I’ve always loved TV shows that feature obstacle courses. As a child I watched Fun House, Finders Keepers, and several incarnations of Double Dare. (Even if I hadn’t mentioned my twenty-seventh birthday, this information would place my date of birth squarely in the mid-eighties.) In recent years I’ve gotten into Wipeout and Ninja Warrior. The former makes me laugh, the latter makes me get off my butt and do push-ups during commercials, and both, naturally, make me think about physics- especially Newton’s Laws of Motion.
1) Newton’s First Law: Objects at rest tend to stay at rest; objects in motion tend to stay in motion.
That’s the version you’ve always heard. A more specific version would go like this: An object moving at a constant velocity (i.e. not changing speed or direction) will continue to move at that velocity unless pulled or pushed by a nonzero external net force.
First Law is particularly evident during Wipeout.
When a contestant stops and hesitates before jumping to the next obstacle, she is at rest, so she has a constant velocity of zero. Thus she won’t move unless acted upon by a nonzero external net force. There are absolutely forces acting on her. Gravity is pulling down on her and the platform is pushing up on her, but these forces cancel each other out, resulting in a net force of zero. She is safe and at rest ten feet above the water. If no other forces came into play, the contestant could stand there forever and the show would be cancelled. However, since it’s Wipeout, after a moment a giant foam-padded plank will swing out and apply a third horizontal force to the contestant. This will cause her to accelerate forward, pushing her off the platform. At the instant she leaves the platform, the only force acting on her is the downward force of gravity, so that downward force is now the external net force on her.
The next installment of this series will discuss how Newton’s Second Law of Motion applies to Ninja Warrior and Wipeout.