Dynamics: Direction of friction on a bicycle wheel.

A bicycle (with rider) is speeding up. In which direction does the friction act on the front wheel? On the rear wheel? Don't use your gut instinct. Instead, justify your answer based on physical principles.

[willratcliff]

Thank you so much! I've been looking to understand this for a long time

[codehere]

“An expert is a person who has made all the mistakes that can be made in a very narrow field.” – Neils Bohr

[thecarman]

After thinking about the initial question for a few moments I actually came to the correct conclusion for each of the frictional force vectors. The frictional force that acts on any body is commonly pointing in the opposite direction of motion or intended motion --- a stationary block on an incline remains motionless because friction points opposite to how it would slide. Taking the bike's front wheel off the bike and allowing it to roll on its own (assuming it remains vertical) one would expect the wheel to eventually slow to a stop, hence friction points to the left for the front wheel. As for the rear wheel, if one considers the wheel to be on ice it would just spin (while pedaling), so the motion for the bottom point is to the left, hence friction must be to the right. But while coasting both wheels will have friction pointing to the rear, since the rear wheel becomes just like the front.

[wandilemngoma]

Sir I would like to thank you I found this video very helpful in terms of the way you explained it with principles, however what i would like to understand based on the permutation u have told us to consider for a case where the bicycle slows down or the brakes are rapidly applied(assuming the wheels lock/slide) or in lamens terms the wheel rolls with slipping would the frictional force on the rear wheel be in the same direction as it is in your video lecture analysis or would it differ and why.

[ajmo]

cant we say the point touching the ground have a tendency to move in left direction so the net force at that point is in just opposite drxn and since only friction is there at that point....hence friction is in right

[mustafasarac]

What if the direction of motion on the front wheel when bicycle slows down with its rear wheel locked of braking?

[KhoaNguyen-rvsc]

Sir, allow me to ask why we do not include the force of the driving torque (let's call it Ft) in the set of equations, the direction of it will be opposite of the bycicle direction right?

[thedeathofbirth]

Hi Brianno, You are a brilliant teacher and communicator. As a high school student my teacher does not use the precise words as you do to explain the forces in real world and hence a lot of confusion is created unintentionally. Also for me it is NOT enough that the teachers says friction is always acts in the opposite direction of the motion! I would like to know if you have full lectures like this one, on line for the whole course.( I am hoping your course starts with motion and covers the rest as a typical grade 12 course goes. Of course I assume this is a higher level but your explanations are clear enough that even I can follow and take advantage of.) Can you please give me the URL for your course. I am a highschool student dreaming to be a mechanical engineer, by the time I finished this video I could hardly breath because many of my misconceptions were taken care of and I realized there is a way to learn how to think like a physicist! Many thanks for this video and for the possible information about how to find your lectures.

[presauced]

Hey! Are you still working on Spumone?