Forces And Pressure Online Test 8th Science Lesson 2 Questions in English

Explanation

Opening up a pen, opening a door, kicking a football, striking a carrom coin, making of chapattis etc., all these actions need a force.

Explanation

Force is an ‘action of push or pull’, which makes the bodies to move or brings the moving bodies to rest. It even changes the shape and size of certain bodies.

Explanation

The group of students who pull the rope with a greater force will definitely win. The winners are applying a greater amount of force. Hence, the rope moves in the direction of the greater force.

Explanation

Force is that which changes or tends to change: i) the state of rest or ii) the state of uniform motion of a body or iii) the direction of a moving body or iv) the shape of a body

Explanation

Pushes and pulls are forms of forces. The direction of a force is in the direction in which a push or a pull is applied. Thus, force is a vector quantity, which has magnitude and direction. It is measured with a unit called “newton (N)”.

Explanation

A single pin produces a large pressure over a small area. But, when a large number of pins prick a body, each pin exerts very little pressure on the balloon, as the applied force gets distributed over a larger surface of the body. So, the balloon will not burst. We conclude that the effect of a force depends on the magnitude of the force and the area over which it acts.

Explanation

Thrust is a force acting perpendicularly on any given surface area of a body. It is measured by the unit newton.

Explanation

The effect of force can be measured using a physical quantity called pressure. It can be defined as the amount of force or thrust acting perpendicularly on a surface of area one square meter of a body.

Explanation
Unit of pressure is pascal (Pa) or N m–2.
Pressure = Thrust (or) Force/ Area
The SI unit of pressure is pascal (named after the French scientist Blaise Pascal).
1 pascal = 1 N m–2

Explanation
Average weight of the elephant = 4000 N
Weight of one leg = force exerted by one leg = 4000/4 = 1000 N
Area of the sole of one foot = 0.1 m2
Pressure = Force/ Area = 1000/ 0.1
= 104 N m-2

Explanation
The effect of pressure can be increased by increasing the thrust or by decreasing the area of the surface of the body experiencing the thrust.

Explanation
It is very difficult to walk on sand. But camels can walk easily on it because they have large padded feet, which increase the area of contact with the sandy ground. This reduces the pressure and enables them to walk easily on the sand.

Explanation

The axe, nail, knife, injection needle, bullet etc., all these are having sharp fine edges so as to exert a larger pressure on a smaller area of the body; in order to get the maximum effect from them.

Explanation
 More number of wheels are provided for a heavy goods-carrier for decreasing the pressure; thereby increasing the area of contact on the road.
 Broader straps are provided on a backpack for giving a lower pressure on the shoulders by providing a larger area of contact with the shoulder.
 It is difficult to drive an automobile, which has flattened tyres.

Explanation

You all know very well that air fills the space all around us. This envelope of air is called as atmosphere. It extends up-to many kilometres above the surface of the Earth. All objects on the surface of the Earth experience the thrust or force due to this atmosphere.

Explanation
Atmospheric pressure can be measured using the device called barometer. The barometer was invented by “Torricelli”

Explanation

The amount of force or weight of the atmospheric air that acts downward on unit surface area of the surface of the Earth is known as atmospheric pressure. Atmospheric pressure decreases with altitude from the surface of the Earth.

Explanation

Atmospheric pressure can be measured by the height of the mercury column in a barometer. The height of the mercury column denotes the atmospheric pressure at that place at a given time in ‘millimetre of mercury’. Even if you tilt the tube at various angles, you will see that the level of mercury will not vary

Explanation

At sea level, the height of the mercury column is around 76 cm or 760 mm. The pressure exerted by this mercury column is considered as the pressure of magnitude ‘one atmosphere’ (1 atm).

Explanation

At a higher altitude, due to the lack of atmospheric pressure the boiling point of a substance reduces. So, the water boils even at 80o C. At this temperature, the thermal energy that is produced is not sufficient enough for baking or cooking. So, cooking is difficult at higher altitude.

Explanation

An upward force is exerted by water on a floating or a partly submerged body. This upward force is called buoyant force. The phenomenon is known as “buoyancy”.

Explanation

Buoyant force is not only exerted by liquids, but also by gases. Liquids and gases together are called fluids.

Explanation

Buoyant upward force decides whether an object will sink or float. If the weight of the object is less than the upward force, then the object will float. If not, it will sink. A body floats if the buoyant force > its weight; A body sinks if its weight > buoyant force.

Explanation

Liquids do not have a definite shape. The force acting on unit area of the surface, on which the liquid is placed, is called the static pressure of the liquid. Liquids exert a pressure not only on the base of their container/vessel, but also on its side walls. The pressure exerted by a liquid depends upon the depth of the point of observation considered in it.

Explanation

An instrument used to measure the difference in the liquid pressure is called a “manometer”. You can measure the pressure of fluids enclosed in a definite container using the manometer.

Explanation

Pressure exerted by a liquid on the base of a container depends upon the height of the liquid column. Liquids exert the same pressure in all directions at a given depth. Liquid pressure varies with the depth.

Explanation

Explanation

Pascal’s law: The pressure applied at any point of a liquid at rest, in a closed system, will be distributed equally through all regions of the liquid.

Explanation

Some of the following examples highlight their working according to Pascal’s law
 In an automobile service station, the vehicles are lifted upward using the hydraulic lift, which works as per Pascal’s law.
 The automobile brake system works according to Pascal’s law.
 The hydraulic press is used to make the compressed bundles of cotton or cloth so as to
occupy less space.

Explanation

Explanation

Why are rain drops spherical in nature?
All the above questions have an answer, i.e., “due to surface tension”.

Explanation

Surface tension is the property of a liquid. The molecules of a liquid experience a force, which contracts the extent of their surface area as much as possible, so as to have the minimum value.

Explanation

The amount of force acting per nit length, on the surface of a liquid is called surface tension. Surface tension has the unit N m–1.

Explanation

Water molecules rise up due to surface tension. Xylem tissues are very narrow vessels present in plants. Water molecules are absorbed by the roots and these vessels help the water to rise upward due to “capillarity action”, which is caused by the surface tension of water.

Explanation

For a given volume, the surface area of a sphere is the minimum. This is theReason for the liquid drops to acquire a spherical shape.

Explanation

Water strider insect slides on the water surface easily due to the surface tension of water. For a given volume, the surface area of a sphere is the minimum. This is theReason for the liquid drops to acquire a spherical shape.

Explanation

When a liquid is flowing, there is a frictional force between the successive layers of the liquid. This force which acts in order to oppose the relative motion of the layer is known as viscous force. Such a property of a liquid is called viscosity

Explanation

Viscosity force is measured by the unit called poise in CGS and kg m–1 s
–1 or N s m–2
in SI. When a liquid is flowing, there is a frictional force between the successive layers of the liquid. This force which acts in order to oppose the relative motion of the layer is known as viscous force.

Explanation

You have studied that forces are classified into two types: contact force and non-contact force. Now, you are going to study one of the contact forces, i.e., friction.

Explanation

Frictional force or friction arises when two or more bodies in contact move or tend to move, relative to each other. It acts always in the opposite direction of the moving body. This force is produced due to the geometrical dissimilarities of the surface of the bodies, which are in relative motion.

Explanation

Friction can produce the following effects:

 Friction opposes motion.
 Friction causes wear and tear of the surfaces in contact.
 Friction produces heat.

Explanation

Static friction: The friction experienced by the bodies, which are at rest is called static friction. (E.g.: all the objects rigidly placed to be at rest on the Earth, a knot in a thread.)

Explanation

When a body rolls over another surface, the friction acting between the surfaces in contact is called rolling friction. Rolling friction is less than sliding friction. That is why wheels are provided in vehicles, trolleys, suitcases etc.

Explanation

Explanation
It is easy to pedal your cycle without any load on its carrier. With a load placed on its carrier, it is difficult to move it because the weight on the carrier increases the friction between the surface of the tyre and the road.

Explanation

For a given weight, the friction is directly related to the area of contact between the two surfaces. If the area of contact is greater, then, the friction will be greater too. A road roller has a broad base, so it offers more friction on the road. But a cycle has the least friction, since the area of contact of the tyre with the surface of the road is less.

Explanation

 We can hold any object in our hand due to friction.
 We can walk on the road because of friction. The footwear and the ground help us to walk without slipping.
 Writing easily with a pen on paper is due to friction.
 Automobiles can move safely due to friction between the tyres and the road. Brakes can be applied due to frictional resistance on brake shoes.

Explanation

 Friction wears out the surfaces rubbing with each other, like screws and gears in machines or soles of shoes.
 To overcome the friction an excess amount of effort has to be given to operate a machine. This leads to wastage of energy.
 Friction produces heat, which causes physical damage to the machines

Explanation

A substance, which reduces the frictional force, is called a lubricant. E.g.: grease, coconut oil, graphite, castor oil, etc.

Explanation

Since, the rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. You can see lead shots in the bearing of a cycle hub.