Hi, I am new poster on this forum. Apologies if you have already answered a similar problem before. let me get to the point straightaway. Now consider one example: Take a shopping cart and sit in it. Now jerk your body in any manner you require. The goal is to create a net motion forward. Can you sit in the cart and move it say 1 meter forward by giving the appropriate jerks? remember that you cannot push against the ground or a wall. Your arms and legs are totally inside the shopping cart and not touching any external surfaces. My contention is that this is quite possible. My friend is equally convinced that it is impossible? Who is right ? (and why?) For this problem i think we need to consider the ideal case (where there is no friction involved) and the real world case (where there is friction is involved). I am quite convinced that if i take a walmart shopping cart moving my self forward by simply jerking my body (while not contacting any external surface) is quite do able. I have a theory for this, but i will wait for your answers so as not to bias you in any way. Thanks again and i hope to get some illuminating answers.