Uniform plank question?

  • #1

Homework Statement



A uniform plank 10m long and of mass 40kg is supported by two posts...?
A uniform plank 10m long and of mass 40kg is
supported by two posts, the first 1m from the left
end and the other 6m from the left end. A mass of
10kg is placed 8m from the left end. Find the force
each of the support posts exerts on the plank if it is
in equilibrium.

I'm not looking for just an answer. I would appreciate it you could tell me what needs to be done.

Here is the illustration that goes along with the question. http://i55.tinypic.com/2s61zj8.png


Homework Equations


T= F x ℓ


The Attempt at a Solution


Torque on the left: T = F2 x 1m (CCW)
Torque on the plank: T = -400N x 5m (CW)
Torque by the weight T= -100N x 7m (CW)

I really don't know where to go on from there. This is my first year in physics and i'm having trouble.

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
ideasrule
Homework Helper
2,266
0
Torque on the left: T = F2 x 1m (CCW)
Torque on the plank: T = -400N x 5m (CW)
Torque by the weight T= -100N x 7m (CW)
Before calculating torque, you have to decide which point you're using to calculate torque. You seem to have chosen the leftmost point of the plank, which is perfectly fine, as long as you're aware of it.

Your torque calculations are all correct, provided that you define the counterclockwise direction to be positive. You also need to calculate the force exerted by the second support.

I really don't know where to go on from there. This is my first year in physics and i'm having trouble.
In order for the system to be in equilibrium, torques and forces both have to sum to 0. For forces, write out Newton's second law, taking into consideration the directions of all the forces.
 
  • #3
Thanks you were really helpful
 
  • #4
gneill
Mentor
20,907
2,858
Your terminology is a bit confusing. "Torque on the left", "Torque on the plank", Torque by the weight" are not precise, and it would be difficult to produce equations from them!

A torque is taken about a particular center of rotation. The first thing to do is identify the centers of rotation of interest. In this case it's the support points where the posts meet the planks. Next, identify the weights that will be causing the torques, and their positions.

In this case there are two weights specified: the weight of the 40kg plank and that of the 10kg mass. Assuming that the plank is uniform, the location of its weight can be assigned to its center of mass at location 5m from the left end of the plank. The 10kg mass is located at position 8m from the same left end.

Pick one of the centers of rotation and calculate the net torque. Suppose you choose the right hand support. The center of mass of the 40kg plank is a meter to left of this support, while the "object" of 10kg mass is located 2m to its right. Can you write the equation for the net torque about this rotation center?
 

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