Here is something to try at a sporting event. Show that the maximum height h attained by an object projected into the air, such as a baseball, football, or soccer ball, is approximately given by h ≈ 1.2t2 m, where t is the total time of flight for the object in seconds. Assume that the object returns to the same level as that from which it was launched, as in Fig. 3–58. For example, if you count seconds and find that a baseball was in the air for t = 5.0 s, the maximum height attained was h = 1.2 x (5.0)2 = 30 m. The fun of this relation is that h can be determined without knowledge of the launch speed v0 or launch angle θ0. Why is that exactly? See Section 3–8. <Image>
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Step 1: Begin by analyzing the motion of the object. The object follows a parabolic trajectory due to the influence of gravity. The maximum height h is reached when the vertical velocity component becomes zero momentarily.
Step 2: Recall the kinematic equation for vertical motion under constant acceleration: h = v₀ᵧ² / (2g), where v₀ᵧ is the initial vertical velocity component and g is the acceleration due to gravity (approximately 9.8 m/s²).
Step 3: The total time of flight t is related to the vertical motion. Since the object returns to the same level, the time to reach the maximum height is t/2. Using this, v₀ᵧ can be expressed as v₀ᵧ = g(t/2).
Step 4: Substitute v₀ᵧ = g(t/2) into the equation for h. This gives h = (g²(t/2)²) / (2g). Simplify the expression to find h in terms of t: h = (g(t²)) / 8.
Step 5: The constant 1.2 in the given formula h ≈ 1.2t² m arises from approximating g/8 ≈ 1.2. This approximation works because g ≈ 9.8 m/s², and dividing by 8 gives a value close to 1.2. Thus, the formula h ≈ 1.2t² m is derived without needing the initial velocity v₀ or launch angle θ₀.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Projectile Motion
Projectile motion refers to the motion of an object that is launched into the air and is subject to gravitational force. It can be analyzed in two dimensions: horizontal and vertical. The vertical motion is influenced by gravity, which causes the object to accelerate downwards, while the horizontal motion remains constant if air resistance is negligible. Understanding this concept is crucial for determining the trajectory and maximum height of the projectile.
Kinematic equations describe the relationships between an object's displacement, velocity, acceleration, and time. In the context of projectile motion, these equations can be used to calculate various parameters, such as maximum height and time of flight. The specific equation h = 1.2t² arises from the kinematic relationship that relates the time of flight to the vertical displacement, assuming constant acceleration due to gravity.
In projectile motion, the independence of horizontal and vertical motions is a key principle. This means that the time an object spends in the air (total time of flight) is determined solely by its vertical motion, while its horizontal motion does not affect the time of flight. This principle allows us to derive the maximum height formula without needing to know the initial speed or launch angle, as these factors do not influence the time of ascent and descent.