One, two, then three
The chapter opens with three attempts to push a motor car that has stopped on the road with a mechanical fault.
One person
The force applied was not enough to move the car, so the car did not move.
Two people
On that occasion too the car did not move.
Three people
On that occasion the car moved.
Figure to be added
Figure to be added
Figure to be added
What changed?
The car is the same and the obstruction is the same. All that changed is the size of the force applied. Several people pushing rather than one means all the forces add together to make a larger force in the one direction, and that gets the job done easily.
The direction is not incidental
To push a motor car like this, every force on it must be applied in the direction it has to move. If the three had pushed in three different directions the forces would still add up to something, but the car would not go anywhere.
On the next page this idea of "the forces adding up" gets a name.