The second difference is the equal sign instead of less-than-or-equal to. This means that the friction force remains constant as long as the object is sliding – it is no longer equal to the applied force. This means that the net force is not zero. Run and push the chair hard, the speed of the chair will increase.
Let’s go back to that tug of war. The driver on the right now has an idea: instead of shutting down his engine, he accelerates downwards to maintain static frictional contact with the rail. slow and steady. The person on the left puts it on the floor—and what happens? Its wheels rotate and it receives kinetic friction force. Well, static friction beats kinetic friction, so the right train wins!
This will work even if the train on the left is somewhat heavy. Therefore, it is possible for rail engines to pull more spacious cars. But wait! There is an even more important factor: the moving train carriage is rolling, not slipping. The wheel simply touches the rail at one point and then rolls to another point on the wheel. It’s the magic of wheels: there is no longer an option for towed cars Any Friction with rails.
But there must be kinetic friction somewhere, and indeed there is – it is between the wheel axle and the car itself. To rotate, the spindle must slide on a surface in the housing that holds it in place. But with roller bearings and lubrication, μOf The mass can be reduced from 0.56 to 0.002 for dry steel on steel.
now we’re talking! Thus a locomotive can pull a long train of cars with very high mass. Engine pulling forward using steel-on-steel steady Friction, which is quite high (0.74), giving it good traction. And cars have a resistive kinetic friction force whose coefficient is of small magnitude.
some extra tricks
Still, that massive weight of 10,000 metric tons creates a much greater normal force – like about 100 million newtons. And remember, static friction is greater than kinetic friction. So even if you can get the train running, you may not be able to get it to start.
That’s why trains have a trick called slack action. If you’ve ever been near a train as it starts moving, you’ve probably heard some noises moving down the line of cars. The reason for this is that the connection from one car to another is loose. So when the locomotive pulls the first car, the second car remains stationary until the slack is removed. With this trick, the locomotive can move one car at a time and add it to a group of moving cars. Very smart!
One last good thing. There is also another type of friction called rolling friction. You can see this on trucks with rubber tires: under the weight of the vehicle, the tires flatten downwards. So when the truck is moving, the tires are constantly deforming and returning to their proper shape. This flexing causes the tires to overheat and energy is lost where the heat is. Since energy is conserved, this means the wheels slow down, and the truck has to burn more fuel to maintain its speed. Trains, on the other hand, have very low rolling friction, because their steel wheels hardly deform. This makes trains a more energy-efficient means of transportation.
So, you see- it is actually possible for a locomotive to pull cars of greater mass. You just need to use a little bit of physics.
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