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A light rod of length 2L has 2 small heavy masses (each with mass m) attached at the end and the middle. The axis of rotation is at one end, as shown. If the bar is released from rest what is the magnitude of the initial angular acceleration?

User StuartLC
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8.2k points

1 Answer

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To determine the magnitude of the initial angular acceleration of the system, we can use the principles of rotational dynamics and the equation for the torque.

When the rod is released from rest, the two masses will start to fall due to gravity, creating a torque that causes the rod to rotate. The magnitude of the torque can be calculated using the equation:

τ = I * α

where:

τ is the torque,

I is the moment of inertia of the system,

α is the angular acceleration.

The moment of inertia of the system can be calculated as the sum of the individual moments of inertia of the masses with respect to the axis of rotation.

For the mass at the end of the rod, the moment of inertia is given by:

I1 = m * L^2

For the mass at the middle of the rod, the moment of inertia is given by:

I2 = (1/4) * m * L^2

The total moment of inertia of the system is the sum of these two:

I = I1 + I2 = m * L^2 + (1/4) * m * L^2 = (5/4) * m * L^2

Substituting this into the torque equation, we have:

τ = (5/4) * m * L^2 * α

The torque created by the falling masses is due to the force of gravity acting on them. The magnitude of the torque is given by:

τ = F * d

where F is the force of gravity on one of the masses and d is the lever arm, which is L for the mass at the end of the rod and L/2 for the mass at the middle of the rod.

The force of gravity on one of the masses is given by:

F = m * g

where g is the acceleration due to gravity.

Substituting these values into the torque equation, we have:

(5/4) * m * L^2 * α = m * g * L + (1/2) * m * g * (L/2)

Simplifying the equation:

(5/4) * L * α = g * L + (1/2) * g * (L/2)

(5/4) * α = g + (1/2) * (g/2)

(5/4) * α = g + (1/4) * g

(5/4) * α = (5/4) * g

α = g

Therefore, the magnitude of the initial angular acceleration is equal to the acceleration due to gravity, g.

User Pier Paolo Ramon
by
8.6k points
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