The centre of gravity of an object is the point through which the entire weight of the object appears to act.

(i) H₂SO₄ consists of 2H atoms, 1 S atom and 4 O atoms.
(ii) Hence, Atomicity = 2 + 1 + 4 = 7

A molecule is made up of two or more atoms chemically combined.

One astronomical unit is defined as the average distance between the earth and the sun.
1 AU = 1.4965 10⁶ km = 1.496 \(\times\) 10¹¹m.

One light year is defined as the distance travelled by light in vacuum during the period of one year.
1 Light year = 9.46 \(\times\) 10¹⁵m

Physical quantities which can be obtained by multiplying, dividing or by mathematically combining the fundamental quantities are known as derived quantities.
Ex: Area, volume

Apparatus: Irregularly shaped plate, string, pendulum bob, stand.
i) Make three holes in the lamina.
ii) Suspend the lamina from the optical pin through one of the holes as shown.
iii) Suspend the plumbline from the pin and mark the position of the plumbline on the lamina.
iv) Draw lines on the lamina representing the positions of the plumbline.
v) Repeat the above steps for the holes.
vi) Label the intersection of the three lines as X is the position of the centre of gravity of the lamina.

Take a leaf from any one of trees in your neighbourhood. Place the leaf on a graph sheet and draw the outline of the leaf with a pencil. Remove the leaf. You can see the outline of the leaf on the graph sheet.
i) Now count the number of whole squares enclosed within the outline of the leaf. Take it to be M
ii) Then count the number of squares that are more than half. Take it as N
iii) Next, Count the number of squares which are half of a whole square. Note it to be P.
iv) Finally, count the number of squares that are less than half. Let it be a Q.
v) Now the approximate area of the leaf can be calculated using the following formula.
Approximate area of the leaf = M+(3/4)N+(1/2)P+(1/4)
This formula can be used to calculate the area of any irregularly shaped plane figures.