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A 20.0 -kg child is on a swing attached to ropes that are \(L=1.50 \mathrm{~m}\) long. Take the zero of the gravitational potential energy to be at the position of the child when the ropes are horizontal. a) Determine the child's gravitational potential energy when the child is at the lowest point of the circular trajectory. b) Determine the child's gravitational potential energy when the ropes make an angle of \(45.0^{\circ}\) relative to the vertical. c) Based on these results, which position has the higher potential energy?

Short Answer

Expert verified
Answer: To determine which position has a higher gravitational potential energy, first calculate the child's height at each position by following Steps 2 and 4. Then, calculate the gravitational potential energy at each position using Steps 3 and 5. Finally, compare the potential energies and determine which position has the higher potential energy by following Step 6.

Step by step solution

01

Understand gravitational potential energy formula

The formula for gravitational potential energy is: \(U = mgh\) where \(U\) = gravitational potential energy \(m\) = mass of the object (in this case, the child) \(g\) = acceleration due to gravity (approximately \(9.81 m/s^2\)) \(h\) = height of the object above the reference point (we'll determine this for each position)
02

Calculate the child's height when the ropes are horizontal

When the ropes are horizontal, the child is at the lowest point of the trajectory. The height \(h_1\) above the reference point (when the ropes are horizontal) is given by the length of the ropes: \(h_1 = L\)
03

Calculate gravitational potential energy when ropes are horizontal

Now we can use the formula for gravitational potential energy at this position: \(U_1 = m \cdot g \cdot h_1\) Plug in the given values: \(U_1 = \left(20.0\,\text{kg}\right) \cdot \left(9.81\, \mathrm{m/s^2}\right) \cdot \left(1.50\, \mathrm{m}\right)\) and find \(U_1\).
04

Find the child's height when ropes make a \(45.0^{\circ}\) angle

We need to determine the child's height \(h_2\) when the ropes make a \(45.0^{\circ}\) angle relative to the vertical. In this position, the child forms a right-angled triangle with the vertical and horizontal sides. We can use the sine function to find the vertical side: \(\sin{45^{\circ}} = \frac{\text{opposite side}}{\text{hypotenuse}} \) Using the given length of the ropes, \(L\), as the hypotenuse: \(\sin{45^{\circ}} = \frac{\text{opposite side}}{1.50\,\text{m}}\) The vertical side will be longer than the horizontal side since it forms a \(45^\circ\) angle with the vertical and is equal to: \(\text{opposite side} = L \cdot \sin{45^{\circ}}\) Therefore, the child's height above the reference point is: \(h_2 = L - \text{opposite side}\)
05

Calculate gravitational potential energy when ropes make a \(45.0^{\circ}\) angle

Now we can use the formula for gravitational potential energy at this position: \(U_2 = m \cdot g \cdot h_2\) Plug in the given values and the height found in Step 4: \(U_2 = \left(20.0\,\text{kg}\right) \cdot \left(9.81\, \mathrm{m/s^2}\right) \cdot h_2\) and find \(U_2\).
06

Compare potential energies and determine which position has higher potential energy

Now that we have both gravitational potential energies, \(U_1\) and \(U_2\), we can determine which position has the higher potential energy by comparing their values. The higher value means higher potential energy.

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Key Concepts

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

Pendulum Motion
Pendulum motion describes the back-and-forth swinging of an object attached to a pivot point, like a swing or a simple pendulum. This motion is significant because it exemplifies periodic motion, which means it repeats itself at regular intervals. In the context of a child swinging, the pivot is the point where the ropes are attached to a swing set. When the child is at the highest point of the swing, they momentarily stop before changing direction, indicating the interplay between potential and kinetic energies. At the lowest point, the speed is the highest, and all the potential energy is converted to kinetic energy, showing energy conservation in action. The motion of a pendulum in such a scenario follows a path known as a circular trajectory with a radius equivalent to the length of the swing's ropes. Understanding pendulum motion allows us to explore concepts like gravitational potential energy and kinetic energy, which are fundamental in describing how objects move within a gravitational field.
Energy Conservation
Energy conservation is a crucial principle in physics, stating that the total energy in a closed system remains constant over time. When discussing a swinging pendulum, this principle is illustrated by the transformation of energy between gravitational potential energy and kinetic energy. - **Gravitational Potential Energy (GPE):** At the peak of its swing, the pendulum holds maximum potential energy due to its elevated position. - **Kinetic Energy (KE):** At the lowest point of the swing, the pendulum's speed—and consequently, its kinetic energy—is at its maximum because the gravitational potential energy has been completely transformed into kinetic energy. During the pendulum's motion, energy moves back and forth between these two forms. Potential energy is highest at the highest point, and kinetic energy is highest at the lowest point.
Trigonometry in Physics
Trigonometry is a branch of mathematics that deals with the relationships between the sides and angles of triangles. It is especially important in physics for solving problems involving angles, such as those found in pendulum motion.In the given exercise, understanding the child's position at various swing points involves using trigonometric functions like the sine function. When the child swings and makes an angle with the vertical, it creates a right triangle. The length of the pendulum rope serves as the hypotenuse.- **Finding Height Using Sine:** To find the vertical distance (height) from the reference point when the ropes are at an angle, you can use \( \sin \theta = \frac{\text{opposite}}{\text{hypotenuse}} \).- By substituting the known values, it is possible to solve for the opposite side, which then helps compute the gravitational potential energy at different positions in the swing.By incorporating trigonometry, one can accurately compute the height differences in pendulum problems, crucial for calculating potential energy changes.

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Most popular questions from this chapter

Two masses are connected by a light string that goes over a light, frictionless pulley, as shown in the figure. The 10.0 -kg mass is released and falls through a vertical distance of \(1.00 \mathrm{~m}\) before hitting the ground. Use conservation of mechanical energy to determine: a) how fast the 5.00 -kg mass is moving just before the 10.0 -kg mass hits the ground; and b) the maximum height attained by the 5.00 -kg mass.

A pendulum swings in a vertical plane. At the bottom of the swing, the kinetic energy is \(8 \mathrm{~J}\) and the gravitational potential energy is 4 J. At the highest position of its swing, the kinetic and gravitational potential energies are a) kinetic energy \(=0 \mathrm{~J}\) and gravitational potential energy \(=4 \mathrm{~J}\) b) kinetic energy \(=12 \mathrm{~J}\) and gravitational potential energy \(=0 \mathrm{~J}\) c) kinetic energy \(=0 \mathrm{~J}\) and gravitational potential energy \(=12 \mathrm{~J}\) d) kinetic energy \(=4\) J and gravitational potential energy \(=8 \mathrm{~J}\) e) kinetic energy \(=8 \mathrm{~J}\) and gravitational potential energy \(=4\) J.

A package is dropped on a horizontal conveyor belt. The mass of the package is \(m,\) the speed of the conveyor belt is \(v\), and the coefficient of kinetic friction between the package and the belt is \(\mu_{\mathrm{k}}\) a) How long does it take for the package to stop sliding on the belt? b) What is the package's displacement during this time? c) What is the energy dissipated by friction? d) What is the total work supplied by the system?

a) If the gravitational potential energy of a 40.0 -kg rock is 500 . J relative to a value of zero on the ground, how high is the rock above the ground? b) If the rock were lifted to twice its original height, how would the value of its gravitational potential energy change?

A 1.00 -kg block compresses a spring for which \(k=\) 100. \(\mathrm{N} / \mathrm{m}\) by \(20.0 \mathrm{~cm}\) and is then released to move across a horizontal, frictionless table, where it hits and compresses another spring, for which \(k=50.0 \mathrm{~N} / \mathrm{m}\). Determine a) the total mechanical energy of the system, b) the speed of the mass while moving freely between springs, and c) the maximum compression of the second spring.

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