Push V Pull, you decide….

My cousin asked an innocent question via email today to a few people within my family via cc. The question was, “Why is it easier to pull something than pull it?” Good question.

My cousin David thought that the contrary was true and that William was just doing it wrong. He said that when you push something you get to use your whole body and not just his “stick like lawyer arms”, and therefore, he surmised that my cousin was “doing it all wrong”.

My brother? Well, he’s an engineer….

[Editorial comments (made by me) are in square brackets]

Okay,

There are two separate problems with pushing an object, but both are related to the box tending to rotate.

Consider a cubic box 1m x 1m x 1m filled to the brim with pornographic magazines. I believe Grant [never you mind who “Grant” is, but it’s probably true] has such a box in his garage. This box weighs 1000kg and because its contents is uniform it has a center of gravity located at the cubes center, or .5m above the floor and .5m from the sides.

If Thomas [that’s me – This blog’s author] is contracted to remove this box and he decides to push it the following will probably happen:

Because Thomas is (slightly)  taller than the box [What? Outrageous!!] he will most likely push on the top rear corner. This will have 3 effects:

  1. This will tend to rotate the box about its centre of gravity such that the front bottom corner will dig into the ground
  2. He is pushing slightly downward on the box. This will tend to increase the effective weight of the box and increase the friction resting the motion of the box.
  3. Because he is pushing down on the box he is reducing the weight he is putting on his feet, increasing the likely hood that they will slip

Because Thomas is a savvy business man he will not push the box, he will pull it. If he is truly clever, he will attach the rope to the very bottom front corner of the box. This will have the following 3 effects:

  1. This will tend to rotate the box about its centre of gravity such that the front bottom corner will have less pressure on the ground
  2. He is pulling upward on the box. This will tend to decrease the effective weight of the box and decrease the friction resting the motion of the box.
  3. Because he is pulling up on the box he is increasing the weight he is putting on his feet, increasing the traction available for him.

The other problem with pushing a box is that if you push to one side of the center of gravity the box will tend to rotate away from you rather than sliding away from you. Consider the following top views of a box:

If Thomas pushes to the right of the centre of gravity the box will tend to turn. As it turns the distance, d, which is the distance between the pushing force’s line of action and the centre of gravity will increase. If Thomas keeps merrily pushing away in the exact same direction, then the box will eventually rotate so far that Thomas pushes past the box as it spins out of his way.

However if Thomas fastens a rope to the right of the centre of gravity as shown, the rotation of the box will tend to rotate in such a way that the distance d decreases. Eventually the box will rotate far enough that the pulling force’s line of action goes right through the boxes centre of gravity and it will no longer rotate.

I have many more diagrams if you want to see them.

And so THAT ladies and gentlemen, is why you pull a box instead of push it. There’s no mention of loading it into a truck, bin, trailer that’s facing down hill though. I asked him about it and he said that was next week’s lesson. He’s not a garbage man, so I’ll spare him the graphical representation. DON’T FIGHT GRAVITY. I’m always amazed when I see roofers rolling wheel barrow after wheel barrow of shingles up a hill into a bin.

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1 Comment »

  1. We have to move things every day of the week and when it comes to moving boxes, we always push large ones. However, as an engineer, I can fully appreciate the above theory. However, in practise, you have to also remember your back (and spine in particular) and pulling boxes can put an awful strain on that. The other thing to note is that there’s always the option of pushing from the bottom, rather than the top, where you end up putting a vertical force on the front bottom corner of the box.

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