Static Friction

1
Static Friction

• For next time
• Read Chapter 4, Section 8
• Homework SP31, SP33, SP51

2
HB
BA
AB
TA
TB
3
Review Kinetic Friction

• fA,B ?k NA,B
• Note surfaces must be sliding

4
370
A 70 kg skier skis down a slope that is angled
370 with respect to the horizontal. The
coefficient of kinetic friction between the skis
and the snow is 0.10.What is the skiers
acceleration?
5
700 N
370
Which angle is 37??
6
To find the angle, drive a spike
7
To find the angle, drive a spike
8
To find the angle, drive a spike
9
To find the angle, drive a spike
10
?snow,skier mk Nsnow,skier
560 N
56 N
560 N
37?
700 N
370
420 N
420 N - 56 N (70 kg) ax
ax 5.2 m/s2
11
The Ugly Block
2 N
12
The Ugly Block
13
The Ugly Block
NT,B
TS,B 2 N
fT,B 2 N
WE,B
14
The Ugly Block
4 N
15
The Ugly Block
NT,B
TS,B 4 N
WE,B
16
The Ugly Block
16N
17
The Ugly Block
NT,B
TS,B 16 N
fT,B 16
WE,B
18
The Ugly Block
16.01N
19
The Ugly Block
NT,B
TS,B 16.01 N
fT,B ?
WE,B
20
Its now a kinetic friction
a
NT,B
TS,B 16.01 N
fT,B lt 16 N
WE,B
21
Maximum static friction is greater than kinetic
friction.
ALWAYSthis value
22
Static friction

• There is a maximum value of static friction that
  depends on surfaces and normal force.
• The maximum force is given by fs ?sN.
• The static force adjusts to the demands.
• We typically use Newtons 2nd Law to find the
  value of the static frictional force.

23
Sample Friction Problem
24
Sample Friction Problem
mB 10 kg
ms 0.8
a 2 m/s2
MAX
fs ms NA,B 0.8 (100N) 80 N
Fnet m a (10 kg) (2 m/s/s) 20 N