Why can we neglect forces such as those holding a body together when we apply Newton’s second law of motion?

Newton’s second law of motion states that the acceleration of a system is directly proportional to the net external force acting on the system and is inversely proportional to the mass of the system. Mathematically,

$F_{net}=ma$

Herelocalid="1654165093803" ${}^{F_{net}}$is the net force,localid="1654165395003" ${}^m$is the mass, andlocalid="1654165421915" ${}^a$is the acceleration.

According to Newton’s first law, the change in motion is due to the net external force. Internal forces are responsible for holding together a given body. The internal forces do not affect the motion of a body. The motion of a body is affected only by external forces.

Internal forces acting on a body are equal and opposite in direction, so their sum must be zero. Therefore, there can be no net force from the internal forces.