Two objects (45.0 and 21.0 kg) are connected by a massless string that passes over a massless, frictionless pulley. The pulley hangs from the ceiling. Find
(a) the acceleration of the objects and
(b) the tension in the string.

Respuesta :

a) The acceleration of the objects is [tex]3.56 m/s^2[/tex]

b) The tension in the string is 280.8 N

Explanation:

a)

We start by writing the equations of motion for the two masses attached to the pulley.

For the heavier mass, we have:

[tex]m_1 g - T = m_1 a[/tex] (1)

where

[tex]m_1 = 45.0 kg[/tex] is the mass

[tex]g=9.8 m/s^2[/tex] is the acceleration of gravity

T is the tension in the string

a is the acceleration of the system (here we assumed that the heavier mass accelerates downward)

For the lighter mass, we have

[tex]T-m_2 g = m_2 a[/tex] (2)

where

T is the tension in the string

[tex]m_2 = 21.0 kg[/tex] is the mass

[tex]g=9.8 m/s^2[/tex] is the acceleration of gravity

a is the acceleration of the system (here we assumed that the lighter mass accelerates upward)

From (1) we get

[tex]T=m_1g - m_1 a[/tex]

And substituting into (2),

[tex](m_1 g - m_1 a)-m_2 g = m_2 a\\(m_1 -m_2)g  = (m_1+m_2)a\\a=\frac{m_1 - m_2}{m_1+m_2}g=\frac{45-21}{45+21}(9.8)=3.56 m/s^2[/tex]

b)

From the previous part of the problem we got an expression for the tension in the string:

[tex]T=m_1g - m_1 a[/tex]

Where we have

[tex]m_1 = 45.0 kg[/tex]

[tex]g=9.8 m/s^2[/tex]

[tex]a=3.56 m/s^2[/tex] is the acceleration, found in part a)

Susbtituting, we find

[tex]T=(45.0)(9.8)-(45.0)(3.56)=280.8 N[/tex]

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