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A baseball is thrown from the roof of a 22.0 -m-tall building with an initial velocity of magnitude 12.0 \(\mathrm{m} / \mathrm{s}\) and directed at an angle of \(53.1^{\circ}\) above the horizontal. (a) What is the speed of the ball just before it strikes the ground? Use energy methods and ignore air resistance. (b) What is the answer for part (a) if the initial velocity is at an angle of \(53.1^{\circ}\) below the horizontal? (c) If the effects of air resistance are included, will part (a) or (b) give the higher speed?

Short Answer

Expert verified
(a) and (b) both result in a speed of 24.0 m/s; air resistance reduces this speed in both cases.

Step by step solution

01

Analyze the Initial Conditions

The ball is thrown from a height of 22.0 meters with an initial speed of 12.0 m/s at an angle of \(53.1^\circ\) above the horizontal. We'll use the principle of conservation of mechanical energy, ignoring air resistance.
02

Determine Initial Kinetic Energy

The initial kinetic energy \(KE_i\) is given by \(KE_i = \frac{1}{2}mv^2\), where \(v\) is the initial speed:\[ KE_i = \frac{1}{2}m(12.0)^2 = 72m \] m stands for mass.
03

Calculate Initial Potential Energy

The initial potential energy \(PE_i\) is \(PE_i = mgh\), where \(h = 22.0 \) meters and \(g = 9.81 \text{ m/s}^2\):\[ PE_i = m \cdot 9.81 \cdot 22.0 = 215.82m \]
04

Find Total Initial Mechanical Energy

The total initial mechanical energy \(E_i\) is the sum of initial kinetic and potential energies:\[ E_i = KE_i + PE_i = 72m + 215.82m = 287.82m \]
05

Determine Final Mechanical Energy Just Before Impact

Just before impact, the potential energy is zero, and all energy is kinetic energy. Hence, \(E_f = KE_f\). Since energy is conserved: \[ E_i = E_f = KE_f = 287.82m \]
06

Solve for Final Velocity

The final kinetic energy \(KE_f\) is given by \(KE_f = \frac{1}{2}mv_f^2\). Solving for \(v_f\) gives:\[ \frac{1}{2}mv_f^2 = 287.82m \]\[ v_f^2 = 575.64 \]\[ v_f = \sqrt{575.64} \approx 24.0 \text{ m/s} \]
07

Analyze Changes for Angle Below Horizontal

For an initial velocity at an angle \(53.1^\circ\) below the horizontal, kinetic energy calculations remain the same since speed doesn't change. Thus, \(v_f = 24.0 \text{ m/s}\) as well.
08

Consider the Effect of Air Resistance

Air resistance will reduce the speed of the ball irrespective of its launch angle. Parts (a) and (b) will give the same final speed without air resistance, but with air resistance, the speeds will be lower.

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Key Concepts

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

Understanding Kinetic Energy
Kinetic energy is a fundamental concept in physics that describes the energy an object possesses due to its motion. It's described by the formula \( KE = \frac{1}{2}mv^2 \), where \( m \) is the mass of the object and \( v \) is its velocity. This means that kinetic energy increases with both the mass of the object and the square of its velocity. That is why even a small increase in speed can lead to a larger increase in kinetic energy. In the scenario of the baseball, the initial velocity of the ball plays a crucial role in determining the amount of kinetic energy it begins with as it's thrown from the building. Understanding kinetic energy is key to solving problems involving any moving objects, from as small as a baseball to as large as a car. The more kinetic energy an object has, the more work it can potentially perform when it hits another object.
Exploring Potential Energy
Potential energy is another essential concept in energy conservation, referring to the energy stored within an object due to its position in a force field, typically gravitational. The potential energy due to gravity is calculated with the formula \(PE = mgh\), where \(m\) is mass, \(g\) is the acceleration due to gravity (approximately 9.81 m/s² on Earth), and \(h\) is the height above a reference point.For our baseball example, the initial potential energy is determined by the height of 22.0 meters from which it is thrown. This potential energy is converted into kinetic energy as the ball falls, illustrating a beautiful conservation of energy principle. Even while stationary, objects with height have potential energy which can be transformed into motion, demonstrating the interconnectedness of energy forms.Whether an object is stationary or moving, potential energy is an important concept when considering how forces like gravity affect the motion and energy states of objects.
The Concept of Mechanical Energy
Mechanical energy is the sum of kinetic and potential energies in a system. It represents the total energy available to an object due to its motion and position. The principle of conservation of mechanical energy states that in the absence of non-conservative forces like friction or air resistance, the total mechanical energy of a system remains constant. In the baseball exercise, the mechanical energy at the start, comprising initial kinetic and potential energies, equals the mechanical energy just before the ball strikes the ground. This principle allows us to analyze different stages of motion and simplifies problem-solving as it consolidates various energy forms into a single comprehensive measure. When non-conservative forces are involved, such as air resistance, the total mechanical energy can decrease, converting some energy into other forms like heat, which is not useful for the current motion. Understanding mechanical energy provides a big-picture view of how energy is conserved and transferred within a system.

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