## What is the Universal law of gravitation?

Force of attraction between two masses $$m_1$$ and $$m_2$$ is given by,

$$F = \frac{m_1 m_2}{r^2}$$

Where,
r is the distance between two masses $$m_1$$ and $$m_2$$.
G is a constant, called the Universal gravitational constant.

That is called Universal law of gravitation.

## What does elasticity mean?

The property of a body, by virtue of which it tends to regain its original size and shape when the applied force is removed is known as elasticity.

#### Hooke’s law

Within elastic limit, the strain is proportional to stress. That is Hooke’s law.

Stress ∝ Strain
Stress = k × Strain

Where k is constant, called the Modulus of the elasticity.

Elastic energy per unit volume is given by,
ε = ½ × Y × σ2
Where,
Y is young’s modulus of elasticity.
σ is the strain.

#### Modulus of elasticity

According to Hooke’s law

Stress ∝ Strain

or Stress = k × Strain

$$k = \frac{Stress}{Strain}$$

Where k is a constant, called the modulus of elasticity or coefficient of elasticity.

### Types of Modulus of elasticity

#### 1. Young’s modulus of elasticity or Young’s modulus

Young’s modulus of elasticity or Young’s modulus is given by,

$$Y = \frac{\sigma}{\epsilon}$$

Where $$\sigma$$ is normal stress and $$\epsilon$$ is longitudinal strain.

We know that

$$\sigma = \frac{F}{A}$$ and $$\epsilon =\frac{\Delta L}{L}$$

So $$Y = \frac{F L}{A \Delta L}$$

#### 2. Bulk modulus

Bulk modulus is given by,

$$B = \frac{\sigma_v}{\epsilon_v}$$

Where $$\sigma_v$$ is normal stress and $$\epsilon_v$$ is volumetric strain.

We know that

$$\sigma_v = \frac{F}{A}$$ and $$\epsilon_v = \frac{\Delta V}{V}$$

So $$B = \frac{F V}{A \Delta V}$$

#### Modulus of rigidity

Modulus of rigidity is given by

$$G = \frac{\sigma_s}{\theta}$$

Where $$\sigma_s$$ is tangetial stress and $$\theta$$ is shearing strain.

#### Poisson’s ratio

Poisson’s ratio is given by,
$$p.r. =-\frac{d/D}{l/L}$$
where, l/L is the longitudinal strain and d/D is the lateral strain.