Talking about Hot Wheels™ can lead to some strange discussion.

gunslinger2000  RLC™# 50523

I am working with my son to develop some physics experiments using Hot Wheels for a summer science program for High School students at the local university.

Allow me a "devil's advocate" question, please. Since the acceleration of gravity is independent of mass (weight) and any two objects fall at the same rate in a vacuum regardless of weight, what law says the heavier car will accelerate faster? The laws relative to inertia say it should take more force to accelerate a heavier car and we are talking about the uniform force of gravity, aren't we?

19Mopar68  HWC™# 500730

The cars are rolling, not falling. Maybe that has something to do with it?
I don't know, I never passed grade 10 Science, and I took it twice!

gunslinger2000  RLC™# 50523

LMAO! I taught 10th-grade science twice and I don't understand it either. I have to be honest, I can't see anything in physics which would explain why a heavier car should be faster, and am looking for an answer without appearing dumb. The only thing I can figure out is if it possibly has something to do with the mass of the two attracting objects— earth and the car— but that should be so small it can't be measured.

BeccasWheels  RLC™# 50739

I think that "the greater the weight the faster the car" theory will only work if the track is sloped the entire way to the finish line. I have found that if the car is too heavy, friction will slow the car down. The Ferrari I use in my downhill race will trail the heavier cars for the first 40 feet, but then the last 10, it blows them away.

It would be interesting to graph the time the truck takes to go down the hill relative to the added weight on the truck. There would probably be little difference between trials (less than a tenth of a second per trial). But with enough trials, I think the graph would be pretty cool!

clearSpeed  HWC™# 425062

Gravity, weight and friction all have a part in the equation:
Gravity sends the cars downhill on the track.
Friction is like putting on the brakes.
Weight (mass) is like putting on the gas pedal to overcome the braking.

When adding weight, watch out for the flex of the axles, too much will throw the alignment off.

Don't forget the bearings themselves-- they may be round but not perfectly round. The more perfectly round they are the less friction there is. There may only be one out of a million that has perfectly round bearings (within a thousandth or ten thousandth of an inch out of round), but yes, it does make a difference.

19Mopar68  HWC™# 500730

Maybe a heavier car rolls straighter (doesn't bounce off the sides so much), and a lighter car does the opposite.

Newtons Laws Of Motion... Maybe this helps...

1. Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

2. The acceleration produced by a particular force acting on a body is directly proportional to the magnitude of the force and inversely proportional to the mass of the body.

3. To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

In Newton's second law, shouldn't that mean a heavier car will pick up speed faster? The acceleration produced by a particular force (gravity) acting on a body (the car) is directly proportional to the magnitude of the force and inversely proportional to the mass of the body.

1.eyed.jack  HWC™# 468740

Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

I'm impressed... but mostly I'm compelled to take this body straight to a state of rest.

gunslinger2000  RLC™# 50523

I'll state my confusion and let you guys flame away. I am ashamed to say that I am serious when I say I am confused. First of all, the only forces acting on the car at the top of the track are the force of gravity pulling the car down and the force of the track holding the car up. We have been told for hundreds of years that two objects falling under the effects of gravity in a vacuum will fall at exactly the same rate... The examples usually used are a feather and a rock. Theoretically the car going down the track and one dropped from the same height should hit the ground at the same time. (A bullet fired horizontally will hit the ground at the same time as one dropped from the same height as the muzzle of the gun).

As for the inertia thing, it clearly takes more force to get a heavier object moving than a lighter one. A 300 HP engine will accelerate a rail a lot faster than it would a big limo. But the force acting on a heavy HW is the same as a light one. I am apparently making a false assumption somewhere or trying to apply the wrong law. I should understand this, but must be getting too old to think it through.

19Mopar68  HWC™# 500730

two objects falling under the affects of gravity in a vacuum will fall at exactly the same rate

That's right, but technically the Hot Wheels cars are not falling. They are rolling down a hill. Maybe the heavier car puts enough weight to bend the axle a slight amount, tilting the wheels outward a hair (camber angle), and they have less contact with the track... meaning less friction. Maybe this also explains why redlines are usually faster, too. The pressure on those bent axles could make the wheels tilt slightly more than usual.

gunslinger2000  RLC™# 50523

I think you are on the right track. At this point it looks like it is some factor affected by the weight, rather than the actual weight that is determining the speed. Otherwise the Drag Bus would be the fastest stock machine and we'd all be racing drag buses loaded with lead.

mcokydoky  RLC™# 52427

Yes... Yes... This is all interesting, but how do they hold up in a wind tunnel?