Roll the car so it runs into a thin book or piece of corrugated cardboard.
Observe the car and the penny. What can you learn from this activity about the importance of seat belts?
What difference does a seat belt make? Try again, using a small piece of tape to hold the penny on the car.
Do different coins respond differently? Dimes are lighter than pennies, quarters are heavier.
Note:
You need to push the car very gently, or you'll observe a different form of inertia - the car will move out from underneath the penny and the penny will drop to the table.