When you trip on a crack, you fall forward due to INERTIA.
The first law of Newton stipulates that an object will tend to move at constant speed in a straight line or remain at rest unless an external force acts on the object.
Explanation:
As you walk, your whole body is traveling at the same speed and direction. When you trip on a crack, your foot is suddenly stopped. However, the upper part of the body is still in motion due to inertia (tendency to move at the constant speed your body was moving at and in a straight line). Your body, therefore, continues moving forward. Since your foot (especially forward the foot) is no longer moving with the body (and the forward foot is not available for the next step), the body lunges and you fall forward.
Learn More:
For more on inertia check out;
https://brainly.com/question/1845915
https://brainly.com/question/5056627
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