In a recent stunt, a Ford crew hitched an all-electric F-150 pickup truck to a freight train filled with 42 more F-150s. Then a driver hit the throttle, and the pickup truck towed the 1.3 million-pound train. This raises some interesting questions. How hard is it for a truck to pull a train? Is that even possible? Could a normal truck do this? Of course it’s an impressive feat—but the real limiting factor is friction.
Let’s start with a more idealized situation. That’s what we do in physics—when something is potentially complicated, we make the scenario less complicated to make sure we are on the right track.
The Case With No Friction
So, what would it take to pull a giant train in the case of zero friction? The answer is that any tiny force would move the train. Even an ant could move it. Yes, this is true—it just seems impossible because you’ve never encountered a situation with zero friction. Here is a force diagram for a tiny object pulling a massive object with no friction. I’m going to use boxes to represent the objects, but if you squint real hard you can make that box look like an ant.
That diagram might look complicated, but it’s not too bad. Let me go over all the details. The first thing that might seem puzzling are those arrows over some of the symbols. You don’t really need to know about those, but that means those quantities are vectors. Yes, force is a vector. That means that pulling to the left on an object is not the same as pulling to the right. Direction matters with forces, and forces are vectors.
Next, let’s look at those two forces pulling down on block A and block B. These are the gravitational forces due to the blocks’ interaction with Earth; this is also called “weight.” The gravitational force depends on the mass of the object and the gravitational field (g), which has a magnitude of about 9.8 newtons per kilogram. This means that more massive objects have a greater weight. Oh, but you knew that—you just might not have known why you knew it. So object B has much more mass, and it has a much greater weight.
The upward-pushing force labeled N is called the normal force. This is a force between the object and the surface. If this were a train on a railroad track, the normal force would be from the rails pushing up on the train and preventing it from falling through the surface. It’s called a “normal” force because this force is always perpendicular to the surface—remember that in geometry “normal” means at a right angle. Since object B has a much greater weight, it also has a much greater normal force. It has to so that it doesn’t fall through the tracks. This normal force will become much more important when we add friction.