Net torque and net force in equilibrium Help

In summary, the problem involves finding the distance a person can walk on an overhanging plank before it begins to tip. Using the net torque equation, the distance x can be determined by setting the net torque about the right support equal to zero. This results in the equation -(W of man)(x) + (W of plank)(1,4) = 0. The solution to this equation is x = 0.36m, meaning that the person can walk 0.36m on the plank before it starts to tip.
  • #1
renaldocoetz
33
0
Net torque and net force in equilibrium.. Help please

Homework Statement



The diagram shows a uniform plank of length 5.0m and weight 225N that rests horizontally on 2 supports, with 1,1 m of the plank hanging over the right support.

http://www.postyourimage.com/view_image.php?img_id=Oc9PYdcDMUhBrGB1196709689 [Broken]

To what distance x can a person who weighs 450N walk on the overhanging part of the plank before it just begins to tip?

Homework Equations



Net force on Y-axis = 0

Net torque on right support = 0

Net torque on left support = 0

The Attempt at a Solution



I used these 3 formulas to end up with 3 unknowns in order to work out x but ended up with 0x = 234 :(

Net torque right post = -FNl(3,9) + Wp(1,4) - Wmx = 0

Net torque left post = -Wp(2,5) + FNr(3,9) - Wm(3,9 + x)

FNr + FNl -225 -450 = 0

i used counter-clockwise rotation as positive

I combined these 3 formulas in an attempt to get x but it didnt work quite so well. Is this a wrong approach or might my mistake just be in my algebra?

Please help any1
thxPS
FNl = normal force of left support on plank upwards
FNr = normal force of right support on plank upwards
Wp = weight of plank
Wm = weight of man
 
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  • #2


All you need is one equation: Find torques about the right support. Hint: When the plank is just about to tip, what's the normal force from the left support?
 
  • #3


hmm.. k so when the plank is just about to tip the normal force on left support is -225?

then torque = -(W of man)(x) -(-225)(3,9) ?
 
  • #4


renaldocoetz said:
hmm.. k so when the plank is just about to tip the normal force on left support is -225?
No. Realize that when it starts to tilt, the plank will lose contact with the left support. So what must the normal force be at that point?

Also: Where does the weight of the plank act?
 
  • #5


oh yeah so the normal force is 0 on left suport then? and the weight of the plank acts in the middle i believe
 
  • #6


renaldocoetz said:
oh yeah so the normal force is 0 on left suport then? and the weight of the plank acts in the middle i believe
Exactly. So set up the equation for the net torque about the right support.
 
  • #7


net torque = -(W of man)(x) + (W of plank)(1,4) ?Just wondering.. if the net force is zero on left suport, and the plank has already started to tip is the equation still right considering the question is how far can the person walk BEFORE the plank starts to tip? so obv he has already walkd to far?
 
  • #8


renaldocoetz said:
net torque = -(W of man)(x) + (W of plank)(1,4) ?
Good.

Just wondering.. if the net force is zero on left suport, and the plank has already started to tip is the equation still right considering the question is how far can the person walk BEFORE the plank starts to tip? so obv he has already walkd to far?
We're finding the point where the normal force from the left support just becomes zero, so the man's torque still balances the torque due to the plank's weight. If he keeps walking, his torque will be greater than the torque due to the plank's weight and he'll surely fall.
 
  • #9


that makes sense :) so can i set before-mentioned equation equal to 0? by doing this i get that the man can walk 0,36 m before it begins to tip
 

1. What is net torque and net force?

Net torque and net force are two concepts in physics that are used to describe the overall effect of all the forces and torques acting on an object. Net torque is the sum of all the individual torques acting on an object, while net force is the sum of all the individual forces acting on an object.

2. What is equilibrium?

Equilibrium is a state in which an object experiences no net force or net torque, meaning that it is either at rest or moving with a constant velocity. In other words, all the forces and torques acting on the object cancel each other out, resulting in a balanced system.

3. How do you calculate net torque and net force in equilibrium?

To calculate net torque, you must first determine the individual torques acting on the object. Then, you can use the equation net torque = force x distance from pivot point to find the net torque. To calculate net force, you must determine the individual forces acting on the object and add them together using the equation net force = mass x acceleration.

4. What is the difference between net torque and net force?

The main difference between net torque and net force is that net torque is a rotational force that causes an object to rotate, while net force is a linear force that causes an object to move in a straight line. However, both concepts are related and can affect an object's motion in different ways.

5. What happens when an object is not in equilibrium?

When an object is not in equilibrium, it experiences either a net force or a net torque. This can cause the object to accelerate, change direction, or rotate. In order to return to equilibrium, the net force or net torque must be countered by an equal and opposite force or torque.

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