1. Quiz 4

2. Question 3
There is no friction between block A and the tabletop, but the coefficient of static friction between block A and block B is
What is the largest mass that block C can have so that blocks A and B still slide together when the system is released from rest?
The maximum force of friction: FFrict = μs FN
The surface between B and A: FN = mB g

3. Static Friction
There is no friction between block A and the tabletop, but the coefficient of static friction between block A and block B is
The maximum force of friction: FFrict = μs FN
The surface between B and A: FN = mB g = 50 N
FFrict = μs FN = .75 ∙ 50 N = 37.5 N
What forces act on B ?

4. Forces on B Gravity - mg down Box A - mg up
Friction - Box A pulls Box B to right: N
Fnet =
37.5 N
Fnet = ma = N
a = N / m = 37.5/5 = 7.5 m/s2
Everything accelerates at 7.5 m/s2

5. Forces on A Gravity = mg = 8 ∙10 = 80N Box B pushes down = 50 N
Friction from B = N to left
Table pushes up =
String = T to right
Fnet =
T − 37.5
Fnet = ma = T − 37.5
= 8 kg ∙ 7.5 m/s2 = 60 N = T − N
T = = N
130 N Up

6. Forces on C Gravity = mC g = String = T = 97.5 Net Force = mC g − T
mC a = mC g − 97.5
mC a − mC g = − 97.5
mC (a − g) = − 97.5
mC (7.5 − 10) = − 97.5
mC = − 97.5/(7.5 − 10) = 39 kg
130 N Up