When i straighten my right knee it frequently pops/cracks and is painful for a moment. it also hurts when i uses stair or crouch. torn cartilage?

Cartilage damage-yes: You likely have lost the smooth normal surface of the articular cartilage under your patella (kneecap) along with the cartilage of the trochlea (groove in femur that articulates with your patella). Seek orthopaedic evaluation to determine if this is the case.

Chondrosis: Popping, crunching, cracking, grating and grinding are often noted in the front aspect of normal/asymptomatic knees. The sensation/sound is termed "crepitance". It poorly correlates with the amount of articular cartilage irregularity or "chondrosis". When associated with pain, see your doctor. There is often an imbalance of the muscles and soft tissues controlling the knee.