Question

Explain why bicycles are easier to pedal when their tires are fully pumped up with air.

Answer 1

$answer$

The sufficient air in the tires decreases the point of contact to the ground which in turn decreases the friction.

Decrease in friction means that you'll have to put a less of force to overcome that decreased friction. which is why you have to put a less energy into paddling the bike.

Answer 2

Air resistance pushes the bike in the direction opposite which its moving. Friction and rolling resistance on the chain and tires also make it harder to move forward on a bike.