I had acl surgery five weeks ago and my physical therapist says I should bend my knee by force. Is it safe for me to bend my knee by force?

Best to ask your. Best to ask your surgeon who should be guiding the physical therapist.
Check with your doc . Assuming the PT is communicating with the surgeon, it may be necessary to actively assist motion to regain full motion. The surgeon knows what exactly was done and the strength of the reconstruction and therefore the timing and restrictions of rehab.