Yes, it would be a braking force on the car. You've basically made one of those "magnet in a copper pipe" toys (the pipe is the car). If you make them electrom

Yes, it would be a braking force on the car. You've basically made one of those "magnet in a copper pipe" toys (the pipe is the car). If you make them electromagnets and drive turn just so as the car passes, now it's a linear motor and would work to accelerate the car (this is how some rollercoasters work), but still doesn't magick energy out of the ether: something has to generate that power. There's no such thing as a free lunch, and there's certainly no such thing as a lunch you get paid to eat.