# The reason behind circular motion of vehicles

In all the textbooks and internet resources that I've ever read , most of them explains role if friction in circular motion of an object (let's says a cycle), from the frame of the cyclist i.e. with the help of centrifugal pseudo force. I think they do this because it's more intuitive and easy to understand. However I want to Visualise it from the ground frame. The best explanation I gave myself is that without friction no change in the path of my cycle can be observed (ignoring everything other than gravity) so there has to be something with the help of which I'm able to continuously turn. But it feels dry and it was only to satisfy my thirst. Please share your views on this problem. And possibly explain it without any maths.

• – Paul T. Aug 17 at 20:38

## 2 Answers

This might help, I promise no maths...

We imagine your cycle going straight. To make it turn to the left, we have to either pull it or push it leftwards so it will start turning in a circle. And as long as we wish it to continue going in a circle, we have to keep pushing or pulling it ever to its left.

Where does this leftward force come from? If you had a friend in the middle of the circle with a rope and he tied one end of it to your bike frame and pulled on the other end, he could make you swing around and around in a circle by applying a constant pulling force towards the center of the circle, and around you go. But as soon as he lets go of the rope and thereby stops pulling on the bike, you and the bike will depart in a straight line.

Without a friend with a rope, you must instead rely on the friction force between the bike's tires and the pavement to push sideways on the bike and bend its path into a circle. The process by which a cyclist turns a bike is complex and has been written about extensively here, but the important point you need to know is this:

the bike leans to the left, and as it tracks around into a turn it is pushing sideways towards the right on the pavement- and here it is- the pavement is pushing sideways on the tire to the left, thereby providing the sideways force needed to bend the bike's path into a turn.

• Nice math free answer. +1 from me. – Gert Aug 17 at 21:42
• So that was the important point I was missing. So the cycle has to lean atleast a little bit to even start curving it's path. By leaning we are basically creating a component which can push the road sideways. And then friction will oppose this push and we will use this sidewise stopping force as the step we step in to, to make the turn.(feel what I'm tryna say). This also means that without friction, I'll disbalance if a lean , the instant gravity can't balance my torque, since there is no friction to help. Lol without Friction I can't even start the cycle. – NightKruger Aug 17 at 21:57
• But is circular motion in a cycle even possible without leaning? – NightKruger Aug 17 at 22:00
• certainly, if you have a friend with a rope! – niels nielsen Aug 17 at 22:24

If an object is to change its velocity, it must experience a resultant force, that is a push or pull from another body. For a bicycle on level ground, that resultant force must be horizontal, and the only forces usually available are (a) from the ground; we call that force 'friction' (b) air resistance, which, unless there is a wind blowing, acts in the opposite direction to the cyclist's velocity.

So if you want to increase your speed you need a frictional force from the ground in your forward direction; you obtain this by pedalling at an appropriate rate.

If you want to follow a circular path you steer accordingly, and a 'sideways' frictional force from the road provides the centripetal force needed to change your velocity (because in order to go in a circle the direction of the velocity vector has to keep changing. Although the magnitude of the velocity may stay the same, the change in direction means that there is a change in velocity – an acceleration! And, as you know, for a body to accelerate there must be a resultant force.