In this activity, students will discover the relationship between force and mass using Hot Wheels. Students will work in small groups to build a track and test their hypotheses. You will need toy cars straight race track and connectors for this activity. If you don't have them already, I would check the local online yard sales. You can also apply for a FREE Speedometry set from Hot Wheels if you are a 4th grade teacher. If there are any available, they will ship it to you at no cost.