Lecture 2: More Forces
Physics 101: Lecture 2, Pg 1
Friction(Static)
 No
motion between surfaces (can be rolling)
 Magnitude of frictional force (parallel to
surfaces) is proportional to the normal force.
fstatic <= ms N
 Be
ms coefficient of Static friction
Careful!
Static friction <=, can be any value up to msN
It is reactive just like Normal, Tension
Physics 101: Lecture 2, Pg 2
42
Example: speeding truck
What is the magnitude of the
force of friction on box?
m=10kg
µs = 0.4
a= 2m/s2
Physics 101: Lecture 2, Pg 3
Example: speeding truck
What is the maximum
acceleration of the truck?
m=10kg
µs = 0.4
a= 2m/s2
Physics 101: Lecture 2, Pg 4
Friction (kinetic)

Similar name, but behaves very differently!
fkinetic = mk N
mk coefficient of Kinetic friction
The equals sign means we always know the
magnitude
Direction is always opposite of motion
Physics 101: Lecture 2, Pg 5
Example: pulling a box
What is the acceleration of the box?
40N
m=10kg
•µk=0.3
Physics 101: Lecture 2, Pg 6
Newton’s 3rd law
 Forces
come in pairs
 Same type (gravity, normal, friction)
 Different objects
 Always same magnitude, opposite
direction
Physics 101: Lecture 2, Pg 7
Example: pulling 2 blocks
T
5kg
30N
10kg
frictionless
•Solve for acceleration
•Solve for tension
Physics 101: Lecture 2, Pg 8
Example: pulling 2 blocks
T
5kg
30N
10kg
µk= 0.2
•Solve for acceleration
•Solve for tension
Physics 101: Lecture 2, Pg 9