Although questions with BA (吧) and MA (嗎) both indicate that the speaker has made an assumption of the situation and has an expectation as to what the answer will be, 吧 shows a stronger assumption or expectation than 嗎.
nǐ shì xué fǎ wén dí ba?
You must be studying French, aren't you? (Near certainty.)
nǐ shì xué fǎ wén dí ma?
Are you studying French? (Expecting "yes" to be the answer.)
Reference: YUFA! A Practical Guide to Mandarin Chinese Grammar