1. Unit 5: Equilibrium
Lesson 2: Friction

2. f
Warm-Up:
A 100 N block is at rest on a rough incline. Find the normal force and the friction force. N
100N
30º y
X : 100sin30 = f f N
f =
50 N
Y : 100cos30 = N
10 min
30º
N =
87 N x
100N

3. Definition of coefficient of friction (μ):
Example:
Find the minimum coefficient of friction which prevents this block from slipping. M= 5.0 kg , θ=30° f N mg
30° y N f mg
30°( x x:
10 min
f = (5.0 x 9.8) sin30° = 24.5 N
N = (5.0 x 9.8) cos30° = y:
42.4N
0.58
f = mg sinθ
N = mg cosθ
= tanθ

4. Brain Break!

5. 2. Find the maximum value for mass B if the system is in equilibrium and mass A is 20 kg. The pulley is frictionless. a) no friction b) µ = 0.2 T N mg y x
37° ( f A . T mg B A B
37°(
a) T = (20 x 9.8) sin37° =
TB = TA
118N
= mg = 118N
= 12kg
15 min
b) N = (20 x 9.8) cos37° =
f = µN
156.5N
= 0.2 x 156.5N
= 31.3N
X: T = f + mg sinθ
= 31.3N + 118N
= 149N
mg = T
= 15kg

6. Try This!
20N
40° (
a) A 20 N mass is held by a string as shown. Find the tension in
the string. (Assume no friction.)
b) If the tension is 5.0 N, find the coefficient of friction. T N mg y x
40°( f
T = mg sinθ
= 20 x sin40°
= 12.9 N
X: T + f = mg sinθ
f = 20 sin40° - 5.0
= 7.9 N
Y: N = mg cosθ
= 20 cos40°
= N
20 min
= 0.51

7. Lesson 3: Practice with Newton’s First Law
Unit 5: Equilibrium
Lesson 3: Practice with Newton’s First Law

8. Try This!
A box 200 N is held in place by a 150 N horizontal force. Find the minimum coefficient of friction. Y N N f f x
F = 150N
200N F
) 60°
) 30°
) 30° mg mg
y: N = mg cos30° + F cos60°
= 200 cos30° cos60°
= N
x: f sin60° = 200 sin30°
f = 200 sin30° sin60°
= N
Friction is going the other way!
15 min
= 0.12

9. Brain Break!

10. Practice Time!
Work on the Newton’s First Law Problem Set. (You should be able to do all of the questions.)
40 min

11. Lesson 4: More Practice with Newton’s First Law
Unit 5: Equilibrium
Lesson 4: More Practice with Newton’s First Law

12. Practice Time!
You have today’s class to finish working through the Newton’s First Law Problem Set.
60 min