How can a 5,000-pound truck tow 10,000 pounds?

Physics, Driving and You

Take it in, fellas. This is where the torque is produced.
Take it in, fellas. This is where the torque is produced.
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There are two kinds of frictional forces that are working against you as you drive your truck. Static friction is the friction your tires will encounter before they reach the threshold of motion. Once your wheels begin to move, the threshold of motion has been crossed and your tires must now deal with kinetic friction -- or in the case of a wheel, rolling friction. To accelerate, static friction must be overcome through applied force, but this isn't the case with rolling friction. Instead, the goal is to accelerate until the applied force equals the amount of rolling friction applied to the tires. Once the amount of applied force matches the amount of rolling friction, you've reached the point of constant velocity. You may know it as cruising speed -- that point where you're not speeding up or slowing down, just traveling happily along.

All of this physics talk wouldn't amount to much if it weren't for the way your car uses applied force from the engine to propel your truck down the road. It does so by producing torque, which is the energy that rotates a wheel on its axis. The applied force created by your engine is distributed to the wheels of your truck through the transmission, which turns the drive shaft and distributes the torque to the wheels.

Torque is different from the energy it takes to move something along a horizontal plane. Think of it like this: Let's say that you have a quarter standing on its edge that you intend to roll down your hallway. You can push on the edge with your finger in a top-down motion to get it to move forward or a bottom-up motion to make it roll backward. You've just applied torque. Now try to move the quarter forward without rolling it. Doesn't work very well, does it? The quarter just skids along the surface which makes it difficult to control -- not a very efficient way to move. This is the challenge that's presented to your truck every time you drive: moving forward without skidding.

It seems simple enough; you push the gas pedal, and the engine distributes torque to the drive shaft which spins the axle and, in turn, the wheels. But if the engine produces too much torque, your tires will overcome the rolling friction they meet from the road and will skid uselessly (and possibly dangerously). What you want is for your tire never to leave the road.

It sounds a bit strange, but when your truck is driving along properly, the bottom of the tire --literally where the rubber meets the road -- remains at rest. What constitutes the bottom of the tires changes since all points on the tread have the opportunity to serve as the bottom of the tire as it completes a full rotation. So does the location of the bottom of the tire in relation to the road. But, as far as gravity and the normal force are concerned, the bottom of the tire's at rest since it never leaves the road.

So what the heck does all of this have to do with towing? Plenty. You'll see what we mean on the next page.