How to Solve Moment and Friction Problems in Mechanics

[Chantry09]

Homework Statement

http://img237.imageshack.us/img237/7580/mechhelpui7.jpg

2. The attempt at a solution

(i) Moment of F = 70 x 800 = 56000
Moment of weight = 500 x 270 = 135000

Therefore the trolley remains at rest. Is this right?

(ii) Can you help me with this question please?

 

[sokratesla]

# Hi
# for (i) Yes you are right. The thing will not start to rotate until the moment due to force exceeds the moment of weight.
# (ii) For calculating the moment one looks at the perpendicular distance between the force vector and the pivot point and they are the same for figure I and II. but the pivot points are different. A in I and B in II. Hope this helps.

 

[Chantry09]

Hi, thank you for confirming (i) is correct.

Im still a little confused about (ii). It gives me a coefficient of friction value of 0.5 for the wheel. I know:

F = Coefficient of friction X Resultant
F = 0.5 x 500
F = 250N = Friction force acting in the opposite direction of the force pushing the trolley

Is this corrent so far? I don't really know where to go from here, could anyone help me out?

 

[Doc Al]

2. The attempt at a solution

(i) Moment of F = 70 x 800 = 56000
Moment of weight = 500 x 270 = 135000

Therefore the trolley remains at rest. Is this right?

What about the forces at B?

 

[Chantry09]

Am i not accounting for the weight at B by finding the moment of the weight force? If I am doing something wrong can you please set me straight, its the first question on this past exam paper and its driving me nuts.

 

[Doc Al]

Am i not accounting for the weight at B by finding the moment of the weight force?

No. There is a reaction force of the ground on the object at point B. Figure out that normal force. Then you can compare the applied force F to the static friction force.

 

[Chantry09]

Since the weight is central, does that mean the weight on point A and point B will be equal? If this is the case, that means there is a reaction force of 250N at point B.

Is that correct?

 

[Doc Al]

No. You can't just forget about the applied force F. Consider moments of all forces to find the normal force.

 

[Chantry09]

Force F as in the friction force on the wheel or the force pushing the trolley?

I really appreciate your help Doc Al, but I am having a hard time understanding the process i have to go through to get the rigth answer. Can you possibly go into a little more detail so i am able to understand?

 

[Doc Al]

Force F as in the friction force on the wheel or the force pushing the trolley?

Force F as in the applied force on the object. (Look at the diagram!)

The wheels are frictionless; friction will only act at point B.

Draw yourself a diagram showing all forces acting on the object. Don't neglect the forces acting at point B.

Hint: Consider moments about point A.