So I am doing a physics science fair project. My original idea was "How does heat affect friction in shoe polymers?" but how would I know how to identify shoe polymers and where would I get 5 or 6 different pairs of shoe polymers. I changed it to "How does different temperatures affect friction in different shoe brands?" but doesn't that have two variables which isn't allowed. Someone please help me fix one of the two problems. I need help so badly.