Many science fiction movies and TV shows have depicted using a nuclear weapon to destroy asteroids. What problem does Plait see with this plan?

This question may refer to the exelent TED talk of Phil Plait: How How to defend Earth from asteroids.

In it, he says that the problems may be timming: Because you should detonate the nuclear bomb in the exact time (with a tolerance of miliseconds) to move the asteroid, which is a really hard thing because the asteroids move really fast, and if you detonate the weapon a few seconds earlier or latter, you will miss the asteroid.

f=ma newton's second law 7x8=56n

molecules are free to separate from each other and will take on the shape of the container. molecules move completely at random and high speeds. they vibrate in a fixed position and do not move around much. molecules expand to the edge of their container.