Portlandjib.com CENTER OF GRAVITY
 Home about Roosevelt HS STEM PROGRAM BRIDGES CRANES BRIDGE BEAMS ENGINEERING middle school projects crane projects Engineer's kit Portland Transformer Junior builder's kit montessori bridge beams Montessori Children's House scissors lift new crane new bridges contact FAQ

This wood beam is 22" long and weighs 4 pounds  (4#), and
the steel block (3" wide) weighs 7.375#
We attach slings with the hook centered over the middle of
the beam and lift----oops!!! We have to find the combined Center of Gravity--- CG.
First the wood beam-If we assume the density of the beam
is uniform throughout its length, then the CG will be in
the geometric center You knew that already, just like the CG of a 12" ruler is
centered at 6" Same with the steel block. It's 3" wide so the CG is 1 1/2" in

from the edge.

But when we put the 2 together, everything changes and we'd better

know where the combined CG is or our load will tip and could injure someone.

Here's how- let's assume the beam and the block are balanced on a pivot point

at 1 end and would therefore try to rotate- like a see-saw--  only with just 1 side.

Remember forces that cause rotation can be computed using the basic formula for Moment-

M = Force x Distance

For the pivot point we'll use one end of the wood beam-the end opposite the steel block.

To compute the Moment of the beam    M = F x D

The force is the weight (4#) and the distance is measured from the line of force to the pivot

(at a right angle). The line of force (gravity remember) goes through the geometric

center of the beam- so the Dist is 11"

M = F x D

M = 4# x 11"

M = 44inch pounds-44"#

Now the steel block- wt = 7.375#.and the distance from the pivot is 20 1/2" M = F x D

M = 7.375# x 20.5"=151.18"#

Here's the formula to find the combined CG

MOMENTbeam  + MOMENT block divided by combined weight of

beam and block.

44"# + 151.18"# divided by 7.375# + 4# =

195.18"# /11.3745# =

17.1"

So now we place the hook directly above the CG (17.1" from pivot) and lift.  Enter content here