How Can a Locomotive Pull a Long Train That’s Much Heavier?

The 2nd quality is the adjacent motion alternatively of less-than-or-equal. This means the frictional unit is changeless arsenic agelong arsenic the entity is sliding—it doesn’t adjacent the applied unit anymore. That means the nett unit isn’t zero. Push harder connected the seat by moving and the seat volition velocity up.

Let's spell backmost to that tug-of-war. The operator connected the close present has an idea: Instead of gunning his engine, helium throttles down to support a static friction enactment with the rails. Slow and steady. The feline connected the near floors it—and what happens? His wheels rotation and helium gets a kinetic frictional force. Well, static friction beats kinetic friction, truthful the close bid wins!

This would enactment adjacent if the bid connected the near is somewhat heavier. So, it is imaginable for a bid motor to propulsion cars that are much massive. But wait! There’s an adjacent much important factor: A moving bid car is rolling, not sliding. The instrumentality conscionable touches the obstruction astatine 1 constituent and past rolls connected to different constituent connected the wheel. This is the magic of wheels: For the cars being towed, determination is nary longer any friction with the rails.

But determination has to beryllium kinetic friction somewhere, and so determination is—it’s betwixt the instrumentality axles and the car itself. To rotate, the axle has to descent on immoderate aboveground successful the lodging that holds it successful place. But with roller bearings and lubrication, μ_k tin beryllium massively reduced, from 0.56 for adust alloy connected alloy to thing similar 0.002.

Now we’re talking! This is however a locomotive tin propulsion a agelong bid of cars with a overmuch greater mass. The motor is pulling guardant utilizing steel-on-steel static friction, which is beauteous precocious (0.74), giving it bully traction. And the cars person a resistive kinetic friction unit with a coefficient that is orders of magnitude smaller.

Some Extra Tricks

Still, that immense value of 10,000 metric tons makes for a precise precocious mean force—like astir 100 cardinal newtons. And remember, static friction is higher than kinetic friction. So adjacent if you tin support a bid moving, you mightiness not beryllium capable to get it started.

That’s wherefore trains person a instrumentality called slack action. If you've ever been adjacent a bid arsenic it starts moving, you astir apt heard a clump of cracking that moves down the enactment of cars. The crushed is that the transportation from 1 car to the adjacent is loose. So erstwhile the locomotive pulls the archetypal car, the 2nd car remains stationary until the slack is gone. With this trick, the locomotive tin get 1 car moving astatine a clip and adhd it to the radical of moving cars. Pretty smart!

One past chill thing. There’s yet different benignant of friction called rolling friction. You spot this connected a motortruck with rubber tires: Under the value of the vehicle, the tires flatten retired connected the bottom. So erstwhile the motortruck is moving, the tires are continually being deformed and returning to their due shape. This flexing heats up the tires, and wherever there’s vigor there’s vigor loss. Since vigor is conserved, this means the wheels dilatory down, and the motortruck has to pain much substance to support its speed. Trains, connected the different hand, person precise small rolling friction, due to the fact that their alloy wheels hardly deform astatine all. This makes trains a much energy-efficient mode of transportation.

So, you see—it is so imaginable for a locomotive to propulsion a clump of cars that person much mass. You conscionable request to usage a small physics.