This physics video tutorial provides a basic introduction into the concept of density. It also explains why objects sink or float in water and why warm air rises but cold air sinks.
Density Problems: https://www.youtube.com/watch?v=9CKDQE35qXQ
Buoyant Force: https://www.youtube.com/watch?v=xh6V4sEeUYs
Buoyancy: https://www.youtube.com/watch?v=i1pGInNc-hg&t=1s
