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### College Physics (Urone)

Book edition 1st Edition
Author(s) Paul Peter Urone
Pages 1272 pages
ISBN 9781938168000

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

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.

See the step by step solution

## Step 1: Concept of 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.

## Step 2: Explanation for neglecting the internal forces

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.