Science Question Answer (5 Marks)

The drag is the other name for fluid friction. It resists the motion of any object that travels in fluid.

Examples:

1. A boat travelling in water.
2. An aeroplane moving in the sky.

If we oil between the two surfaces, say part1 and part 2 of a machine, then the oil layer will allow less contact of the two surfaces with each other as you can see in the figure below:

In this way, the friction between the two surfaces is decreased.

Suppose, a person is pushing a heavy box, then he is applying say force F1.

Now if the same person pushes two boxes of same mass as the previous case. Then, in this case he will have to apply more force to overcome the larger friction as in the earlier case. Thus is because friction depends on the mass of the object. Heavier the mass, more the force is exerted by the object on the surface and thus more friction is produced.

The cars, aeroplanes and rockets are streamlined so as to reduce the fluid friction. The air drag obstructs the vehicle to move forward and the fuel is consumed mainly for opposing this opposite frictional force. Therefore, the objects are given special shape called streamline shape, which itself cuts the air friction.

1. A pencil is made up of graphite. The paper surface offers more friction to the pencil when we write on the paper, in this way when the pencil is rubbed on the paper, some bits of graphite come on the paper and we are able to write. But glass offers no friction to the pencil, therefore we can’t write on a glass with pencil.
2. Climbing a greasy pole is very difficult because the greasy pole offers very less friction to the feet of the person climbing on it. Du to which, the person slips down again and again.

The force that opposes the motion of an object is called friction. Example, when we walk, friction acts in opposite direction we apply force to walk.
The factor affecting friction are:

1. The weight of the object.
2. The type of contact between the two surfaces.

Disadvantages of Friction:

1. Friction always opposes he motion.
2. Friction causes wearing and tearing.

Advantages of Friction:

1. Friction help us to walk.
2. A horse can’t pull a cart without friction.

The cause of friction is surface roughness and adhesion.
When we apply force, F on a book, the book slide (Fig A). Then acts a force of friction, f (Fig B).

The friction is produced due to the contact of the two surfaces of the table and the book. It acts in opposite direction of the force to stop it. And, we see after some distance, the book again comes to the rest (Fig C).

When the body is in stationary state or is at rest, the frictional forces acting on the body is known as static frictional forces.
But when the object is pushed and it moves, the friction that comes into action is sliding friction.

The static friction is greater than sliding friction because more force is required to remove the object from rest state into dynamic state. Once the object comes into motion, it is easier to keep the object moving by applying comparatively lesser force.

The friction depends on the roughness as well as smoothness of the surface. If the surface is rough, then the surfaces in contact have more interlockings and the thus the friction is more. Thus, the object is not able to move easily due to more friction.

If the surface is smooth, then the surfaces in contact have very less interlockings and the thus the friction is less. Thus, the object is able to move easily due to less friction.

The sliding friction is less than static friction because, in sliding friction, the interlinking of the two surfaces is for very short time.
Friction mainly arises due to the interlocking of irregularities between two surfaces, as shown below in the diagram:

In smooth surfaces, there are very fewer irregularities, thus friction is very very less. But, on rough surfaces, the irregularities are very large.
In sliding friction, the time of interlocking is very less, as one object slides over another object. Whereas, in static friction, one object is moved over another object thus the irregularities time is more. Thus, the two surfaces could not properly come in contact with each other in sliding due to which the magnitude of static friction is more than sliding friction.

When no person is sitting on the mat, then the mat and floor are not pressed together.

Thus, there is no friction.
But, when a person is sitting on the mat.

Then due to the weight of the person, the mat and floor are pressed together harder, increasing the friction too much. Thus, then it is very difficult to drag the person.

1. Sliding friction.

2. Static friction.

3. Rolling friction.

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