This fun activity is an amazing way to introduce momentum and collisions to your students. Not only will they be studying collisions (yes, crashing cars!) and analyzing what they see, but through a series of steps they will attempt to create equations to support elastic and inelastic collisions as well as determine the law of conservation of momentum.
My students absolutely love this activity. Instead of a lecture, consider giving this activity a shot. Even if they can't figure out exactly what the equation should look like (many of them will be able to) then once you lecture it will ALL make sense and there will be some huge "Ah ha" moments.
Pasco carts are perfect for this activity (https://www.pasco.com/prodCompare/classic-dynamics-carts/index.cfm) but you could use the phet collision simulation or the physicsclassroom momentum simulation.
(This lesson incorporates the NGSS and common core math standards of creating equations.)