A puzzle about the open road
Draw a free body diagram for a car traveling down the road at constant velocity. Many of my kids originally want to say things like “the engine pushes the car forward” but with a question or two, they see that that same engine won’t do much on ice, so it must be the road pushing the car forward. This leads them to this FBD:
But I have a bit of a problem with this I’ve never been able to fully resolve. Kids also want to draw a backward frictional force in addition to the drag force. And then they get all hung up on how the road can both be pushing the car forward and backward at the same time.
I’ve read about rolling resistance, and some great articles on what makes a car go (this makes a lot of sense, and really digs into a understanding of energy). But I’m still a bit perplexed about whether there should be a frictional force opposite the motion of a car driving down the highway (I think there should), and whether this force would act on all four tires, or just the drive wheels. If this rolling resistance force only acts on the non drive wheels, it would seem to be you could escape this force entirely with all wheel drive, which doesn’t make sense. Also, how big is this force compared to the drag force? My guess is that at highway speeds, rolling resistance is much smaller than the drag force.
So I’d welcome any of your insights to this problem. Can the road exert two frictional forces on the tire in opposite directions?