# How do I calculate length of bridge on this one?

• Oct 27th 2010, 02:03 PM
How do I calculate length of bridge on this one?
A hiker who weights 985 N is strolling thru the woods and crosses a small horizontal bridge. The bridge is uniform, weights 3610N and rests on two concrete supports, one at each end. He stops 1/5th of the way along the bridge. What is the magnitude of the force that a concrete support exerts on the bridge First at the near end and at the for end???
How do I do this with no length of the bridge.
I am lost on this one.

Can someone help please.

I know he goes 1/5th, but how does that give me a distance??
• Oct 27th 2010, 02:14 PM
TKHunny
You need relative distance, not actual distance. The actual length has nothing to do with it, except perhaps it would change the bridge's newtons.
• Oct 27th 2010, 02:15 PM
skeeter
Quote:

Originally Posted by bradycat
A hiker who weights 985 N is strolling thru the woods and crosses a small horizontal bridge. The bridge is uniform, weights 3610N and rests on two concrete supports, one at each end. He stops 1/5th of the way along the bridge. What is the magnitude of the force that a concrete support exerts on the bridge First at the near end and at the for end???
How do I do this with no length of the bridge.
I am lost on this one.

Can someone help please.

I know he goes 1/5th, but how does that give me a distance??

let L = length of bridge

$\displaystyle \sum F_y = 0$

$\displaystyle \sum \tau = 0$

L will cancel in your calculations
• Oct 27th 2010, 03:09 PM
Have not been shown how to do that?
• Oct 27th 2010, 03:09 PM
How do I do torque if I have no distance. I am not sure what you mean? I looked thru my book and show nothing.
Can you explain a bit more please.
• Oct 27th 2010, 03:29 PM
skeeter
Quote:

Originally Posted by bradycat
How do I do torque if I have no distance. I am not sure what you mean? I looked thru my book and show nothing.
Can you explain a bit more please.

let $\displaystyle F_1$ = upward support force on the near end of the bridge

$\displaystyle F_2$ = upward support force on the far end of the bridge

forces up = forces down

$\displaystyle F_1 + F_2 = 3610 + 985$

let the near end be the pivot point ...

torque clockwise = torque counter-clockwise

$\displaystyle \displaystyle 985 \cdot \frac{L}{5} + 3610 \cdot \frac{L}{2} = F_2 \cdot L$

note that each term has L = bridge length as a factor ... like I said previously, they will all cancel.

all set up ... solve the system.
• Oct 27th 2010, 04:53 PM