# How to guess direction of friction in any case

Here , in case of a death well situation as an example.

Q How is direction of friction going to be opposite to mg or in upwards direction ?

Why not:

1. In case of like circular turning , direction of friction is toward the centre. Why is it not the same here ?

2. Online it says that when f=mg , then only net force is towards the centre. How is that possible ? Normal is pushing the bike. How is it still on the track then if f and mg cancel out.

3. Friction should be in a direction opposite to motion I.e since the bike is moving forward I,e towards you in picture. Then , friction should be backwards.

Similarly , there are many situation in which it becomes difficult to guess the direction of friction. For example , banking of road , vertical circular motion of a car or bike.

Especially in cases of banked raids, direction of friction changes as magnitude of velocity changes .

Q:What is the way with which I can always and I’m almost every situation , guess the direction of friction or how can you find the direction of friction in an intuitive way.

The easiest way to find is the direction of friction between two contacting bodies is to consider the direction of sliding (i.e. relative movement).

The key concept is that there needs to be relative movement between two surfaces (during the rolling of the wheel there is no relative movement, therefore in the direction of movement of a free rolling wheel there is no friction).

Also, it is important to note that there does not have to be actual sliding. Even if there are resultant forces towards a direction, which could cause sliding, then that force direction could be considered the direction of sliding (actually you would have had sliding if not for the friction).

The direction of friction is always opposite to the direction of sliding.

Q How is direction of friction going to be opposite to mg or in upwards direction ?

mg would move the bicycle downwards (ie. it would slide against the wall if there was no friction.

Why not: In case of like circular turning , direction of friction is toward the centre. Why is it not the same here ?

In circular turning the reaction is vertical and unless there is superelevation does not contribute to turning. In the death well, the reaction from the floor is what actually turns the motorcycle.

Online it says that when f=mg , then only net force is towards the centre. How is that possible ? Normal is pushing the bike. How is it still on the track then if f and mg cancel out.

The normal force is the net force and from Newton's laws:

$$\sum F = m \cdot a$$

For a constant velocity, the only acceleration is the centripetal acceleration. So the normal force from the side of the death well is what keeps the motor on track.

Friction should be in a direction opposite to motion I.e since the bike is moving forward I,e towards you in picture. Then , friction should be backwards.

This statement is completely wrong. When a wheel is turning without sliding then there is no friction. In order to visualize this consider the following. Imagine you are pedaling with a bike on a flat surface, then stop to pedal.

1. if the wheel is rolling freely you will go a long way.
2. if you hit hard on the brakes then the wheel will slide on the floor. On that case you have friction between the wheels and the flat surface.

In the case no 1. eventually you will stop only because of the deformation of the wheel, not because of the friction on between the wheel and the surface.

• Ok. It is useful. Thanks a lot.I would also like to know if for the death well situation there is an intuitive way to think about the problem ?. Why is friction opposite to sliding. For example , friction is the bike up. That doesn’t sound like Sth possible for friction. May 20 at 15:47
• It is possible, because the only forces (apart from friction) are the weight, the reaction from the death well. The weight would be dragging the motorcycle downwards (that is the would be sliding direction), so the friction opposes that sliding. if you are confused about the direction of the friction, then consider a mass on an inclined plane (the friction is pointing upwards). May 20 at 15:51

The keys are 1) the wheel must in contact with a surface, 2) having the noraml force pushing it against the surface, 3) the friction is always in the reversed direction of the motion.

In the case shown in the picture, there could be no friction because conditions 1 and 2 above do not exist. Or there could be two frictional forces, one pointing backward against the direction of motion, and one pointing up against the inertial/gravitational force. This phenomenon happens on cars running on a curved road with superelevation towards the center, the side friction is pointing toward the center to resist the centrifugal force that tends to throw the car out of the driving lane.

Friction is the resistance of instantaneous bonds between the molecules of the two contact surfaces against their sliding.

Any effort or force trying to slide one of the surfaces against the other will cause a reaction exactly equal and in the opposite direction of that force. (static friction)

This static friction will keep increasing equal to applied force until it becomes greater than the bond between the two surfaces.

When this force exceeds this level the bond will break with a sudden jerk and the two surfaces will slide against each other with less resistance, kinetic friction.

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