Science questions in short. [3 marks ]

When a book slides down on the tilted writing desk, a frictional force acts between the book and the surface of the desk. The direction of frictional force on the book is opposite to the direction of its motion and acts in upward direction, as shown in the following figure. Concept insight: Friction opposes the motion.

As we know that nothing is frictionless and so is the ground and our feet, when we walk we push the ground backwards by our foot and the ground in reaction pushes us forward, because of friction present on the ground we are able to push the ground without slipping and hence it is easy to walk on the ground and also pushing it.

A very heavy machine can be moved from one place to another with the help of wheels. Because then rolling friction comes into action and its value is the least among static and sliding friction. The following diagram shows, how a heavy cylinder can be rolled easily.

Cartilage is present in the joints of our body, reduces friction during movement of joints. But on wearing off this cartilage, the force of friction increases due to which the smoothness of movement decreases and one feels the joint pain.

Lubricants (oil or grease) are applied to rubbing surfaces of machines because when the parts rub against each other, heat is produced. To avoid the wearing and tearing of the machine parts, lubrication is done. Because it reduces the friction and thus less heat is produced.

When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. This opposing force is known as the drag force. This frictional force depends on the shape of the body. By giving objects a special shape, the force of friction acting on it can be minimised. Hence, it becomes easier for the body to move through the fluid.

The steps of foot over bridges at railway station gets wear out by time because when we walk the soul of our shoes gets plain and thin by time because it gets worn out and also the stairs or road also gets worn out due to friction there is rubbing between these surfaces and as a result there is worn out of both the surfaces by time but the material of shoes is softer than the stairs, so it gets worn out easily than that of stairs.

When we pull the brake lever the calipers attached to the tyre grabs the rim of the tyre or the disk of the tyre (if there are disk brakes) and as the motion of the tyre is opposed by the friction always, so the friction occurring between the surfaces of the rim and the rubber calipers of the brakes as a result of contact developed between them makes the tyre slow by time and ultimately stops it.

Marble will move the shortest distance on the layer of sand because it will exert a greater force of friction on the marble and other two surfaces like silk cloth and glass sheet will exert lesser friction force comparatively.

Car tyre spins on icy road because of less friction on icy road and due to less friction the force exerted backward by the wheels does not get suitable forward reaction force by the floor and the car does not move forward properly and thus the car tyre slips rather than move along with the car and hence the tyre keeps on exerting force on the floor by spinning and slipping.

A speedboat has a streamlined shape so as to reduce the fluid friction as it moves in the waters.

When step forward and walk, we are applying force (denoted by “F”) on the ground. Now, in return ground also applies the reaction force to us. But we can fall if friction (denoted by “f”) is not present as the large ground can apply a large force. This is explained in the diagram:

While rubbing with sandpaper, more frictional force is produced between the layers of metal sheet and sandpaper which causes more force on dust particles and they are removed easily, so it will shine more. But in case of ordinary paper, the force of friction is not sufficient to remove all the dust, so it will shine less in this case.

A streamline shape is the one in which the body is pointed at the ends and broad-shaped in the middle. Example: Speedboat has streamlined shape.

Whenever we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.

Friction acts always in opposite direction to the direction of motion of body because friction is a reaction force occurring due to the irregularities on the surfaces in contact, so it opposes the relative motion between two surfaces in order to keep the body at rest. Friction is zero when the body is at rest.

Yes, there is always a force of friction between the wheels of a moving train and iron rails. The name of this friction is rolling friction, since the wheels are rolling on the track.
On introducing air cushion, the frictional force becomes less, since there is no contact between rails and wheels.

Ball bearings are used in machineries because due to friction, ball bearings change the type of friction experienced by the machines which means the sliding friction is replaced by rolling friction and as we know that rolling friction is always less than sliding friction, so ball bearings are used as they behave as wheels to roll and not to slip.

When the car is at rest, we have to apply greater force to set the car in motion which value is more. As the car starts moving, the friction changes into the rolling friction which is always less than the previous one. So, we have to exert the lesser force to keep it in motion.

Frictional force is of three types:
Static Friction: It is the frictional force acting on a body when the body has not started moving yet and is at rest, also this friction is equal to the amount of force applied on the body and is variable to a certain limit.
Sliding Friction: It is the friction acting on the body when it is in motion and this friction is constant and is always less than the maximum value of static friction.
Rolling Friction: It is the constant friction acting on the body through wheels, it is the least amount of friction of all and is only applied when the body is moving.

The handle of a cricket bat or a badminton racquet is rough, so that while playing, the bat or badminton racquet does not slip away from the hands of the player. Roughness is responsible for the frictional force between handle of the bat or a badminton racquet and hands, without which gripping is not possible.

Due to the friction between cutting edge of a knife and stone, the temperature of the knife and hence stone increases and it increases to such a level that the sparks are produced which can be seen while sharpening it.