Day 46: The Most Difficult Concept
But if the forces are balanced, wouldn’t the bowling ball be at rest? How could it be moving?
The point that is overlooked is that the ball is already moving. Which I tried to help kids see by sequencing the diagrams as speed up, constant speed, then slow down. Interestingly, more kids are OK with the frictionless scenario (they already have first-hand experience with not hitting the bowling ball) than the friction scenario. My best explanation asked them to imagine hitting a rolling bowling ball on a frictionless surface but a split second after they hit it forward (to speed it up), someone else taps it in the opposite direction with the same strength (to slow it down). You hit, they hit. You hit, they hit. What happens to the speed of the ball? (It never changes). Friction is just like those opposing taps.
Lastly, to my delight, the kids suggested 2 more possible force diagrams for the bottom right scenario — which then lead another kid to ask which of the 3 possibilities would be most effective at stopping the ball.
You can download a copy of the sheet here: BalancedUnbalancedFBDMotionInteraction 2014