1. Friction & Inclined Planes
Alex Tsantilis & Ken Ni

2. What is Friction?
It is a force opposing the motion of one surface over another
results from contact between surfaces
opposing force of motion (always opposide direction)
Formula: Ff = μFN

3. Examples of Friction If the lawn mower's horizontal force is
being pushed at
136N on dry
concrete, what
is the frictional
force?
Use Ff = μFN!

4. Defining the Formula: Ff = μFN
µ = coefficient of kinetic or static friction
kinetic friction occurs when an object is in motion
static friction occurs when an object is stationary
STATIC FRICTION IS ALWAYS GREATER THAN KINETIC FRICTION!!
Ff = the frictional force (obviously)
FN = normal force (equal to Fg when on horizontal plane)

5. Inclined Planes is a plane set at an angle to the horizontal plane. θ
An inclined plane
is a plane set at
an angle to the
horizontal plane. θ

6. Properties of Inclined Planes
Parallel Force can be calculated by mg(sinθ). It is parallel to the incline of the ramp.
Normal Force has the same magnitude of perpendicular force, and is in the opposite direction.
Perpendicular Force can be calculated by mg(cosθ). It is perpendicular to the incline of the ramp.
Frictional Force can be calculated by multiplying the coefficient of friction by perpendicular force.

7. Sample Problem 1 A 2 kilogram wooden block was placed on
a wooden ramp at an angle of 25 degrees.
Calculate:
a) normal force
b) parallel force
c) perpendicular force
d) frictional force

8. Answer 1 a) FN = Fperpendicular = mgcosθ = (2kg)(9.81m/s2)cos25
b) Fparallel = mgsinθ = (2kg)(9.81m/s2)sin25
= 8.29N
c) Fperpendicular = mgcosθ = (2kg)(9.81m/s2)cos25
d) Ff = µFN = (0.42)(17.78N) = 7.47N

9. Sample Problem 2
When a 12-newton horizontal force is applied to a box on a horizontal tabletop, the box remains at rest.
What is the force of static friction?

10. Answer 2
The force of static friction is 12 Newtons, since the box does not move in response to the applied force.

11. Sample Problem 3
Which vector best represents the direction of the normal force acting on the box

12. Answer 3
(C), because the Normal force is always perpendicular to the surface on which the object is moving on

13. Sample Problem 4
The strongest frictional force between two surfaces is:
(1) static friction
(2) kinetic friction
(3) sliding friction
(4) rolling friction

14. Answer 4
(1) Static friction, because the force that is required to initiate movement is greater than the required force to continue movement

15. Sample Problem 5
A 40.-newton object requires 5.0 newtons to start moving over a horizontal surface.
Calculate the coefficient of static friction

16. Answer 5
Ff = µFN = µFg,
5N = µ(40N),
µ = 8

17. Sources:
Landa, Kiefer. Reviewing Physics: The Physical Setting "Third Edition", (2007)
Mr. Wala Mr. Wala Mr. Wala