Newton's third law of motion:- It states that every action, there is an equal and opposite reaction.
Let us consider two particles in an isolated system, their masses are m1 and m2 velocity are v1 and v2. Consider that they are moving along the same line and they interact. Their velocities change due to interaction. Thus, their momenta will also change in time Δt.
Let the change in momentum of the first particle
= ΔP1 (Change in momentum of the second particle)
= ΔP2
According to the law of conservation of linear momentum, in an isolated system with has no external force.
ΔP1 + ΔP2 = 0 or ΔP1 = -ΔP2
Dividing it by Δt, we have,
ΔP2/Δt = -ΔP1/ΔT
⇒ dP2/dt = -dP1/dt
⇒ F2 = -F1
That is force acting on m2 = -force acting on m1
or Action = -Reaction
This is Newton's third law of motion. When we press the sandy ground in backward direction, the sand gets pushed away and as a result, we get only very small reaction from the ground making it difficult for us to walk.