6.4 How cyclists, swimmers and parachutists reduce friction?
In the same way, swimmers also wear tight clothes and place the arms in front of them to make their bodies streamlined. In this way, they can swim faster. Similarly, the parachutists with a small parachute can reach the land earlier than the one with a large parachute. This is because the small parachute face less air friction.
6.5 Gravitational Force
In our daily life, we observe that a ball thrown upward always comes down. An apple or walnut detached from the tree always falls to the ground. What do you think? Why does everything fall down? In fact, Earth attracts all the bodies towards itself. This force of attraction is called gravitational force or simply force of gravity.
The gravitational force not only exists between the Earth and the other objects but also exists between all the things around us. For example, gravitational force also exists between two little objects like balls lying on the ground but it is too small to be noticed. The effect of gravitational force is visible in heavenly object like Sun, Earth and other planets due to their greater masses.
The gravitational force of the Sun is greater than the planets, that is why all the planets move around the Sun in their orbits.You might have known that there is no gravitational force in space. Why? This is because gravitational force reases when we move far from the centre of the Earth. The satellites ma by scientists revolve around the Earth due to its gravitational force. So Thus, we conclude that gravity between two objects depends upon their masses and the distance between their centers. The greater the mass of an object, the heavier the objects, the greater the force gravity. The larger the distance between the centers of two objects, the smaller will be the gravitational force.
Advantages of gravitational force Earth
- 1:- Trees, buildings, animals, and water etc., are all kept at their places due to gravitational force of the Earth.
- 2:- Gravitational force makes rainfall on the Earth.
- 3:- Due to gravitational force, the streams flow down into rivers.
- 4:- Artificial satellites move around the Earth due to gravitational force. They send information to us. We can see programs and sports played in different countries on our televisions with the help of these satellites.
6.6 Mass and Weight
In our daily life, we use the words mass and weight as a same quantity but it is not true. Mass and Weight in science have two different quantities.
Mass
Massively we know that all things are made from the matter. Some things are heavier while the other are lighter. The amount of substance in the body is called mass. It is measured in kilograms (kg).Every Quantity or thing in the universe is the same. For example, you weight is 40 kilograms on Earth, if you go to the moon it will be 40 kilograms. The object with greater mass have a stronger gravitational force. Mass is measured by ordinary balance.
Weight
The force by which Earth attracts an object towards its centre is called its weight. In other words, weight is the force of gravity of an object. It is measured in Newton (N).
As we move away from the centre of the Earth, the gravitational force decreases. Therefore, weight is more near the centre of Earth and less at high altitudes. For example, our weight becomes less when we move up and it becomes 1/6 on the Moon. The weight is measured by a spring balance.
6.7 Balanced and Unbalanced Forces
Balanced Forces
Have you ever watched a tug of game? In this game, two teams pull at a rope in opposite directions as shown in Fig. 6.9.
When both teams pull equally, the rope does not move in any direction. Such forces are called balanced forces. These forces are equal but opposite in different.When two balanced forces act on an object, they cancel the effect of each other and due to this, the object does not change its position. If the object is at rest, it remains in the position of rest. If it is moving, it will continue to move in the same direction with same speed.
For example, a book lying on a table remains staying on table. It does not fall or move. Why?
Two balanced forces are at work on the book. The upward push of the table is equal to the downward force i.e., weight of book. Both the forces are equal in size and opposite in direction, so, they cancel the effect of each other. The result is that the book remains saying on the table.
Similarly, due to balanced forces ship floats on water. The force of gravity i.e., (weight of ship) pushes the ship in downward direction while the water pushes the ship in upward direction. The ship will continue to float until the push of water remains same as that of ship's weight.
