Anyway, I thought he should choose a heavier vehicle, though he thinks lighter ones would be faster. I have a postal scale to weigh them down to the ounce.
When I did pinewood derby as a kid, we always added weights on the bottom to make them heavier(within the weight limits, that is). Same idea? Heavier is better? He’ll probably choose a few cars.
Total weight doesn’t matter that much (within a reasonable range).
The important things to win Pinewood derby are minimizing friction (make sure the wheels spin freely) and getting the weight as far back in the car as you can go. Because the cars start on a slope, weight in the back of the car means more potential energy at the start, which means more speed at the bottom.
The optimal Pinewood derby car is as long as possible (because you win when the front crosses the finish line, but you get potential energy based on the center of mass, which will be higher for a longer car), has most of its weight in the back, and has just three well-lubricated wheels to reduce friction (Seriously: see if you’re allowed to break off one of the front wheels).
Why don’t you and he do some experiments putting cars head to head on a test slope. Doesn’t have to be anything fancy. Good way to figure out which of his cars is best, and make some hypotheses, collect some data. . . . It’s a much better way to learn than, “I asked the internet; here’s what they said.”
Aren’t the weights of pinewood derby cars strictly controlled? Which is why the derby participants try to gain advantage by distributing the weight in different ways?
The car is going to have a fixed amount of wind resistance. Rolling friction will be a function of the weight of the car, as is potential energy. It’s possible that a vehicle could have such a poor suspension and wheel setup that friction would rise with weight faster than the gain in potential energy, but for a hot wheels in a reasonable weight range I would bet that heavier is better.
I agree that the best thing to do is test all the cars he has.
Weight will make a difference if the wheels have significant friction. If two cars have the same amount of friction, the heavier one will roll better. And friction increases as the cars get older. So try putting some lube on the wheels/axles and see if they roll better.
If the only forces involved were gravity, normal force, and dry friction, then the weight of the car would be irrelevant, since all of those forces are proportional to mass.
But those aren’t the only forces. Air resistance is also present, and extremely relevant. And it is not proportional to mass. So you want the mass as great as possible, to reduce the relevance of air resistance.
That said, eschrodinger has the best answer: Do the experiment. Learning about how science works is a far more valuable lesson than just learning which car is best. If he wonders why the heavier car wins, that’s the time then to explain about air resistance.
+1 to doing some testing. Like real cars, how the Hot Wheel (forget Matchbox <grin>) weight distribution may matter more than weight. My 50 year old memories are that even identical cars can differ in speed. As I recall, the original Camaro, on the first to be introduced was amongst the fastest and the Stilletto was among the slowest.
More important than weight, make sure the wheels are straight and true in line with each other and put a tiny drop of 3in1 oil or probably better some new lightweight spray lube or even graphite on the axle. The oil/graphite may be viewed a cheating, but aligning the wheels is part of regular tuning. There was/is a tool for for.
Edit: Like pinewood derby, watch the lanes. One will always be faster than the other.
True, there may well be differences in things like wheel alignment that would make one car faster than another, completely independent of weight. Finding those is another advantage to experimental testing.
And as an added bonus, it gives you an excuse to play with Hot Wheels with your kid.
I think also that the rolling resistance isn’t very close to the simple friction model (where the frictional force is directly proportional to the normal force/weight). I strongly suspect there’s a pretty big constant term (independent of the normal force), which would also add to the speed-weight dependence.
But again, best way to answer this is the Mythbuster’s way: try it out!
There’s other kinds of friction than dry friction and air resistance. The axle may be corroded or there may be dirt in the axle-wheel interface. Older cars will have greater amounts, as a general rule. Lube will help but not completely eliminate them.
Rolling resistance might not be proportional to weight, either, but I’m pretty sure that it goes the other way, such that it’d be totally insignificant for a toy car. Rolling resistance comes from changing deformation of the wheels, and I highly doubt that the deformation of a Hot Wheels car’s wheels under its weight is even measurable.
I always thought that the old-style Hot Wheels cars with spring suspension were the best. Each wheel spun independently of the other, and any Hot Wheels car could beat any Matchbox car.
Oh, one other point that could theoretically be relevant (but probably isn’t very): You want as low a proportion as possible of the weight to be in the wheels. Weight in the wheels will result in some energy being wasted in rotational kinetic energy, instead of translational kinetic energy. To see this effect most clearly, roll four objects down a ramp: A ring or hollow cylinder, a solid cylinder (like a hockey puck), a hollow sphere (like a basketball), and a solid sphere (like a super-bounce ball). Size and density don’t even matter. The solid ball will always win the race, followed by the solid cylinder, the hollow ball, and in last place the ring, because the solid ball has most of its mass close to the axis of rotation, while the ring has all of its mass far from the axis.
