1. Definition- Force is defined as an effect which causes a body to change its state.

• Force on 1 kg mass in the presence of gravity $\left(g=9.8 \mathrm{~m} / \mathrm{s}^2\right)$ is 1 kg-f=9.8 N

• Unit of force- 

1.  In SI unit- Newton(N)
2.  In CGS- 1 dyne (1 newton = 100000 dyne)

• 1 Newton(N) is the force needed to accelerate an object with a mass of 1 kg at a rate of 1 m/s²     (1 N = 1 kg · m/s²)

2.    Types of forces-

a) Contact forces- 

• Contact forces are due to direct physical contact between objects. 

Types of contact forces-

• Tension
• Normal reaction
• Spring force
• Friction

b)   Non-contact forces- 

• These forces act without the necessity of physical contact between objects. 

• They depend on the presence of a “field” in the region of space surrounding the body under consideration. 

Types of non-contact forces-

• Gravitational force
• Electrostatic force
• Magnetic force

Note:- Gravitational and Electrostatic force have infinite range

c)    Weak forces-

• Vanderwaal force

d)   Nuclear forces

$
\begin{aligned}
& F_{\text {nuclear }}>F_{\text {electro }}>F_{\text {gravitation }} \\
& F_e / F_g=10^{43}
\end{aligned}
$

Therefore, $F_e \gg F_g$