# Thread: If my arm would be much more massive than the rest of my body would my body actually move backward instead of my arm forward

1. When I move my arm my body stands still. If my arm would be much more massive than the rest of my body would my body actually move backward instead of my arm forward?

2.

3. What's forward, what's backwards? It's all about perspective

4. Originally Posted by hgfhjgfh
When I move my arm my body stands still. If my arm would be much more massive than the rest of my body would my body actually move backward instead of my arm forward?
Your body does also move in the opposite direction when your move your arm. It's just too small a movement to notice. If your arm were much more massive, then you would notice it. It wouldn't move instead of your arm; your arm would still move too.

5. (I will use abstract approximate 1 dimension positions for example) I mean let's say I stay at position x=0, now I move my arm forward(positive value change), my arm is at position x=5 any my body is still at position x=0 cause friction of my body and ground is much greater than my body movement backward (caused by moving arm forward).

Now let's say my arm weight 100x more than the rest of my body(it is just example), when I try to move my arm forward will instead my body be at position like x=-5(backward movement) and my arm like x=0 (or a very small positive value) ? (It seems a bit strange - I based it on understanding that less massive part moves much more than more massive)

6. If you anchor the body to the ground by friction, then it isn't going to move at all. At some point, the force of acceleration will overcome the frictional force and your feet will lose traction. But you haven't provided enough information to determine that, like the coefficient of friction, and the rate of acceleration.

If you can ignore friction, then you can use conservation of momentum to solve the problem. m1v1 = m2v2 where m1 is the mass of the body, m2 is the mass of the arm, v1 is the velocity of the body and v2 is the velocity of the arm in the opposite direction.
After the arm is fully extended, both masses will come to a stop. The center of mass of the system will not have changed, so the heavier mass will be moved just a little bit from its initial position, and the lighter mass will be moved further. The distance of each mass from the center of mass will be inversely proportional to its mass.

7. It seems you are right but I just can't imagine that if I would hold barbell 2x heavier than me when trying to move my arms forward I would actually move my body much more backward (it's almost like pushing away from a wall)

8. It would be pretty hard to hold a barbell twice your own weight and push it straight away from your body, but you could set it on a wheeled cart and push it. Now, if you were on a wheeled cart yourself when you did this, what do you think is going to happen?

9.

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