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Today in our Physics class, we learned about static equilibrium of solid rigid. The teacher presented us with an example in which a man was walking up an inclined board held up by the ground on one end and by his truck on the other (I hope this picture makes sense).

At any given moment, the teacher represented all the forces that were acting on the board: its weight, the vertical normal force from the ground,the horizontal normal force from the truck, and then the (vertical) weight of the man on the board.

Can someone please explain to me why we consider that the force the man exerts on the board is its weight and not the reaction (Newton's 3rd law) to the normal force the board exerts on the man? What would happen if instead of a man we had a ball?

I am really confused and any help would be really appreciated.

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Assume that the man is moving up the slope at constant velocity.
This means that the net force on the man must be zero.
You can imagine that there are three forces acting on the man:

  • the force on the man due to the gravitational attraction of the Earth (the weight of the man)
  • the normal reaction on the man due to the plank
  • the frictional force on the man due to the plank.

The sum of the frictional force on the man and the normal reaction on the man must be equal and opposite to the weight of the man.

enter image description here

Looking at the plank and using Newton's third law:

  • The normal reaction on the plank due to the man must be equal and opposite to the normal reaction on the man due to the plank
  • The frictional force on the plank due to the man must be equal and opposite to the frictional force on the man due to the plank.

So the normal reaction on the plank due to the man plus the frictional force on the plank due to the man must equal the weight of the man.

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  • $\begingroup$ Thank you. Your explanation is very clear and straightforward. What would happen if friction was not considered? $\endgroup$ – Bee May 20 '17 at 12:59
  • $\begingroup$ The man could not go up the ramp. $\endgroup$ – Farcher May 20 '17 at 12:59

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