Chapter 4, Problem 8Q

(a) Why do you push down harder on the pedals of a bicycle when first starting out than when moving at constant speed? (b) Why do you need to pedal at all when cycling at constant speed?

To determine

Why do you push harder on the pedals of a bicycle when starting to move at a constant velocity? Why do you need to keep pedaling when cycling at a constant speed?

Answer to Problem 8Q

Solution:

a)Because the force of static friction is greater than the force of kinetic friction. You need a force to overcome the static friction.

b)To overcome the losses of aerodynamic drag, frictional and mechanical losses the pedaling necessary to move in a constant speed

Explanation of Solution

a)Because the force of static friction is greater than the force of kinetic friction. You need a force to overcome the static friction.

In order to accelerate from rest, the applied force must be greater than zero. When cycle is moving at a constant speed, net force equals zero. When starting to move, it is necessary to push the pedal to overcome static friction which is larger than kinetic friction.

b)Because your bicycle is not an ideal system, and there will be forces which slow you down directly like aerodynamic drag and indirectly as friction and mechanical losses in the pedal mechanism. You will deaccelerate if you do not provide sufficient force to overcome these forces. If you provide force enough to counter these, you maintain a constant speed.