Finding the Center of Mass: Balancing a Plank with an 800-N Person

[gcombina]

1. A horizontal, 10-m plank weighs 100 N. It rests on two supports that are placed 1.o m from each end. How close to one end can an 800-N person stand without causing the plank to tip?

 

[nil1996]

Hi gcombina

so far how you have attempted the problem?

 

[gcombina]

I know is a torque
that is all I know and I don't know where to start

 

[haruspex]

Your diagram is not the most useful since it shows the person right at one of the supports. Redraw it with a bit more overhang for the person to stand on. Draw in the relevant forces. No need to draw a stick figure, just indicate the force of the weight of the person. What equations do you know for static equilibrium?

 

[gcombina]

for static equilibrium? I know ΣF = 0

 

[gcombina]

 

[gcombina]

1. SO, I got 4 forces
F1 - W + F2 - W = 0
F1 - 100N + F2 - 800N = 0

i DONT EVEN KNOW WHY AM I DOING WHAT I AM DOING
I AM TRYING TO FIND THE DISTANCE OF THE PERSON

IM SO LOST, SORRY

 

[hav0c]

what does the fact that the body is in static equilibrium indicate to you?

 

[gcombina]

that there are no external forces??

 

[hav0c]

mmm no.
what do you know about torques in static equilibrium ?

 

[gcombina]

I know torque = force x level arm

when a rigid body is in equilibrium then it means that the sum of torques = 0

 

[hav0c]

try to write both
1)∑F=0
2)∑τ=0
for your system

 

[gcombina]

I got it, thank you!