Hint: The law of conservation of angular momentum states that when no external torque acts on an object, no change of angular momentum will occur. This means that when the object is spinning or rotating on its axis it has a particular momentum and that momentum will not change unless the external torque is applied.
Complete step by step answer: Conservation of angular momentum is quite similar to conservation of linear momentum. In the case of linear momentum, we know that the momentum of an isolated system will remain constant and that if we want to change the total momentum, we must apply an external force to the system.
In the case of angular momentum, as the body is in rotational motion, the total momentum of an isolated system will be constant unless an external torque acts on the system. Here the external force of the linear momentum is replaced by the torque because torque is the turning effect of the force.
When we ride a bike, it has angular momentum due to the spinning motion of the wheels and to a smaller extent, the pedals. For this momentum to be constant we have to consider it with respect to both size and direction of the spinning objects of the bike. The direction of the angular momentum vector lies along the axis of rotation that is the axle of the wheel. So, as long as no external torque is present, the axle of the wheels will remain horizontal, and as a result, the entire bike will remain vertical.
When we paddle faster the wheels spin faster in this case the greater will be the angular momentum so, more torque will be needed to produce a change. This is why little we find that once we get a good start on the bike, we can ride it without fear
Note: • The torque is the rotating effect of the force • When we ride a bike the spinning or rotating object have their momentum it may increase for one part or may increase for another but the net total remains constant so there is no change in the axis of rotation thus bike remains vertical. • If we apply external torque or in other words external force then there will be a change in total momentum which might change the axis of rotation leading to instability of the bike.