What does Newton's third law say about why momentum is conserved in collisions? Equal forces act in equal times, so the change in momentum for both objects must be equal. Unequal forces act for unequal times, so the change in momentum for both objects must be unequal. Unequal forces act for unequal times, so the change in momentum for both objects must be equal. Equal forces act for unequal times, so the change in momentum for both objects must be equal.