Problem A block with mass m, 7.70 kg are connected by a light string that passes over a frictionless pulley, as shown in Figure 4.23a. The coefficient of kinetic friction between the block and the surface is 0.300. 3.90 kg and a ball with mass m2 %3D %3D

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Example 4.13 Connected Objects
Goal Use both the general method and the system approach to solve
a connected two-body problem involving gravity and friction.
Problem A block with mass m, = 3.90 kg and a ball with mass m,
%D
7.70 kg are connected by a light string that passes over a frictionless
pulley, as shown in Figure 4.23a. The coefficient of kinetic friction
between the block and the surface is 0.300.
(a) Find the acceleration of the two objects and the tension in the
string.
m2
(b) Check the answer for the acceleration by using the system
approach.
Figure 4.23a Two objects connected
by a light string that passes over a
frictionless pulley.
y
Strategy Connected objects are handled by applying Newton's
second law separately to each object. The free-body diagrams for the
block and the ball are shown in Figure 4.23b, with the +x-direction to
the right and the +y-direction upwards. The magnitude of the
acceleration for both objects has the same value, |a,| = |a,| = a.
The block with mass m, moves in the positive x-direction, and the
ball with mass m, moves in the negative y-direction, so a, = -aŋ2:
Using Newton's second law, we can develop two equations involving
the unknowns T and a that can be solved simultaneously. In part (b),
treat the two masses as a single object, with the gravity force on the
ball increasing the combined object's speed and the friction force on
the block retarding it. The tension forces then become internal and
don't appear in the second law.
m2
T
mog
Figure 4.23b Free-body diagrams for
the objects.
Solution
Transcribed Image Text:Example 4.13 Connected Objects Goal Use both the general method and the system approach to solve a connected two-body problem involving gravity and friction. Problem A block with mass m, = 3.90 kg and a ball with mass m, %D 7.70 kg are connected by a light string that passes over a frictionless pulley, as shown in Figure 4.23a. The coefficient of kinetic friction between the block and the surface is 0.300. (a) Find the acceleration of the two objects and the tension in the string. m2 (b) Check the answer for the acceleration by using the system approach. Figure 4.23a Two objects connected by a light string that passes over a frictionless pulley. y Strategy Connected objects are handled by applying Newton's second law separately to each object. The free-body diagrams for the block and the ball are shown in Figure 4.23b, with the +x-direction to the right and the +y-direction upwards. The magnitude of the acceleration for both objects has the same value, |a,| = |a,| = a. The block with mass m, moves in the positive x-direction, and the ball with mass m, moves in the negative y-direction, so a, = -aŋ2: Using Newton's second law, we can develop two equations involving the unknowns T and a that can be solved simultaneously. In part (b), treat the two masses as a single object, with the gravity force on the ball increasing the combined object's speed and the friction force on the block retarding it. The tension forces then become internal and don't appear in the second law. m2 T mog Figure 4.23b Free-body diagrams for the objects. Solution
Solution
(a) Find the acceleration of the objects and the
tension in the string.
Write the components of Newton's second law for
the cube of mass m1.
EF, = T- f = m,a1
EF, = n – m,g = 0
%3D
The equation for the y-component givesn = m,g.
Substitute this value for n and f,
equation for the x-component.
T- HM,g = m1a (1)
Mkn into the
Apply Newton's second law to the ball, recalling
that a, = -a1.
EF, = -m,g + T = m,a, = -mza, (2)
Subtract Equation (2) from Equation (1),
eliminating T and leaving an equation that can be
solved for a, (substitution can also be used).
m2g – Hm19 = (m, + m2)a,
m1 + m2
Substitute the given values to obtain the
acceleration.
(7.70 kg) (9.80 )- (0.300) (3.90 kg) (9.80 )
3.90 kg + 7.70 kg
m/s2
a
Substitute the value into Equation (1) to find the
T =
tension T.
(b) Find the acceleration using the system
approach, where the system consists of two blocks.
Apply Newton's second law to the system and solve
for a. (Use m,, m,, µµ n, and g as appropriate.)
(m, + m2)a = m2g – Hn = m29 – HkM19
%3D
Transcribed Image Text:Solution (a) Find the acceleration of the objects and the tension in the string. Write the components of Newton's second law for the cube of mass m1. EF, = T- f = m,a1 EF, = n – m,g = 0 %3D The equation for the y-component givesn = m,g. Substitute this value for n and f, equation for the x-component. T- HM,g = m1a (1) Mkn into the Apply Newton's second law to the ball, recalling that a, = -a1. EF, = -m,g + T = m,a, = -mza, (2) Subtract Equation (2) from Equation (1), eliminating T and leaving an equation that can be solved for a, (substitution can also be used). m2g – Hm19 = (m, + m2)a, m1 + m2 Substitute the given values to obtain the acceleration. (7.70 kg) (9.80 )- (0.300) (3.90 kg) (9.80 ) 3.90 kg + 7.70 kg m/s2 a Substitute the value into Equation (1) to find the T = tension T. (b) Find the acceleration using the system approach, where the system consists of two blocks. Apply Newton's second law to the system and solve for a. (Use m,, m,, µµ n, and g as appropriate.) (m, + m2)a = m2g – Hn = m29 – HkM19 %3D
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