11 2-2 (d) A rod on a rough surface and a smooth wedge
Method
- Forces on the rod: weight , reaction of the wedge (perpendicular to the rod, since the wedge is smooth), the ground reaction and friction at the lower end.
- Use , and moments about the foot of the rod.
- At the point of slipping, .
✍️ Worked examples
Example 16:
- A uniform bar of weight 5 kg·wt and length 30 cm rests with on rough ground and touches a smooth wedge at , 12.5 cm above the ground, at to the horizontal.
- It is about to slip.
- Find the reaction of the wedge and the coefficient of friction.
Solution:
- cm. The weight acts 15 cm from .
- Moments about : , so kg·wt.
- The wedge reaction is perpendicular to the bar, so it makes with the vertical. Horizontal: .
- Vertical: , so .
- .