Explain why a fast-moving freight train takes a few kilometers to stop.?

Question

Explain why a fast-moving freight train takes a few kilometers to stop.

Answer 1

its really heavy and if you ever rode amtrak, then you know that you always get delayed because freight trains always need the track and its harder to stop a heavy train

Answer 2

cause it weighs hunderds of tons

Answer 3

Ah yes.  We can thank Sir Isaac Newton for the law of inertia, but we can thank George Westinghouse for getting trains stopped, since it is his design of air brakes that makes the latter possible.

As has been pointed out, freight trains have much mass, as much as 16,000 tons for unit trains in the US.  But, let's put that number into perspective.

A US Naval destroyer of the WWII vintage weighed about 3,000 tons, so our train is equivalent to 5 of these large ships, on roller bearings, moving at appreciable speed, with very little friction between the wheels and the rails they ride on.  Add a downgrade and the recipe for something that really wants to keep moving is complete, thanks to Newton.

Now let's do a little time / distance caluclation.  Let us assume we are on a train 8,000 feet in length traveling at 60 MPH, or one mile per minute.  The brakes are applied by a reduction in air pressure through the brake pipe.  But, this takes time to accomplish.  At a "service" rate of reduction, that air flows (called "propagation time") at 500 feet per second.  So, when we begin slowing and apply the brakes, it takes a full 16 seconds for the brake application to even begin to affect the last car in our train.  Since we are traveling at 60 per, we have traveled over 1/4 of a mile just to get the brakes applied and we haven't even slowed one mile per hour yet !

The combination of speed, tonnage, length of train, tons per operative brake, curvature and grade conditions will dictate the distance needed to stop, as it varies from train to train.  A nice, smooth controlled stop of the train can take up to five miles or more.  Even at an emergency application rate of air flow, which is 1,000 feet per second, it still takes a mile or more for your average train moving at speed on flat ground to stop.

So, and this is the important part of the answer, if I am an engineer on an average train, moving at nearly any speed, if I can SEE you, it is already too late to STOP for you, if in the way.  So, stay alive and stay in one piece by staying out of the way.  Your friends, family and loved ones will thank you for it.

And so will the train crew.  Someone's brains spattered on the front of your engine can be unnerving........

Answer 4

all good answers and correct

try looking at this in a different perspective

lets say a train is a mile and a half long, with decent power it can get to 50 MPH in that distance.

Also in an emergency brake application it can stop in that distance.

There is not a high performance sports car in the world that can get to 50 mph or stop from 50 mph in it's own length.

Answer 5

The locomotive will weigh somewhere between 100 & 150 tons,each freight car can weigh around 100 tons fully loaded. In the US a train of 120+ freight cars isn't unheard of,with several heavy locomotives doing the hauling,so you can figure a weight for the whole ensemble of up to 12,000 tons or so. In the UK freight trains are much shorter with the heaviest weighing in at 2,000 tons or thereabouts.

The momentum each train will have will obviously be astronomical and to ensure safety stopping distances have to be generous.This will stop wheels locking & skidding causing damage to the wheels & track as the dymanics of the wheel to rail interface are different to the tyre to road interface on a car or truck.The total area of surface contact between the wheel & the rail is surprisingly small & because it is a steel to steel contact on an often slippery surface it will have to take a long way to stop.The stopping distance also stops the loads within the freight cars shifting.

Answer 6

because of one of newton's laws of physics "an object in motion tends to stay in motion, unless a force acts upon it...", and because of the property of velocity, if a bowling ball is thrown at you it's gonna be pretty hard to stop quickly but if a tennis ball is thrown at you it can be stopped very easily.

Answer 7

a locomotive weighs around 160 tons

times that by 2 or 3.

then 1 trailer car may weigh around 25 tons.

times that by say 30.

add the cargo of each trailer, say coal at 10 to 15 tons.

times that by 30.

then add the fact that trains use air brakes and they don't come on the same a hydraulic brakes.

there is a big lack before all are applied and stopping the train.

so yeah,

huge weight  doesn't stop on a dime !

something like that.

all ballpark figures.

Answer 8

When the Engineer applies Brakes, the moment he does so, the weight behind him keeps pushing along.  Until all energy is used up in Braking procedure.

Answer 9

So the guy in the caboose  doesn't get his nose bashed everytime the train comes to a halt. Are you SERIOUS?

Answer 10

Like any other large vehicle (Tractor-trailer, large seagoing ship, etc.), Everything obeys Newton's First Law of Motion:

Newton's first law of motion is often stated as

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. (Unbalanced force here being the brake force applied to the train's wheels and the friction between the wheels and the tracks/grade.)

The stopping distance of that object is directly proportional to it's mass. The greater the mass, the greater the inertia.  (resistance of an object to change it's present state) and momentum (weight times velocity). The faster it's going-the longer distance it needs to stop.