In the beginning of the movie, Marty is at Doc Brown's house. When he plays a chord with the volume all the way up, he is thrown backwards. The cable that was attached to Michael J. Fox is clearly seen. Wasn't there a way to hide that?
In addition to what you are saying, the speaker should have blown when he plugs in the guitar. There's usually a pop (speaker jumps) when you plug in the cord.
There are a lot of reasons why he flew backwards the force maybe from the amplifier (however you spell it) also it could have scared him and he flew back and tripped. There are a lot of explanations. Also it is a movie to us we have a volume system on our T.V. it could have been a lot louder in the movie. The force might actually exert someone to fly back some sort of distance.
I agree with Larman that it´s the cord from the guitar to the amp. Nevertheless I wounder why the cord comes right out of the speaker and not beside it.
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