Reply #114: It Won't Have Twice The Braking Force, Though [View All]

The breaking force is a function of the shoes gripping the rim, not just of the tires on the ground. Unless a larger object has an proportional amount of decelerating forces on the wheels themselves the difference on the ground is less important. So, having twice the mass doesn't work unless the wheels are capable of completely LOCKING UP when force is applied. At enough speed, getting all the wheels on a semi-tractor to lock simultaneously is mechanically difficult and requires massive hydraulic or pneumatic force.

The heavier the object, the more rotational momentum, since something that weighs 30 times as much doesn't have wheels 30 times larger in circumference. That lost surface area requires a compensation of greater force to stop the wheel.

In addition, we can debate this all we want, but if you've never seen the length of skid marks from a truck accident, it's all academic. My dad was in one when another semi jackknifed in front of him. His skidmarks were WAY, WAY longer than any of the cars behind him. They were all stopped at least 50 feet behind where hit the other truck.

BTW: He was essentially ok. Broken collarbone and broken patella. But, otherwise survived just fine. And you know what? A semi full of milk makes a BIG mess.
GAC