Torque/Static Equilibrium/Forces problem....

aliaze1

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

1. The problem statement, all variables and given/known data


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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2. Relevant equations

τ=Frsinφ
F=ma

3. 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 doesnt 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 cant 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

so do i pretend support 2 doesnt exist? and then just find the torque for support 1 using [80kg+(2/3*100)] as the mass?

radou

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