In the (Figure 1) the coefficient of static friction between mass m_A and the ta

Question

In the (Figure 1) the coefficient of static friction between mass m_A and the table is 0.42, whereas the coefficient of kinetic friction is 0.18. Ignore masses of the cord and the (frictionless) pulley. Figure 1 What minimum value of m_A will keep the system from starting to move? Express your answer to two significant figures and include the appropriate units. What value of will keep the system moving at constant speed? Express your answer to two significant figures and include the appropriate units.

Explanation / Answer

(a)

If we draw the FBD for block mAthen
we get
Friction force = usmAg = T where T is the tension in the string.
If mass is not moving then T = mB*g = 2*9.81
usmAg = 2*9.81
mA = 2/0.42 = 4.76 kg
this mass will keep the system from starting to move.
(b) if the system is moving with constant speed then the net force on the mass B is zero therfore
T - mBg = 0
T = mbg
when we write the force equation of mass A
ukmAg = T = mBg
mA = mB/uk = 2 / 0.18 = 11.11 kg

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