Michelle Obama attends Ford 400

First Lady Michelle Obama and Jill Biden received a mixed welcome to NASCAR's season-ending Ford 400 Sunday, where they served as grand marshals to help raise awareness for their "Joining Forces" initiative, which supports military families. Scattered boos could be heard from the crowd as Obama and Biden were introduced and invited to give the traditional command, "Gentlemen, start your engines." Earlier, Obama and Biden had addressed the drivers' meeting at Homestead-Miami Speedway, the Sun Sentinel reported. "I know this is a big day for all of you so we're not going to take up your time because you need to be focused," Obama said, according to the newspaper. "We just wanted to stop by and say thank you. NASCAR has just been amazing in terms of its support -- not just today, but every day -- of military families." Obama and Biden were also scheduled to attend a barbecue lunch with service members and their families, the Sun Sentinel reported. "We want to make sure these families know that America has their back," Obama said during a television interview with an ESPN reporter. When asked how she felt about the sights, smells and sounds of NASCAR, Obama replied, "It's awesome!"