Torque/Static Equilibrium/Forces problem

[aliaze1]

Torque/Static Equilibrium/Forces problem...

I have been working on this problem for a few hours, but I just can't get it ...

Homework Statement

Problem:

A 3.0-m-long rigid beam with a mass of 100 kg is supported at each end. An 80 kg student stands 2.0 m from support 1. How much upward force does support 1 exert on the beam?

here is an image:

http://photo.ringo.com/208/208651783O865465193.jpg

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Homework Equations

τ=Frsinφ
F=ma

The Attempt at a Solution

I figured out the torques kinda, but do i use the mass as (2/3*100)+80? or (2/3*100)+40...either way i kept getting a wrong answer...

and the center of mass i assume is where the person is right? bec the 80kg addition would be the most massive point right?

since the angles are 90 degrees, the sine of φ doesn't matter right?, i mean it would only affect the sign +/- of the answer...since τ=Frsinφ, and φ is either 90 or -90 degrees...

idk...basically i just can't get this...

i got solutions ranging from 76.666.., 1306.666... etc...

 

[radou]

Assume the weight of the beam acting at the center of the beam, and set the sum of all torques with respect to support B equal zero.

 

[aliaze1]

Assume the weight of the beam acting at the center of the beam, and set the sum of all torques with respect to support B equal zero.

so do i pretend support 2 doesn't exist? and then just find the torque for support 1 using [80kg+(2/3*100)] as the mass?

 

[radou]

so do i pretend support 2 doesn't exist? and then just find the torque for support 1 using [80kg+(2/3*100)] as the mass?

Yes, you can pretend support 2 doesn't exist. Do you know how to draw a free body diagram? You have three forces acting - one is the weight of the bar, the second one is the weight of the person, and the third one is the force at support 1 you need to find.

 

[aliaze1]

lol...yea i got it, thanks, i went to a tutor today bec i was making small errors...i for the torque of the person, i was using the weight as (person+segment of rod), when its only supposed to be person

thanks for the help, greatly appreciated