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If I am running on a surface with friction with no air resistance and also accelerate then net force acting on me is F=ma. The friction acts opposite to direction of motion and is less than the forces that push me ahead giving net positive force and acceleration.
My question is how the forces that propel me become greater than frictional force while I am accelerating and are equal to frictional force when I am running with constant velocity?
Considering only net force when walking or running is misleading. Otherwise running with a constant velocity in a flat lane would be as effortless as biking, because the air resistance is small at that speeds.
If we run completely upright, at each step the resultant force direction between foot and soil can't be horizontal. If it were, there would be a torque and we would fall backwards.
That means there is a vertical component besides the horizontal one. We run making jumps and it is that work against gravity that makes us tired.
But we can run tilted forward, so the vertical component is smaller. The torque due to the horizontal friction force is balanced by our body inclination. But even so, there is always a vertical component, and we work against gravity.
Of course, there is an extra effort when accelerating, but most of the effort is related to keep a bigger velocity (higher or more frequent jumps) instead of small one.
Neglecting air resistance, on level ground, when running, without your shoes slipping on the ground, you only have static friction between the shoes and the ground. When accelerating you do not normally exceed the maximum of the static friction coefficient, if you do, the shoes slip on the ground and you have kinetic friction instead of static friction. So the force accelerating you is equal and opposite to the kinetic or static frictions.
