Calculate the spring constant of a spring or rubber band and use that to calculate the mass of a foreign object.
For the foreign object, I used a roll of pennies attached to a paper clip. It weighed 125 grams.
Roll a marble off a table and predict where it will land. The marble acts as a horizontally-launched projectile
For an apparatus, I use a tissue box and two meter sticks. I push the meter sticks together with about a half cm of space in between for