Question: when is the mechanical energy of a system conserved
[{'answer': 'elastic collisions', 'provenance': [{'wikipedia_id': '859234', 'title': 'Mechanical energy', 'start_paragraph_id': 1, 'start_character': 770, 'end_paragraph_id': 1, 'end_character': 788, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '859234', 'title': 'Mechanical energy', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1041, 'bleu_score': 0.7902994189983371, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9916420916632489865}, 'section': 'Section::::Abstract.'}]}]