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A projectile is fired straight upward at 141 \(\mathrm{m} / \mathrm{s}\) . How fast is it moving at the instant it reaches the top of its trajectory? Suppose that it were fired upward at \(45^{\circ}\) instead. Then its horizontal component of velocity would be 100 \(\mathrm{m} / \mathrm{s}\) . What would be the speed of the projectile at the top of its trajectory?

Short Answer

Expert verified
The projectile moves at 0 m/s at the top of its trajectory when fired straight up. When fired at a 45-degree angle, it moves at 100 m/s at the top of its trajectory.

Step by step solution

01

Analyze the vertical motion of the projectile

For a projectile fired straight up, at the top of the trajectory, the vertical component of velocity is zero due to gravity. The horizontal velocity component does not affect the vertical motion, so the speed at the top will be 0 m/s for the vertical component.
02

Examine the horizontal motion for a 45 degree launch

When the projectile is fired at a 45-degree angle, it has both horizontal and vertical components of velocity. At the top of its trajectory, the vertical velocity component is 0 m/s, but the horizontal component remains unaffected by gravity and remains constant throughout the flight.
03

Determine the horizontal component at 45 degrees

The problem states that if fired at a 45-degree angle, the horizontal velocity component is 100 m/s. Since there's no vertical velocity at the top, the horizontal component remains 100 m/s.
04

Calculate the total speed at the top of the trajectory for the 45-degree launch

The total speed at the top of the trajectory is equal to the horizontal component of velocity since the vertical component is 0 m/s. Therefore, the speed at the top is 100 m/s.

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Key Concepts

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

Vertical Component of Velocity
Understanding the vertical component of velocity is essential when studying the motion of projectiles. In the context of an object launched directly upward, like in our exercise, the vertical component of velocity is the speed at which the projectile moves along the vertical axis. It is crucial to remember that gravity acts to decelerate the projectile as it rises and to accelerate it as it falls back down.

At the very top of its trajectory, the vertical component of velocity is zero because the object has come to a momentary stop before it starts to fall back to the ground. This is why, in the given exercise, irrespective of its initial launch speed of 141 m/s, the projectile's vertical speed at its peak height is 0 m/s. It's also important to note that the vertical velocity affects the maximum height reached and the time the projectile stays in the air.
Horizontal Component of Velocity
The horizontal component of velocity refers to the speed at which a projectile moves along the horizontal axis. For a projectile that is launched at an angle, this component remains constant throughout its flight, assuming there is no air resistance. This is because gravity does not influence horizontal motion; it solely affects the vertical motion of the projectile.

In the latter part of our exercise, the horizontal component is given as 100 m/s for a launch angle of 45 degrees. This constant velocity is what allows us to determine the projectile's range or how far it will travel horizontally. Even at the peak of its trajectory, where the vertical component is zero, the horizontal component maintains this constant speed of 100 m/s, thereby defining the projectile's speed at the highest point in its path.
Trajectory of a Projectile
The trajectory of a projectile is the path that it follows through the air. This path is parabolic when air resistance is negligible, a consequence of the independent effects of gravity on the vertical motion and the constant horizontal velocity.

When projected straight upwards, as in our exercise, the trajectory is a straight line. However, when launched at an angle, the trajectory becomes a curved path. The highest point on this path is where the projectile reaches its maximum height, and as discussed, the vertical component of velocity becomes zero, but the horizontal component (if any) remains unchanged. Understanding the trajectory helps in predicting where and when the projectile will land.
Kinematics in Physics
Kinematics is the branch of physics that describes the motion of objects without considering the forces that cause this motion. When analyzing projectile motion, kinematic equations are tools that allow us to calculate various aspects such as displacement, velocity components, acceleration, and time.

In projectile motion, we often use these kinematic equations to relate the vertical and horizontal components of velocity and to determine the trajectory. For instance, when an object is launched upward at an angle, kinematic principles can be applied separately to vertical and horizontal motions to predict the entire flight path of the projectile, as in calculating the height, range, or the speed at various points of the trajectory.

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Most popular questions from this chapter

Why are satellites normally sent into orbit by firing them in an easterly direction, the direction in which Earth spins? And discuss why locations near the equator are favored.

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