Concept explainers
Consider the three connected objects shown in Figure P5.88. Assume first that the inclined plane is friction-less and that the system is in equilibrium. In terms of m, g, and θ, find (a) the mass M and (b) the tensions T, and T2. Now assume that the value of Af is double the value found in part (a). Find (c) the acceleration of each object and (d) the tensions T1 and T2. Next, assume that the coefficient of static friction between m and 2m and the inclined plane is m, and that the system is in equilibrium. Find (e) the maximum value of M and (0 the minimum value of M. (g) Compare the values of T2 when M has its minimum and maximum values.
(a)

The expression for the mass of M of the object.
Answer to Problem 5.88AP
The expression for the mass M is 3msinθ.
Explanation of Solution
The free body diagram of the three connected objects is shown in Figure below,
Figure (1)
From Figure (1), the equilibrium forces acts on the object of mass 2m in inclined plane is,
∑Fnet=T1−2mgsinθ
Here,
m is the mass of object.
g is the acceleration due to gravity.
θ is the angle of inclined plane.
T1 is the tension in the rope of mass 2m.
From the Newton’s second law of motion, the net force on the object of mass 2m is,
∑Fnet=(2m)a=2ma
Here,
a is the acceleration of the object.
Substitute T1−2mgsinθ for ∑Fnet in the above equation.
2ma=T1−2mgsinθ (1)
From Figure (1), the equilibrium forces act on the object of mass m in inclined plane is,
∑Fnet=T2−T1−mgsinθ
Here,
T2 is the tension in the rope of mass m.
From the Newton’s second law of motion, the net force on the object of mass m is,
∑Fnet=ma
Substitute T2−T1−mgsinθ for ∑Fnet in the above equation.
ma=T2−T1−mgsinθ (2)
Add the equation (1) with equation (2).
2ma+ma=T1−2mgsinθ+(T2−T1−mgsinθ)3ma=T2−3mgsinθT2=3m(a+gsinθ)
From Figure (1), the equilibrium forces act on the object of mass M is,
∑Fnet=Mg−T2
Here,
M is the mass of hanging object.
Substitute 3m(a+gsinθ) for T2 in the above equation.
∑Fnet=Mg−3m(a+gsinθ)
From the Newton’s second law of motion, the net force on the object of mass M is,
∑Fnet=Ma
Substitute Mg−3m(a+gsinθ) for ∑Fnet in the above equation.
Mg−3m(a+gsinθ)=MaM(g−a)=3m(a+gsinθ)M=3m(a+gsinθ)g−a
The system is in equilibrium so value of the acceleration is zero.
Substitute 0 for a in the above equation.
M=3m(0+gsinθ)g−0=3mgsinθg=3msinθ
Conclusion:
Therefore, the expression for the mass M is 3msinθ.
(b)

The expressions for tensions T1 and T2.
Answer to Problem 5.88AP
The expression for the tension T1 is 2mgsinθ and the expression for the tension T2 is 3mgsinθ.
Explanation of Solution
From part (a), the expression for the mass of M is,
M=3msinθ
From part (a), the equilibrium forces act on the object is,
∑Fnet=Mg−T2
The system is in equilibrium, the value of acceleration is zero so the net force acts on the system is also zero.
Substitute 0 for ∑Fnet in the above equation.
Mg−T2=0T2=Mg
Substitute 3msinθ for M in the above equation.
T2=(3msinθ)g=3mgsinθ
Thus, the expression for the tension T2 is 3mgsinθ.
From part (a), the equation (2) is,
ma=T2−T1−mgsinθ
Substitute 0 for a in the above equation.
m(0)=T2−T1−mgsinθT2−T1=mgsinθ
Substitute 3mgsinθ for T2 in the above equation.
3mgsinθ−T1=mgsinθT1=2mgsinθ
Thus, the expression for the tension T1 is 2mgsinθ.
Conclusion:
Therefore, the expression for the tension T1 is 2mgsinθ and the expression for the tension T2 is 3mgsinθ.
(c)

The acceleration of each object.
Answer to Problem 5.88AP
The acceleration of each object is gsinθ1+2sinθ.
Explanation of Solution
Given info: The value of mass M is double.
From part (a), the expression for the mass when it is double represents as,
M=2(3msinθ)=6msinθ
From part (a), the equation (1) is,
2ma=T1−2mgsinθ
Rearrange the above equation.
T1=2ma+2mgsinθ
From part (a), the equation (2) is,
ma=T2−T1−mgsinθ
Rearrange the above equation.
T2−T1=ma+mgsinθ
Substitute 2ma+2mgsinθ for T1 in the above equation.
T2−(2ma+2mgsinθ)=ma+mgsinθT2=3ma+3mgsinθ (3)
From Figure (1), the equilibrium forces act on the object of mass M is,
Ma=Mg−T2T2=M(g−a)
Substitute 6msinθ for M in the above equation.
T2=6msinθ(g−a) (4)
Subtract the equation (3) from equation (4).
T2−T2=6msinθ(g−a)−(3ma+3mgsinθ)6masinθ+3ma=6mgsinθ−3mgsinθa(1+2sinθ)=gsinθa=gsinθ1+2sinθ
Conclusion:
Therefore, the acceleration of each object is gsinθ1+2sinθ.
(d)

The expressions for tensions T1 and T2.
Answer to Problem 5.88AP
The expression for the tension T1 is 4mgsinθ(1+sinθ1+2sinθ) and the expression for the tension T2 is 6mgsinθ(1+sinθ1+2sinθ).
Explanation of Solution
From part (c), the expression for the acceleration is,
a=gsinθ1+2sinθ
From part (c), the equation for tension T1 is,
T1=2ma+2mgsinθ
Substitute gsinθ1+2sinθ for a in the above equation.
T1=2m(gsinθ1+2sinθ)+2mgsinθ=2mgsinθ(11+2sinθ+1)=2mgsinθ(2+2sinθ1+2sinθ)=4mgsinθ(1+sinθ1+2sinθ)
Thus, the expression for tension T1 is 4mgsinθ(1+sinθ1+2sinθ).
From part (c), the equation (3) is,
T2=3ma+3mgsinθ
Substitute gsinθ1+2sinθ for a in the above equation.
T2=3m(gsinθ1+2sinθ)+3mgsinθ=3mgsinθ(11+2sinθ+1)=3mgsinθ(1+1+2sinθ1+2sinθ)=6mgsinθ(1+sinθ1+2sinθ)
Thus, the expression for tension T2 is 6mgsinθ(1+sinθ1+2sinθ).
Conclusion:
Therefore, the expression for the tension T1 is 4mgsinθ(1+sinθ1+2sinθ) and the expression for the tension T2 is 6mgsinθ(1+sinθ1+2sinθ).
(e)

The maximum value of M.
Answer to Problem 5.88AP
The maximum value of M is 3m(sinθ+mscosθ).
Explanation of Solution
Given info: The coefficient of static friction between mass m, 2m and the inclined plane is ms.
The static friction forces on the masses m and 2m is shown in Figure below,
Figure (2)
From the Figure (2), the normal force on the object of mass 2m is,
n=2mgcosθ
The expression for the static friction force on the object of mass 2m and mass m is,
fs=msn (3)
Here,
ms is the coefficient of static friction.
n is the normal force.
Substitute 2mgcosθ for n in the above equation.
fs=ms(2mgcosθ)=2mmsgcosθ
From the Figure (1), the equilibrium forces acts on the object of mass 2m is,
2ma=T1−2mgsinθ−fs
Substitute 2mmsgcosθ for fs in the above equation.
2ma=T1−2mgsinθ−2mmsgcosθ (4)
Rearrange the above equation for T1.
T1=2ma+2mgsinθ+2mmsgcosθT1=2m(a+gsinθ+msgcosθ)
Substitute 0 for a in the above equation.
T1=2m(0+gsinθ+msgcosθ)=2mg(sinθ+mscosθ)
From the Figure (2), the normal force on the object of mass m is,
n=mgcosθ
Substitute mgcosθ for n in equation (3).
fs=msmgcosθ
From the Figure (1), the equilibrium forces acts on the object of mass m is,
ma=T2−T1−mgsinθ−fs
Substitute msmgcosθ for fs in the above equation.
ma=T2−T1−mgsinθ−msmgcosθ
Add equation (4) with the above equation.
2ma+ma=T1−2mgsinθ−2mmsgcosθ+T2−T1−mgsinθ−msmgcosθ3ma=−3mgsinθ−3mmsgcosθ+T2T2=3ma+3mgsinθ+3mmsgcosθT2=3m(a+gsinθ+msgcosθ)
Substitute 0 for a in the above equation.
T2=3m(0+gsinθ+msgcosθ)=3mg(sinθ+mscosθ)
The equilibrium forces on the block of mass M when mass is maximum represents as,
T2=Mmaxg
Substitute 3mg(sinθ+mscosθ) for T2 in the above equation.
3mg(sinθ+mscosθ)=MmaxgMmax=3m(sinθ+mscosθ)
Conclusion:
Therefore, the maximum value of M is 3m(sinθ+mscosθ).
(f)

The minimum value of M.
Answer to Problem 5.88AP
The minimum value of M is 3m(sinθ−mscosθ).
Explanation of Solution
From part (e), the expression of tension T2 for the minimum mass is,
T2=3mg(sinθ−mscosθ)
The equilibrium forces acts on block for minimum mass of M is,
T2=Mming
Substitute 3mg(sinθ−mscosθ) for T2 in the above equation.
3mg(sinθ−mscosθ)=MmingMmin=3m(sinθ−mscosθ)
Conclusion:
Therefore, the minimum value of M is 3m(sinθ−mscosθ).
(g)

The difference between the tension T2 for maximum and minimum mass.
Answer to Problem 5.88AP
The difference between the tension for maximum and minimum mass is 6msmgcosθ.
Explanation of Solution
From part (e), the expression for maximum mass is,
Mmax=3m(sinθ+mscosθ)
From part (f), the expression for the minimum mass is,
Mmin=3m(sinθ−mscosθ)
From part (e), the expression for the tension for maximum mass is,
T2,max=Mmaxg
From part (f), the expression for the tension for maximum mass is,
T2,min=Mming
Compare both the above equation.
T2,max−T2,min=Mmaxg−Mming
Substitute 3m(sinθ+mscosθ) for Mmax and 3m(sinθ−mscosθ) for Mmin in the above equation.
T2,max−T2,min=(3m(sinθ+mscosθ))g−(3m(sinθ−mscosθ))g=6msmgcosθ
Conclusion:
Therefore, the difference between the tension for maximum and minimum mass is 6msmgcosθ.
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