09 2-2 (b) Beams on supports
Procedure
- Draw all forces: weights (a uniform rod's weight acts at its midpoint), loads, reactions of supports, tensions.
- Write "vertical forces balance".
- Take moments about one support to find the other reaction directly.
✍️ Worked examples
Example 11:
- A uniform wooden board, 6 m long, has mass 30 kg per metre.
- It rests horizontally on supports at its ends and and a box of 240 kg·wt is on the board 4 m from .
- Find the reaction at each support.
Solution:
- Mass of board kg, so its weight is 180 kg·wt at the middle (3 m from each end).
- Vertical balance: .
- Moments about : .
- kg·wt and kg·wt.
- As the box moves closer to , increases and decreases.
Example 12:
- A uniform rod of length 90 cm and weight 60 N is hung horizontally by two vertical strings at its ends.
- Where must a 150 N weight be hung so that the tension at is twice the tension at ?
Solution:
- Let the tension at be and at be .
- Vertical balance: , so N.
- Moments about : .
- , so cm from .
Example 13:
- A motorcycle of mass 200 kg has its weight acting midway between the wheels, 1.4 m apart.
- A rider of 84 kg·wt sits with the weight acting 1 m behind the front wheel.
- Find the ground reactions on each wheel (a) without the rider, (b) with the rider.
Solution:
- (a) By symmetry each wheel carries kg·wt.
- (b) Moments about the front wheel: , so kg·wt.
- Vertical balance: kg·wt.