Lecture 2:
More Forces
Physics 101: Lecture 2, Pg 1
Springs
Similar to tension, normal
 Reactive
 Changes length according to tension
 F = -k Δx

k=50 N/m
10kg
Physics 101: Lecture 2, Pg 2
Free Body Diagrams
 Choose
Object (book)
 Label coordinate axis
 Identify All Forces
Hand (to right)
Gravity (down)
Normal (table, up)
Normal
friction
hand
Friction (table, left)
Gravity
Physics
y
x
Physics 101: Lecture 2, Pg 3
40
Clicker Question:
A box is hanging in an elevator by a rope.
The elevator is accelerating up at 2 m/s2
a= 2 m/s2
•10kg
Compare the magnitude of the
Tension to the magnitude of the
Force of gravity
A |Fg| < |T|
B |Fg| = |T|
C |Fg| > |T|
Physics 101: Lecture 2, Pg 4
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 5
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 6
Example: speeding truck
What is the maximum
acceleration of the truck?
m=10kg
µs = 0.4
a= 2m/s2
Physics 101: Lecture 2, Pg 7
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 8
Example: pulling a box
What is the acceleration of the box?
40N
m=10kg
•µk=0.3
Physics 101: Lecture 2, Pg 9
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 10
Clicker Question
Two blocks are being pulled along a
frictionless floor by a 30N force
T
5kg
30N
10kg
frictionless
What is the magnitude of the Tension, T,
on the rope between the blocks?
A T = 30N
B T < 30N
C T > 30N
Physics 101: Lecture 2, Pg 11
Example: pulling 2 blocks
T
5kg
30N
10kg
frictionless
•Solve for acceleration
•Solve for tension
Physics 101: Lecture 2, Pg 12
Example: pulling 2 blocks
T
5kg
30N
10kg
µk= 0.2
•Solve for acceleration
•Solve for tension
Physics 101: Lecture 2, Pg 13