Identify the internal and external forces acting on the following systems. (a) Earth alone as a system (b) Earth and Sun as a system (c) Our body as a system while walking (d) Our body + Earth as a system

Explanation

(a) Earth alone as a system: Earth orbits the Sun due to gravitational attraction of the Sun. If we consider Earth as a system, Force exerted then Sun.s gravitational force is an by the sun external force. If we take the Moon into account, it also exerts an external force on Earth.

(b) (Earth+Sun) as a system: In this case, there are two internal forces which form an action and reaction pair-the gravitational force exerted by the Sun on Earth and gravitational System force exerted by the Earth on the Sun.
(c) Our body as a system: While walking, we exert a force on the Earth and Earth exerts an equal and opposite force on our body. If our body alone is considered as a system, then the force exerted by the Earth on our body is external.
(d) (Our body + Earth) as a system: In this case, there Our body as a system are two internal forces present in the system. One is the force exerted by our body on the Earth and the other is System the equal and opposite force exerted by the Earth on our body.