How can trains move on rails? if the wheels are smooth and the rails are also smooth, how can it be enough friction for it to move?
In: 2785
f = uN
Where f is Friction force, u is coefficient of friction and N is normal force. As you might notice, contact area is not really a factor because less contact area at same force means more pressure. It’s a lot of pressure between the wheel and the rail. The train is heavy and while smooth steel to smooth steel does not have very high coefficient of friction, it’s not that low either and it’s not like rails are built on a steep incline to require all that much friction. You have plenty of time to get going or to stop.
The real important part about trains is not the amount of friction between rail and wheels, the wheels might slip only at the most extreme situations. The important part is how little the wheels and rails deform as the train rolls over, that translates to very little rolling friction. It takes very little energy to keep the train moving compared to something like trucks or cars.
f = uN
Where f is Friction force, u is coefficient of friction and N is normal force. As you might notice, contact area is not really a factor because less contact area at same force means more pressure. It’s a lot of pressure between the wheel and the rail. The train is heavy and while smooth steel to smooth steel does not have very high coefficient of friction, it’s not that low either and it’s not like rails are built on a steep incline to require all that much friction. You have plenty of time to get going or to stop.
The real important part about trains is not the amount of friction between rail and wheels, the wheels might slip only at the most extreme situations. The important part is how little the wheels and rails deform as the train rolls over, that translates to very little rolling friction. It takes very little energy to keep the train moving compared to something like trucks or cars.
The simple answer is that you need a lot less force to move than you probably think. A strong person can move a train carriage by hand if it is on perfectly flat ground – all you have to is overcome the rolling resistance and let it accelerate very slowly.
In order to get something on wheels moving, you need to overcome the rolling resistance (how hard is it to make the wheels turn), the angle of the slope it’s on and any bumps it has to climb over.
Trains basically work by minimising all 3 of these factors. The tracks are very smooth so basically no bumps, the steel wheels running on greased bearings have very little rolling resistance and tracks are made as flat as possible (freight trains aim for less than 1.5% slope, or 1.5cm of height change per 1m of distance)
Once you’ve overcome the factors preventing the train from moving, it’s just mass x acceleration and trains generally don’t accelerate very quickly so they can move a lot of mass with a relatively low force
Edit: unit error…
The simple answer is that you need a lot less force to move than you probably think. A strong person can move a train carriage by hand if it is on perfectly flat ground – all you have to is overcome the rolling resistance and let it accelerate very slowly.
In order to get something on wheels moving, you need to overcome the rolling resistance (how hard is it to make the wheels turn), the angle of the slope it’s on and any bumps it has to climb over.
Trains basically work by minimising all 3 of these factors. The tracks are very smooth so basically no bumps, the steel wheels running on greased bearings have very little rolling resistance and tracks are made as flat as possible (freight trains aim for less than 1.5% slope, or 1.5cm of height change per 1m of distance)
Once you’ve overcome the factors preventing the train from moving, it’s just mass x acceleration and trains generally don’t accelerate very quickly so they can move a lot of mass with a relatively low force
Edit: unit error…
Steel on steel actually creates a lot more friction than people in this comment section are making it out to be….
Steel on steel actually creates a lot more friction than people in this comment section are making it out to be….
Because friction is only very low, but it is not zero. We actually want low friction to save energy.
Trams and trains generally carry sand to put on the tracks to increase friction temporarily (e.g. steep inclines in ice and snow, or emergency breaking).
Because friction is only very low, but it is not zero. We actually want low friction to save energy.
Trams and trains generally carry sand to put on the tracks to increase friction temporarily (e.g. steep inclines in ice and snow, or emergency breaking).
Trains are heavy, monstrously heavy, so that weight helps with the lack of friction somewhat.
The engines also have grit blowers (basically sand) in front of the driving wheels to increase friction and thus traction so the machine can get moving.
Once a train gets moving, it takes little energy to keep them moving, so there are quite a few benefits to using steel on steel to move goods around.
There are other methods as well, especially for dealing with inclines and declines but that is getting deep into the process.
Trains are heavy, monstrously heavy, so that weight helps with the lack of friction somewhat.
The engines also have grit blowers (basically sand) in front of the driving wheels to increase friction and thus traction so the machine can get moving.
Once a train gets moving, it takes little energy to keep them moving, so there are quite a few benefits to using steel on steel to move goods around.
There are other methods as well, especially for dealing with inclines and declines but that is getting deep into the process.
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