Final answer:
The backward force experienced when running into a wall is due to Newton's third law, which states that every action has an equal and opposite reaction. The wall applies this force in response to the force exerted by running into it, not due to a backward force acting on the person.
Step-by-step explanation:
The backward force you experience when you run into a wall is a result of Newton's third law, which states that for every action there is an equal and opposite reaction. When you collide with the wall, you apply a force to it, and in turn, the wall applies an equal and opposite force against you. This is why you feel a sudden backward force that can be several times larger than your weight.
Similarly, an observed force during activities, like being pushed back into the seat during an airplane's takeoff or being pushed to one side when taking a tight turn in a car, can be explained through Newton's first law. It states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
Therefore, when running into a wall, the sudden deceleration is not due to a backward force but rather the wall applying a forward force, stopping your motion