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?
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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…
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 use of steel wheels on smooth steel tracks creates a very low rolling resistance – that is, the “braking” force that needs to be overcome to get a wheeled object to move. This low RR gives railway travel a very high efficiency.
Steel/steel friction is low but not zero. Trains are also very heavy, this weight somewhat improves the traction that can be applied. Trains therefore tend to accelerate (and decelerate) fairly slowly. For an electric train, if you’re not paying attention when the train sets off you will sometimes not realise you’re moving, at least for a few seconds.
The use of steel wheels on smooth steel tracks creates a very low rolling resistance – that is, the “braking” force that needs to be overcome to get a wheeled object to move. This low RR gives railway travel a very high efficiency.
Steel/steel friction is low but not zero. Trains are also very heavy, this weight somewhat improves the traction that can be applied. Trains therefore tend to accelerate (and decelerate) fairly slowly. For an electric train, if you’re not paying attention when the train sets off you will sometimes not realise you’re moving, at least for a few seconds.
There’s even less contact than you might think because the wheels are not flat to the rail, but rather are at an angle.
Each wheel tapers in slightly towards the center. It what keeps the wheels on the track and also allows the solid axle wheels to negotiate curves.
There’s even less contact than you might think because the wheels are not flat to the rail, but rather are at an angle.
Each wheel tapers in slightly towards the center. It what keeps the wheels on the track and also allows the solid axle wheels to negotiate curves.
The resulting movement between the wheel and the track is a function of both the force applied by the train on the track (ie. The weight of the train) and the coefficient of friction between the 2 surfaces. It is also a function of the size of the contact area between the 2 surfaces, however this doesn’t matter much in this discussion as I will clarify below.
As you mentioned, both surfaces are smooth metal surfaces, so the coefficient of friction is very low. That being said, the weight of the train is so massive, that even with this low coefficient of friction movement is achieved. Now that friction is achieved, we just need to ensure that the engine provides enough power to move the train in the horizontal direction.
The contact area between the wheel and the track can theoretically* also be increased if the weight of the train cant possibly increase enough (*this never applies in real life since the track is of constant size). This is however what many race cars must do, as they are looking to minimize weight.
The resulting movement between the wheel and the track is a function of both the force applied by the train on the track (ie. The weight of the train) and the coefficient of friction between the 2 surfaces. It is also a function of the size of the contact area between the 2 surfaces, however this doesn’t matter much in this discussion as I will clarify below.
As you mentioned, both surfaces are smooth metal surfaces, so the coefficient of friction is very low. That being said, the weight of the train is so massive, that even with this low coefficient of friction movement is achieved. Now that friction is achieved, we just need to ensure that the engine provides enough power to move the train in the horizontal direction.
The contact area between the wheel and the track can theoretically* also be increased if the weight of the train cant possibly increase enough (*this never applies in real life since the track is of constant size). This is however what many race cars must do, as they are looking to minimize weight.
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