I am trying to understand how to create a correct Free Body diagram and understand formula used to forces acting on object (i.e. a robot with 4 wheels) that is "placed" on vertical surface such as a steel wall.

Normally, I have seen a lot of example on internet regarding a books being "pushed" onto the wall and the book is 'motionless' or stuck.

Q1) However, what would happen if a magnet attached to the object created a pulling force (towards the wall, assuming the wall is made of steel).

Q2) Also, is 'motionless' is same as equilibrium? Can object be in equilibrium but still be moving where we can evaluate force them Sigma Fx = 0 and Sigma Fx=0

Q3) Also, how to incorporate in free body diagram that the object is attempting to move upwards and will accelerate in the upward direction with its own effort (motors etc) no external forces but torque.

Q4) Also, would Friction Force on the wheels of the object would act in opposite direction of the direction of moving wheels?

Q5) Can you determine Torque required to move the wheels on the object to move the object using Friction Force x Wheel radius = Torque? Where: Friction Force, Fa=μ*Na Na is Normal force i.e force of surface on object /wheels μ = coefficient of friction Is this a real formula?

Q6) Also,is Fa=μ*Na applicable even when the object is moving?

The setup of the concept and my solution via formula is attached via photo attached here.

Q7) How to calculate acceleration if you know end velocity and can you assume 0 to end velocity is reached in 1 sec?

Q8)I hope someone can answers my questions so I understand the concepts and theory.

Q9) If you know a Force in terms of Nm how can you convert that more practical format like Kg or Psi etc? Or not possible?

I thank Jared for explaining the implications of Position 1,2 and 3 from by previous post and have restated the questions and providing formula relationship between friction and normal force and suggesting to use Free Body Diagram.

My Free Body Diagram and Formula I use for solution - page1 My Free Body Diagram and Formula I use for solution - page2 My Free Body Diagram and Formula I use for solution - page3

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    $\begingroup$ What would be different between the force of gravity and the force of a magnet? Having said that, as shown your crawler won't be able to negotiate the transition between the wall and the ceiling. It's almost guaranteed to fall off, unless you make the transition curved or you build some sort of movable magnetic latch to hold on while it transitions. $\endgroup$
    – CuriousOne
    May 29, 2015 at 21:35
  • $\begingroup$ Its just a concept. Have not considered the transition.Just wanted to understand the forces acting the directions. $\endgroup$ May 31, 2015 at 14:59
  • $\begingroup$ Fair. Good look with your project! $\endgroup$
    – CuriousOne
    May 31, 2015 at 15:01
  • $\begingroup$ Also.I wanted to add : The Force of the magnet is the force of attraction between the magnet fitted on the robot and the steel wall. Its pulling the robot towards the wall. The force of gravity is pulling the machine down. $\endgroup$ May 31, 2015 at 15:19

1 Answer 1


The frictional force is determine by the normal force: $|F_{fr}| \approx \mu_s |N|$.

In Position 1, the normal force must cancel the attraction due to the magnetic field. Gravity adds nothing to the normal force. In Position 1 gravity pulling down will work to make the crawler roll downward so to get it moving upward, you need extra torque.

In Position 2, The magnetic force directly opposes gravity. Therefore gravity is subtracted from the magnetic field to find the normal force. In this case we need the least amount of torque to get moving because gravity does not oppose motion and the normal force is at it's minimum.

If Position 3 was flat on the floor this would represent the situation where the normal force is the greatest (since you would add gravity and the magnetic field rather than subtract).

  • $\begingroup$ @ Jared , Thank you for your help. Based on your feedback I have attached a revised Free Body Diagram and my solutions. I was hoping you can take a look at it and critique it. $\endgroup$ May 31, 2015 at 15:27
  • $\begingroup$ I am still confused about the direction of Friction force. What direction does friction force act? Opposite to the movement of the wheel? $\endgroup$ May 31, 2015 at 15:28

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