FRICTION ON WHEELS by fRancis Chong

What is the direction of the friction force exerted on the bicycle tires if the bicycle is moving to the right? [Assume that there is no slipping between the surface of the wheels (tires) and the surface of the ground]

Fellow Students,

As discussed in our remedial lecture, the answer is

So how do we arrive at this answer?

Before we start, let's define the rear wheel as the driven wheel and the front wheel as non-driven wheel.

We have no doubt that the bicycle is driven by the rear wheel and it rotates in clockwise direction when the bicycle is moving to the right. The surface of the tire tends to slip to the left across the surface of the road.. Therefore, the direction of the friction on the the tyre is to the right. It is this frictional force on the driven wheel that causes the bicycle to accelerate to the right.

With this explanation and looking at the diagram above, does not mean that the front (non-driven) wheel is rotating in anti-clockwise direction?

I believe some of you have already experimented on your bicycle wheels. Here's an animated view of a moving bicycle.

Yes, in order for the bicycle to move to the right, both bicycle wheels MUST rotate in the same clockwise direction. So why is the friction exerted on the front (non-driven) wheel directed to the left?

The first thing to note is that the front wheel is NOT the driven wheel. We can see the motion of front wheel as someone pushing a wheeled cart. There is a force that pushes the cart forward causing the wheel to rotate. If there is no friction exerted to the left on the tire of the front wheel, the front tire will slide to the right. The driving force is provided by the rear (driven) wheel which pushes the whole frame of the bicycle forward. A free body diagram of the forces acting on the front (non-driven) wheel is shown below.

In circumstances when the bicycle is moving at constant velocity or accelerating, the frictional force on the undriven wheel will always be in the opposite direction to the motion of the bicycle. UNLESS brakes are applied to the undriven wheel.

Hope this would enlighten you! God bless!

PLS NOTE THAT THIS IS A SIMPLIFIED EXPLANATION, FOR A MORE DETAILED EXPLANATION, YOU MAY WANT TO VISIT THE FOLLOWING URL

Static Friction on Wheels

 

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