What should be the direction of friction acting on both tyres of a running bicycle? My friend said that the direction of friction will be opposite for both tyres. My view is different, I think the direction of friction will be the same for both tyres as their slipping tendency is towards the same direction. Which of us is correct?
If a bicycle is coasting along, the front tire and rear tire will have friction in the same direction. If you are pedaling, you are applying torque to the rear wheel that propels the bike (friction pointing forwards); the front wheel meanwhile continues to experience only rolling friction, and would therefore experience a net force of friction that slows the bike down.
So "it depends". Coasting = same; pedaling = opposite.