10 2-2 (c) Tilting and limiting positions
Idea
- A beam on two supports is about to tilt about one support when the reaction at the other support becomes zero.
- Take moments about the support that remains in contact.
✍️ Worked examples
Example 14:
- A uniform beam of length 6 m and weight 40 kg·wt rests on supports at (1 m from ) and (1 m from ).
- A man of weight 100 kg·wt walks from the middle towards .
- How far from is he when the beam is about to tilt?
Solution:
- At the limit the reaction at is zero, so take moments about (5 m from ).
- The beam's weight is 2 m to the left of : anticlockwise moment .
- The man at distance from is to the right of : clockwise moment .
- , so m from (0.2 m from the end ).
Example 15: Two men carry a body of 90 kg hanging from a light rod 60 cm long, with the body 20 cm from man . Find the load each carries.
Solution:
- Moments about : , so kg·wt.
- kg·wt: the man nearer the load carries more.