Month: October 2016

1002 Gravitron

When we see the bodies pinned against the wall, it is tempting to imagine that there are some centrifugal forces pushing those people outward against the wall. But there is none. Those people are moving in circles, which required a centripetal force. The normal contact force that the wall exerted on the bodies provided this required centripetal force.

The motion is fully explained by the presence of a centripetal force, not a centrifugal force.

When we see the people struggling to move towards the centre of the spinning drum, we are again tempted to think that some centrifugal forces are flinging them outward towards the wall. Wrong again. What’s happening is that when those people leave the wall, they can only count on the friction on their shoes to provide the required centripetal force. Their shoes obviously did not provide enough friction, hence they were not able to keep up with the circular motion of the spinning drum. They thus moved in a somewhat tangential direction, and ended up looking as if they were moving along the radius of the spinning drum.

Again, the motion is fully explained by the insufficient or absence of a centripetal force. Let’s banish the centrifugal force forever.

1001 Wall of Death

If done on a horizontal surface, it would be the normal contact force holding up the weight of the bike, and the frictional force providing the required centripetal force.

When done on a vertical surface (as in this video), it is the frictional force holding up the weight of the bike, and the normal contact force providing the required centripetal force.

The key to pulling off this stunt successfully is to ride at a high speed. This forces the bike and wall to be pressed hard into each other, resulting in a large normal contact force. A large normal contact force is crucial because the amount of friction depends on it.

1000 NASCAR

Surely you noticed that the track was slanted at the bends? These are called banked curves.

If you had watched F1 racing before, you should know that F1 race tracks are flat. The race cars have to slow down (F1 fans love to watch and hear the acceleration) when they go into the curves . If they do not, the required centripetal force (mv2/r) will be too large. The tires just do not have enough grip to produce the required amount of friction.

NASCAR race cars do not slow down when they turn. Banking allows the cars to utilize the normal contact force that the track surface exerts on the cars to provide the required centripetal force. In fact, at particular speed and angles, the cars can negotiate the bends without the tires providing any sideway frictional force at all. This allows NASCAR to turn at full speed, which NASCAR fans love.

(Worked example explained at xmtutor)