3. What is Friction?
Force that acts
oppose the relative
motion of two
surfaces
High for dry and
rough surfaces
Low for smooth and
wet surfaces
4. What Causes Friction?What Causes Friction?
Friction is the force that opposes the motion
between two surfaces that touch.
The surface of any object is rough.
Even an object that feels smooth is covered with tiny
hills and valleys.
The contact between the hills of valleys of two
surfaces causes them to stick, resulting in friction.
5. The amount of friction depends on:
Roughness of the surfaces
Force pushing the surfaces together
What Causes Friction?What Causes Friction?
6. Kinetic friction occurs when force is applied to
an object and the object moves.
Examples:
Sliding Friction: pushing an object across a
surface
Rolling Friction: between wheels and a surface
Fluid Friction: opposes the motion of objects
traveling through a fluid (air or water)
Types of FrictionTypes of Friction
7. Static friction occurs when force applied
to an object does not cause the object to
move.
Types of FrictionTypes of Friction
11. Types of Friction
I better be
safe Ump!!
To initiate motion of the box
the man must overcome the
Force of Static Friction
Upon sliding, the baseball
player will come to a
complete stop due to the
Force of Kinetic Friction
12. Static & Kinetic Friction Coefficients
Material Coefficient of
Static Friction µS
Coefficient of
Kinetic Friction µS
Rubber on Glass 2.0+ 2.0
Rubber on Concrete 1.0 0.8
Steel on Steel 0.74 0.57
Wood on Wood 0.25 – 0.5 0.2
Metal on Metal 0.15 0.06
Ice on Ice 0.1 0.03
Synovial Joints in
Humans
0.01 0.003
13. PROBLEMS
A 75 kg crate is to be pushed up an incline
plane 5 m long that makes an angle of 20° with
the horizontal. If the coefficient of static friction
between the crate and the inclined plane is
0.20, how much force must be given to get it
started up the incline? If the coefficient of
kinetic friction is 0.15, how much applied force
is needed to keep it going at a constant speed
up the incline.
14. Given: m=75 kg; l=5 m; θ=20°; μs=0.20; μk=0.15
Fw=mg
Ff
Fn=mg(cosθ) Fa
θ
Fp=mg(sinθ)
16. Take into account the last problem. After you
have the crate moving up the incline at a
constant speed, you want to give it an
acceleration of 1.7 m/s2
. What is the new
force needed to accomplish this task?
18. A 300 kg sled is pulled at constant speed over a
level, horizontal, snow covered surface. The
rope that is used to pull the sled makes a 30°
angle with the horizontal. If the coefficient of
friction is 0.10, find the force required.
19. Given: m=300 kg; θ=30°; μk=0.10
Fn
Fx (cos θ)
Ff
Fw
Fa
θ
Fy (sin θ)
Fax-Ff=max *note: ax is 0 because we are at a
constant speed
Fay+Fn-Fw=may *note: ay is 0 because the sled
never moves up and down
