How can a locomotive pull a long, much heavier train?

The second difference is the equal sign instead of the less than or equal sign. This means that the force of friction remains constant as long as the object is sliding, and is no longer equal to the applied force. This means that the net force is not zero. Push harder on the chair by running and the chair will accelerate.

Let’s get back to that tug of war. The driver on the right now has an idea: instead of starting his engine, he downshifts to maintain static friction interaction with the rails. Slow and steady. The man on the left decks – and what’s going on? Its wheels rotate and it gets kinetic friction force. Well, static friction trumps kinetic friction, so the correct train wins!

This may work even if the train on the left is somewhat heavier. Therefore, it is possible for the train engine to pull larger cars. But wait! There is a more important factor: the moving train car 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: for cars that are towed, there are no wheels anymore any Friction with bars.

But there must be kinetic friction somewhere, and there already is, between the wheel axles and the car itself. To rotate, the axis must slide along some surface in the structure that holds it in place. But with roller bearings and lubrication, M_your It can be reduced significantly, from 0.56 for dry steel on steel to Something like 0.002.

Now we’re talking! This is how a locomotive can pull a long train of cars with a much greater mass. The engine pulls forward using steel on steel fixed Friction, which is very high (0.74), gives it good traction. The two cars have a resistive kinetic friction force whose coefficient is much smaller.

Some additional tricks

However, this huge weight of 10,000 metric tons creates a very high normal force, approximately 100 million Newtons. Remember that static friction is higher than kinetic friction. So, even if you can keep the train moving, you may not be able to start it.

That’s why trains have a trick called slow motion. If you’ve been near a train when it starts moving, you’ve probably heard a set of crackles as it moves down the line of cars. The reason is that the communication from one car to another is weak. So when the locomotive pulls the first car, the second car remains stationary until the slack is gone. With this trick, the locomotive can move one car at a time and add it to the group of moving cars. Very smart!

One last great thing. There is another type of friction called rolling friction. You see this on a truck with rubber tires: Under the weight of the vehicle, the tires flatten underneath. So when the truck is moving, the tires are constantly deforming and returning to their correct shape. This flexing heats up the tires, and where there is heat, energy loss occurs. Since energy is conserved, this means that the wheels slow down, and the truck must burn more fuel to maintain its speed. Trains, on the other hand, have very little friction, because their steel wheels hardly deform at all. This makes trains a more energy efficient means of transportation.

So, as you can see, it is indeed possible for a locomotive to pull a group of cars with a larger mass. You just need to use a little physics.