/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 70 A 2.7 -kg ball is thrown upward ... [FREE SOLUTION] | 91Ó°ÊÓ

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A 2.7 -kg ball is thrown upward with an initial speed of 20.0 \(\mathrm{m} / \mathrm{s}\) from the edge of a 45.0-m-high cliff. At the instant the ball is thrown, a woman starts running away from the base of the cliff with a constant speed of 6.00 \(\mathrm{m} / \mathrm{s} .\) The woman runs in a straight line on level ground, and air resistance acting on the ball can be ignored. (a) At what angle above the horizontal should the ball be thrown so that the runner will catch it just before it hits the ground, and how far does the woman run before she catches the ball? (b) Carefully sketch the ball's trajectory as viewed by (i) a person at rest on the ground and (ii) the runner.

Short Answer

Expert verified
The ball should be thrown at an angle of approximately 53° above the horizontal. The woman runs 51.9 m before catching the ball.

Step by step solution

01

Define Variables and Equations Needed

First, list the known quantities: mass of the ball (2.7 kg), initial speed of the ball (20.0 m/s), height of the cliff (45.0 m), speed of the woman (6.00 m/s). We remember that the trajectory of the ball can be described using kinematic equations and that the range (horizontal distance traveled by the ball) should equal the distance run by the woman. We also consider equations related to projectile motion such as range and time of flight.
02

Determine Time of Flight

The time it takes for the ball to fall to the ground after being thrown from a height of 45.0 m is crucial. We use the vertical motion equation: \[ h = v_{y0} t + \frac{1}{2} g t^2 \] where \( v_{y0} = v_0 \sin \theta \), \( g = 9.8 \ m/s^2 \), and \( h = -45.0 \ m \) because it travels downward. Solve this quadratic equation to find the time, \( t \).
03

Express Horizontal Range of the Projectile

The horizontal range \( R \) of the projectile is given by the formula \( R = v_{x0} t \), where \( v_{x0} = v_0 \cos \theta \). The woman must run this distance at her constant speed, in the same amount of time, \( t \).
04

Set up the Equation for the Horizontal Motion

Since the total distance run by the woman is equivalent to the range of the ball, set \( 6.00 \ m/s \times t = v_0 \cos \theta \times t \). Simplify it to \( v_0 \cos \theta = 6.00 \ m/s \).
05

Combine the Equations to Solve for \( \theta \)

Substitute \( v_0 \sin \theta \) from the vertical equation and \( v_0 \cos \theta \) from the horizontal equation back into their respective expressions and use the Pythagorean identity \( \sin^2 \theta + \cos^2 \theta = 1 \). Solve these equations to find the angle \( \theta \).
06

Calculate the Distance the Woman Runs

Once \( \theta \) is known, you can use either the time solved in Step 2 and the given speed of the woman to calculate the distance she covers: \( \text{Distance} = 6.00 \ m/s \times t \).
07

Sketch the Trajectories

(a) As viewed by a stationary observer on the ground, draw the parabolic trajectory of the ball from the cliff to the ground. (b) As viewed by the runner, sketch the trajectory as a straight line angled downward, reflecting the relative motion of the ball as she runs parallel to the ball's fall.

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Key Concepts

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

Kinematic Equations
To understand projectile motion, we must rely on kinematic equations. These equations help us analyze the motion of objects in physics when they are in free fall or thrown into the air like a ball. In this problem, the ball's trajectory involves both horizontal and vertical motion. - **Horizontal motion**: Here, velocity remains constant because there's no acceleration.- **Vertical motion**: This is affected by gravity, where acceleration due to gravity is denoted by \( g = 9.8 \, \mathrm{m/s^2} \).The equations we can use include:1. The basic kinematic equation for vertical motion: \[ h = v_{y0} t + \frac{1}{2} g t^2 \] where \( h \) is the height, \( v_{y0} \) is the initial vertical velocity, and \( g \) is the acceleration due to gravity.2. The horizontal range equation: \[ R = v_{x0} t \] where \( R \) is the range, and \( v_{x0} \) is the initial horizontal velocity. These equations allow us to calculate the horizontal range and the time of flight, given the initial speed and angle of projection.
Horizontal Range
The horizontal range of a projectile is the total distance it travels horizontally. For the ball to meet the runner at the right spot, both the horizontal range of the ball and the distance the woman runs need to be the same.To find out the range, we need to determine the initial horizontal velocity \( v_{x0} \) using:- The formula: \[ v_{x0} = v_0 \cos \theta \]Next, the horizontal range \( R \) is calculated by multiplying the initial horizontal velocity with the time of flight \( t \):- \[ R = v_{x0} \times t \]Since the woman runs with a speed of 6.00 m/s to catch the ball, her distance covered is equal to the ball’s range when their speeds adjust for time \( t \).This relationship is crucial because it helps us know the exact location where the ball and woman meet, given she maintains a constant speed.
Time of Flight
Time of flight is the duration for which the projectile remains in the air. It’s influenced by the initial speed, angle of projection, and the height from which it is thrown. To calculate the time of flight, we solve the vertical motion equation:- \[ h = v_{y0} t + \frac{1}{2} g t^2 \]In our scenario, the height \( h \) is \(- 45\) meters, indicating a downward motion. The initial vertical speed \( v_{y0} \) relates to the angle \( \theta \) through \( v_{y0} = v_0 \sin \theta \).The equation is quadratic in form, and solving it gives us the time \( t \) when the ball reaches the ground level from the cliff’s height.Knowing the time of flight is necessary as it directly affects how far the projectile will travel horizontally, which is equal to the distance the woman runs to catch the ball.

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Most popular questions from this chapter

The Champion Jumper of the Insect World. The froghopper, Philaenus spumarius, holds the world record for insect jumps. When leaping at an angle of \(58.0^{\circ}\) above the horizontal, some of the tiny critters have reached a maximum height of 58.7 \(\mathrm{cm}\) above the level ground. (See Nature, Vol. 424 , July \(31,2003,\) p. \(509 . )\) (a) What was the takeoff speed for such a leap? (b) What horizontal distance did the froghopper cover for this world- record leap?

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