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When you are walking forwards, 
    you step with a foot in front then when it lands on the ground, using that foot, you exert a force on the ground backwards so your body moves forward.
On the other foot after, you did the same thing again which makes you move forward. 
When you walk backwards,
   everything is reversed, you step with a foot at your back and exert a forward force which makes your body move backward and you did the same thing again on the other foot which makes you walk backwards continuously.
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