The Physics of Backing Up Towed Vehicles
Several factors make backing up a towed vehicle more complicated than just putting the car into reverse and going for it:
- The towed vehicle adds more length to your overall setup.
- The towed vehicle and towing apparatus adds more weight to the setup.
- All of the extra weight is located behind the tow vehicle (unless something has gone very wrong).
- The towing apparatus includes a pivot, which affects how the tow vehicle makes turns.
- The two vehicles apply force to one another, which changes the way the tow vehicle can accelerate, brake and turn.
To tow a vehicle safely, it's good to have a basic understanding of physics. Here's a quick rundown on the physics of towing a vehicle:
Because the tow vehicle has to pull or push more weight than normal, it must deal with a different level of inertia. Inertia is the tendency for an object to resist a change in its state of motion. It's not a constant value -- objects with different masses have different values of inertia. An object with greater mass will resist changes to its state of motion more than an object with less mass.
To overcome inertia, the tow vehicle's engine must perform more work than it would for the vehicle on its own. The change in motion is called acceleration. Acceleration is a vector quantity, which means it has both a direction and a magnitude. With acceleration, that magnitude is a change in velocity.
In the case of backing up a towed vehicle, the direction of acceleration would be backward -- from the perspective of someone sitting in the driver's seat of the tow vehicle. To move in reverse safely, the driver must press down on the accelerator pedal carefully.
Once in motion, assuming both vehicles are on a level surface, the driver must continue to press the accelerator so that the engine will provide a constant force to the towed vehicle. That's because of friction. Friction is a force exerted by a surface as an object moves across it. In this case, friction applies a force opposite to the one applied by the driver. If the driver did not apply a constant force through the accelerator pedal, the two vehicles would eventually stop on their own.
When you're backing up a towed vehicle, you have to be careful not to move too quickly, because inertia also plays a factor when you apply the brakes. Remember, inertia refers to an object's resistance to changes in motion. If an object is moving, it will resist forces trying to stop it. When you're dealing with an object with a lot of mass -- two vehicles joined together, for example -- the inertia involved means more work is required to slow and stop the object's motion.
Next, we'll look at some techniques for backing up while towing a vehicle.