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The simple 2 -kg pendulum is released from rest in the horizontal position. As it reaches the bottom position, the cord wraps around the smooth fixed pin at \(B\) and continues in the smaller arc in the vertical plane. Calculate the magnitude of the force \(R\) supported by the pin at \(B\) when the pendulum passes the position \(\theta=30^{\circ}\).

Short Answer

Expert verified
The force supported by the pin is approximately 50.9 N.

Step by step solution

01

Analyze the Initial Energy

Since the pendulum is released from the horizontal position, it initially has potential energy only. The potential energy of the pendulum at the horizontal position can be expressed as \( PE = mgh \), where \( h \) is the initial height of the pendulum. Since the pendulum length \( L \) is the length to rotate vertically, initially, \( h = L \). Therefore, \( PE = mgL \).
02

Calculate Speed at the Bottom

At the bottom of the swing, all the initial potential energy has converted into kinetic energy. At the bottom, \( KE = \frac{1}{2} mv^2 \). According to the conservation of energy, \( mgL = \frac{1}{2} mv^2 \). Solve for \( v \) to find the velocity at position \( B \): \( v = \sqrt{2gL} \).
03

Analyze the Pendulum at \( \theta = 30^{\circ} \)

At \( \theta = 30^{\circ} \), the pendulum has both kinetic and potential energy. The height at this position is \( h = L(1-\cos(30^{\circ})) \). Calculate the potential energy \( PE = mgL(1-\cos(30^{\circ})) \). The remaining energy is kinetic: \( KE = mgL - PE \).
04

Calculate Velocity at \( \theta = 30^{\circ} \)

Using \( KE = \frac{1}{2} mv^2 \), solve for \( v \) at \( \theta = 30^{\circ} \): \( v = \sqrt{2gL - 2gL(1-\cos(30^{\circ}))} \). So the simplified velocity is \( v = \sqrt{2gL\cos(30^{\circ})} \).
05

Determine Centripetal Force at \( \theta = 30^{\circ} \)

The centripetal force \( F_c \) required to keep the pendulum in circular motion at \( \theta = 30^{\circ} \) is expressed as \( F_c = \frac{mv^2}{r} \). Substitute the velocity from Step 4 and \( r = L \): \( F_c = \frac{m(2gL\cos(30^{\circ}))}{L} \). This simplifies to \( 2mg\cos(30^{\circ}) \).
06

Calculate the Normal Force at the Pin

The normal force \( R \) exerted by the pin at \( B \) equals the gravitational component along with the centripetal component. Thus, \( R = mg\cos(\theta) + F_c \). Therefore, \( R = mg\cos(30^{\circ}) + 2mg\cos(30^{\circ}) \), which simplifies to \( R = 3mg\cos(30^{\circ}) \).
07

Compute Magnitude of Force at Pin

Substitute \( m = 2 \) kg and \( g = 9.81 \) m/s² into the equation found in Step 6: \( R = 3 \times 2 \times 9.81 \times \frac{\sqrt{3}}{2} \). Calculate \( R \) to find the magnitude of force at \( B \). This results in \( R \approx 50.9 \) N.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Pendulum Motion
Imagine a simple pendulum swinging back and forth. A pendulum consists of a heavy bob attached to the end of a string or rod. It moves in a circular arc under the influence of gravity. In this exercise, we studied a pendulum that starts from a horizontal position. It hangs and swings due to force of gravity.
As it descends, its speed increases, reaching maximum speed at the lowest point of the swing. Here, potential energy is entirely converted into kinetic energy. After passing the lowest point, the pendulum climbs again, losing speed as it ascends. It wraps around a pin at a certain point, continuing in a smaller, circular path. This pin doesn’t hinder the motion but changes the path radius because the cord wraps around it.
Understanding the motion of a pendulum is crucial in studying oscillations and periodic movement. This illustrates the conservation of energy and the transformation between potential and kinetic energy.
Conservation of Energy
The principle of conservation of energy is vital in physics. It states that energy cannot be created or destroyed, only transformed from one form to another. In the context of a pendulum, as it swings, there’s a continuous conversion between potential energy (PE) and kinetic energy (KE).
At its highest point, the pendulum has maximum potential energy given by the formula: \(PE = mgh\), where \(m\) is mass, \(g\) is acceleration due to gravity, and \(h\) is height. As the pendulum falls, this energy is converted to kinetic energy, according to \(KE = \frac{1}{2}mv^2\). At the bottom of the swing, potential energy is at its minimum, and kinetic energy is at its maximum.
This cycle repeats with each swing of the pendulum. Applying energy conservation allows us to calculate velocities and energy states at various positions of the pendulum's path.
Centripetal Force
When an object moves along a curved path, a force called centripetal force acts on it. This force keeps the object in circular motion, directed towards the center of the circle.
For the pendulum in this exercise, as it wraps around the fixed pin and continues in a circular path, centripetal force becomes significant. The force necessary to sustain the pendulum's circular path can be described by the formula:\(F_c = \frac{mv^2}{r}\), where \(m\) is the mass of the pendulum, \(v\) is its velocity, and \(r\) is the radius of its path.
This centripetal force increases with velocity and decreases with a larger radius. Understanding centripetal force is essential in dynamics, describing how objects move in circles, such as planets orbiting stars or cars navigating curves.
Gravitational Potential Energy
Gravitational potential energy is the energy an object possesses due to its position in a gravitational field. For a pendulum, this energy is dependent on its height above a reference level, commonly when it's at rest, at the lowest point of its swing.
At the pendulum’s starting point (horizontal position), the gravitational potential energy is at its peak. Calculated with the formula \(PE = mgh\), it decreases as the pendulum falls, converting into kinetic energy when it gains speed. As it rises again after passing the lowest point, the pendulum's speed reduces, and potential energy increases again.
Understanding gravitational potential energy in pendulum motion helps in appreciating energy transfers within systems influenced by gravity.

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