# Do internal forces cause change in velocity?

For the earth sun system, the force of gravitation is an internal force ( forces* a newtons third law pair) and this INTERNAL force is causing the earth to accelerate. . it is providing the necessary centripetal force.

Reasoning behind the question: centripetal forces dont do any work and hence dont cause a change in mechanical energy, dont cause a change in kinetic energy therefore and dont cause a change in velocity.

QUESTION question 1) but the earth is undergoing a change in velocity?

so, is the the gravitational force on the earth an external force? but ive considered our system containing the sun too. shouldn't that be an internal force then?

2) in the earth sun system if for simplicity we call the earth's movement uniform circular motion, is momentum conserved in this system? how?

i suppose ive mixed up too many things

• Internal forces do not accelerate the center of mass of the complete system. They do accelerate the parts of the system they act on. – alephzero Nov 7 '18 at 10:41
• Related question by OP physics.stackexchange.com/q/439417 – Aaron Stevens Nov 7 '18 at 11:28

Centripetal force does change velocity of the object, but not the magnitude of the velocity. Only the velocity direction changes.