Confusing FUTURE TENSE: 'be going to' VS 'will'

Try this question;
>> I don't want to do my homework now. I (will/ am going to) do it later.


*The answer is 'will'

Now, try another question;


>> My mother (will / is going to) come on Friday.




*The answer is 'is going to'

WHY????
WHAT'S THE DIFFERENCE BETWEEN 'BE GOING TO' AND 'WILL'?????

Well, 
'be going to' is used for something that has been planned before
while
'will' is used for decisions made now or at the moment of speaking.

SIMPLE, RIGHT???
Now, try this;

>> Ali is wounded. He_(isn't going to / won't )__ play at Wimbledon.



The answer is 'isn't going to'....
because Ali knew that he was wounded so he had made the decision (before) not to play at Wimbledon!.haha.. ;P