I'm a high school physics teacher and I need a way to demonstrate net force = zero when something is moving at a constant speed. Usually some get it right away, some don't care and a few just really can't get their mind around it. I tell them about the parachutist, their car going 60 mph in a straight line on a smooth road, and the airplane at level flight but some still insist more force needs to be applied to keep the airplane from falling out of the sky. I'm doing a demonstration Monday which shows balanced forces with the object at rest using spring scales, masses and pulleys. I'm also doing accelerated motion with a motion detector, car, and a mass hanging over the table, tied to the car and the string going over a pulley. But I can't think of a way to show balanced forces with constant, uniform motion. Any ideas? Even a computer simulation might help.