Final Answer:
The force that causes you to stop when running and decelerating is primarily friction between your feet and the ground.
Step-by-step explanation:
When you run and want to stop quickly, the primary force responsible for deceleration is friction. Friction is a force that opposes the relative motion or tendency of such motion of two surfaces in contact. In the context of running, it arises between the soles of your shoes and the surface you are running on, typically the ground. As you attempt to stop, the frictional force acts in the opposite direction to your motion, gradually bringing you to a halt.
The coefficient of friction between your shoes and the ground plays a crucial role in determining the effectiveness of this force. A higher coefficient of friction results in a stronger force resisting your forward motion. This force is instrumental not only in stopping but also in providing traction, allowing you to change direction quickly.
It's essential to note that other factors, such as the nature of the surface and the design of your shoes, can also influence the effectiveness of the frictional force in deceleration. Ultimately, it is the interaction between your footwear and the ground that generates the frictional force, bringing your running to a quick stop.