The second difference is the equal sign instead of the less than or equal sign. This means that the force of friction is constant as long as the object is sliding – it is no longer equal to the force applied. This means that the net force is not zero. When running, push harder on the chair and the chair will speed up.
Let’s get back to this tug of war. The driver on the right now has an idea: instead of starting the engine, he reduces the throttle to maintain stationary friction with the rails. Slowly and steadily. The guy on the left floor is… and what’s going on? Its wheels rotate and a kinetic force of friction acts on the body. Well, stationary friction overcomes kinetic friction, so the right train wins!
This would work even if the train on the left was slightly heavier. Therefore, the locomotive can pull wagons with greater weight. But wait! There is an even more critical factor: a moving wagon rolls, not slides. The wheel touches the rail at one point and then rolls to another point on the wheel. That’s the magic of wheels: with towed cars, it’s gone everyone friction with the rails.
But there has to be kinetic friction somewhere, and it does exist – between the wheel axles and the car itself. To rotate, the axle must slide on a surface in a housing that holds it in place. But with roller bearings and lubrication, Mk can be reduced significantly, from 0.56 for desiccated steel on steel to something around 0.002.
Now we’re talking! In this way, the locomotive can pull a long train of wagons with a much greater mass. The engine pulls forward using a steel-on-steel method stationary friction that is quite high (0.74), which ensures good grip. Cars have a resistive force of kinetic friction with a coefficient that is an order of magnitude smaller.
A few extra tricks
Nevertheless, this enormous mass of 10,000 metric tons provides a very high normal force – about 100 million newtons. And remember that stationary friction is greater than kinetic friction. So even if you manage to keep the train moving, you may not be able to start it.
That’s why there’s a trick called slack on trains. If you’ve ever been near a train that’s starting to move, you’ve probably heard the crackling sound as it moves along the line of cars. The reason is a loose connection between one car and another. So when the locomotive pulls the first car, the second car remains stationary until the slack is removed. With this trick, the locomotive can set one car in motion at a time and add it to the group of moving cars. Quite clever!
One last nippy thing. There is another type of friction called rolling friction. You can see this on a truck with rubber tires: the weight of the vehicle flattens the tires on the bottom. So when the truck is in motion, the tires constantly deform and return to their correct shape. This flexing heats the tires, and where there is heat, energy is lost. As energy is saved, the wheels snail-paced down and the truck must burn more fuel to maintain speed. Trains, on the other hand, have very little rolling friction because their steel wheels hardly deform at all. This makes trains a more energy-efficient means of transport.
Therefore, the locomotive can actually pull several wagons with greater mass. Just utilize a little physics.