1. This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the
Directorate for Education and Human Resources (DUE-1245025).
WHAT DETERMINES WHETHER A SLOPE WILL
STAY IN PLACE OR FAIL?
Introduction to driving forces and resisting forces

2. DRIVING FORCES VS RESISTING FORCES
GRAVITY FRICTION
promotes sliding resistance to sliding
Which of the following Earth system scenarios
will increase the risk of slope failure?
(a) Increase gravity
(b) Increase friction
(c) Reduce gravity
(d) Reduce friction

3. DRIVING FORCES
Force = mass x acceleration Force: A push or pull in a particular
direction
measured in Newtons (kg m/s2 = N)
F = ma
Weight (force of gravity) = mass x acceleration due to gravity
Gravity is a mass-dependent force
directed downward towards the center
of the Earth. Acceleration due to
gravity = 9.8m/s2
W = mg
W = FG
The downward-directed force is
dependent on the mass of block.

4. DRIVING FORCES
10 kg block 100 kg block
Which block exerts the greater downward
directed force? Why?

5. DRIVING FORCES – ON A SLOPE!
FN = Normal Force = perpendicular to the sloping surface
FS = Shear Force = parallel to the sloping surface
q = Slope Angle
On a slope, the force due to gravity
consists of two components:
Shear Force (FS)
Normal Force (FN)W = FG
FS
FN q
In the diagram above:
Which force increases friction and keeps the block in place?
Which force pulls the block down the slope?

6. CALCULATING DRIVING FORCES– ON A SLOPE!
FN = Normal Force = perpendicular to the sloping surface
FS = Shear Force = parallel to the sloping surface
q = Slope Angle
W = FG
FS
FN q
FS = W sin q
FN = W cos q

7. FORCE VS STRESS
sN = Normal Stress
sS = Shear Stress
q = Slope Angle
Stress is the force acting over a specific area.
In other words: Stress (s) = Force/Area
W = FG
FS
FN q
sS = FS/A
sN = FN/A
A area between the block
and sloping plane
l w