please note the rough surfaces of these wooden blocks having lot of irregularities. When we pull the upper wooden block A over the lower wooden block B by applying a force, then the irregularities of their rough surfaces get entangled (or locked) with one another. This interlocking of surfaces gives rise to force of friction which opposes the motion of upper block A over the lower block B. We are able to move block A over block B because we apply sufficient muscular force while pulling to undo the interlocking of surfaces and overcome the opposing frictional force. The block A moves over block B only when the pulling force applied by us becomes greater than the force of friction holding them together.
Friction is small for smooth surfaces (like glass and ice). Friction is much greater for rough surfaces (like sand paper and concrete).
Before we go further, we should know what a spring balance is because it will be used to perform some activities based on friction. The spring balance is a device which is used for measuring force acting on an object (see Figure). The spring balance contains a coiled spring which gets stretched when a force is applied to its free end (having a hook).
The extent by which the spring gets stretched is a measure of the force applied. Larger the stretching of spring, greater will be the magnitude of force applied. The stretching of spring or magnitude of force is indicated by a pointer attached to the spring which moves on a graduated scale. The reading on the scale of spring balance (as indicated by the position of pointer) gives us the magnitude of force.
When the spring balance is held vertically (as shown in Figure), it is said to measure the weight of an object hung from its hook (because weight of an object is also a force). And when a spring balance is held horizontally (attached to an object and pulled), it can be used to measure the force Figure. A spring being applied to pull the object on a horizontal surface.
Factors Affecting Friction
It has been found by experiments that the friction between two surfaces depends on two factors :
- the nature of the two surfaces (smoothness or roughness of the two surfaces).
- the force with which two surfaces are pressed together.
The force of friction, however, does not depend on the ‘amount of surface area’ of the two objects which is in contact with each other. We will now study how the friction depends on the nature of two surfaces as well as on the force with which the two surfaces are pressed together.
1. Dependence of Friction on the Nature of Two Surfaces
Friction is not the same for all the surfaces. Friction depends on the smoothness or roughness of the two surfaces which are in contact with each other. When the two surfaces in contact are smooth, then the friction between them will be small (because the interlocking of smooth surfaces is less). As the degree of roughness of the two surfaces in contact increases, the friction also increases. And when the two surfaces in contact are very rough, then the friction between them will be very large (because the interlocking of very rough surface is too much). We can study the dependence of friction on the nature of surfaces by performing some activities as follows.
Activity 2
Place a brick on the floor. Tie a string (strong thread) around the brick and connect it to the hook of a spring balance. Apply a pulling force to the brick by pulling the other end of spring balance by hand till the brick just begins to slide (move slowly) on the floor (see Figure). Note down the reading on spring balance when the brick begins to slide. This reading of spring balance will give us the magnitude offeree of friction between the surface of floor and the surface of brick (which are in contact with each other).
(i) Let us now wrap a piece of polythene around the brick and repeat the above activity. We note the spring balance reading when the polythene wrapped brick just begins to slide on the floor. We will find that this reading of spring balance is smaller than the first reading of spring balance (when there was no polythene around the brick) indicating that the force of friction has decreased. From this observation we conclude that wrapping of polythene sheet makes the surface of brick smooth due to which the friction with floor decreases.
(ii) Remove the polythene sheet from the brick. We now wrap a jute cloth around the brick and repeat the above activity once again. We note the spring balance reading when the jute wrapped brick just begins to slide on the floor. In this case we find that the reading of spring balance is greater than the first reading of spring balance (when nothing was wrapped around the brick) indicating that the force of friction has increased. From this observation we conclude that wrapping of jute cloth makes the surface of brick more rough due to which the friction with floor increases.
From the above activity we learn that when the surface of brick is made more smooth by wrapping polythene sheet, the friction with floor decreases. On the other hand, when the surface of brick is made more rough by wrapping a jute cloth, then the friction with floor increases. Thus, the friction depends on the nature of two surfaces That is, the friction depends on the smoothness or roughness of the two surfaces.
Activity 3
Make an inclined plane on a smooth marble floor by keeping a wooden board in tilted position with the help of a brick placed behind the wooden board (see Figure). Mark a horizontal line AB with a ball pen on the upper half of the inclined wooden board (as shown in Figure). Hold a pencil cell on the line AB marked on the inclined wooden board and then release it. The pencil cell will move down rapidly from the inclined board and travel a certain distance on the marble floor before coming to rest (or stopping) (see Figure). The moving pencil cell stops due to friction exerted by the marble floor. We note the distance covered by the pencil cell on the marble floor (from the base of the inclined board). This distance will give us an idea of the friction between marble floor and pencil cell.
(i) Let us now put some water on the marble floor to make it wet. We repeat the above activity by releasing pencil cell from the same line of inclined board. Again the pencil cell will travel a certain distance on wet marble floor before coming to rest. We note the distance travelled by the pencil cell on wet marble floor. We will find that the pencil cell travels a larger distance on the wet marble floor indicating that the friction on wet marble floor is less (than that on dry marble floor).
(ii) We make the wet marble floor dry by wiping it. Let us now spread a sheet of newspaper on the dry marble floor. Repeat the above activity by releasing the pencil cell from the same marked line AB on the inclined board and note the distance travelled by the pencil cell before coming to rest. We will find that the pencil cell travels less distance on the newspaper sheet (than that on the dry marble floor) indicating that the friction exerted by newspaper is more than that exerted by dry marble floor.
(iii) Let us remove the newspaper and spread a towel on the marble floor. Repeat the activity once again by releasing the pencil cell from the same marked line AB on the inclined board and note the distance travelled by the pencil cell before coming to rest. We will find that the pencil cell travels the least distance on the towel indicating that the friction exerted by towel is even greater than that of newspaper.
From the above activity we see that when the surface of marble floor is made more smooth by making it wet with water, the friction with pencil cell decreases. On the other hand, when the surface of marble floor is made more rough by covering it with newspaper or towel, then the friction increases. Thus, the friction depends on the nature (smoothness or roughness) of the two surfaces.
2. Dependence of Friction on the Force With Which Two Surfaces are Pressed Together
Friction is caused by the interlocking of irregularities of the two surfaces when one object is placed over another object. lithe two surfaces of objects are pressed together harder by a greater force, then the friction will increase (because pressing together two surfaces of objects with a greater force will increase the interlocking in the two surfaces). This will become more clear from the following activity.
Activity 4
Suppose we have two boxes of the same size but one box is light and the other box is heavy (see Figure). If we push both the boxes on the floor, one by one, we will find that we have to apply only a small force to make the lighter box move on the floor but a much larger force has to be applied to make the heavier box move on the floor. This shows that there is less friction between the light box
and floor but much more friction between the heavy box and the floor (see Figure). We know that the weight of a box is also a force (which acts in the downward direction). Now, because of its smaller weight, the light box presses on the floor with less force and hence the friction between lighter box and floor is less [see Figure (a)].
This lesser force of friction allows the lighter box to be moved easily by applying a smaller push. On the other hand, because of its greater weight, the heavy box presses on the floor with a greater force. Since the surfaces of heavy box and floor are pressed together harder (with a greater force) the friction between them increases and becomes much greater [see Figure (b)]. This greater friction does not allow the heavy box to be moved on floor by applying a small force. A much larger force of our push has to be applied to make the heavy box move on floor.
From the above activity we conclude that the friction between two surfaces depends on the force with which the two surfaces are pressed together. In general, greater the weight of an object which moves over another surface, greater will be the friction between them. The force of friction increases when the two surfaces are pressed together harder (because of the greater weight of one of the objects) because then the interlocking of the irregularities of their surfaces increases. We will now discuss static friction, sliding friction and rolling friction.