Consider three particles that interact withinternal forces given by ? ⃗ 1→2, ? ⃗ 3→1,etc. The total
force on the system is the sum of these internal forces with anyexternal forces
(? ⃗ i,ext , ?ℎ??? ? = 1,2,3). Bywriting out the sum explicitly, use Newton’s third law to provethat
the total acceleration of the system is only due to the externalforces.
