I was thinking about a situation where a person in standing on the ground with some friction. The frictional force is directly proportional to the normal reaction acting on him by the ground. Assume that he leaned forward i.e his center of mass is not vertically up the point of contact of him with the ground.Then, does the frictional force change?
My thoughts:Although the person has leaned forward the force $mg$ acting on him will be vertically downward. Since normal reaction is always normal to the surface of contact its magnitude will remain same and thus frictional force will remain same.But, I have seen athletes starting the running race with a leaning forward position which would be mostly for increasing the friction between their shoes and the track. So, I'm in a dilemma. Please help.
Best Answer
It's not correct that runners lean forward to begin a race in order to increase friction. They lean forward because otherwise, they would experience no propulsion whatsoever because static friction is zero when the runner is completely upright.
When the runner leans forward and flexes his leg muscles, he exerts a horizontal force on the track in the backward direction. The track responds by exerting an equal and opposite frictional force (unless there is slipping) on the runner in the forward direction that propels him forward.
Generally speaking, the more a runner leans forward at the start, the larger the horizontal component of the force exerted by his legs against the ground, and the larger the frictional force he will experience. As a result, his initial acceleration will be greater.