Both Times a Ball Passes a Ledge

Q. A ball is thrown straight up at 20 m/s from the foot of a wall with a ledge 15 m up it. Taking g as 9.8 m/s², at what times after the throw does it pass the ledge, in seconds? expects 0.990 s and 3.09 s

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Q. A ball is thrown straight up at 20 m/s $v_{0} = 20$ m/s from the foot of a wall $x_{0} = 0$ m with a ledge 15 m $x_{1} = 15$ m up it. Taking g as $a = - g$ 9.8 m/s² $g = 9.8$ m/s², at what times after the throw does it pass the ledge, in seconds?

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Inputs

throw speed

v_{0}

20.0 m/s

height

x_{1}

15.00 m

gravity

g

9.8 m/s²

Outputs

times

Δ t

The solution

Solved with UniformAcceleration

Given

$v_{0} = 20$ m/s
$g = 9.8$ m/s²
$x_{1} = 15$ m
$x_{0} = 0$ m from “from the foot of a wall”
$a = - g$ from “thrown straight up” and “Taking g as”

Find $Δ t$ .

Working

thrown straight up and Taking g as $a = - g$
$a = - 9.8$
$a = −9.8$ m/s²
position after a time(flying) $x_{1} = x_{0} + v_{0} \cdot Δ t + \frac{a \cdot {Δ t}^{2}}{2}$
$15 = 0 + 20 \cdot Δ t + \frac{(−9.8) \cdot {Δ t}^{2}}{2}$
$Δ t = 0.990241822, 3.09139083$ sall of them are taken
$Δ t = 0.99 s and 3.09 s$
$Δ t \approx 0.990, 3.09$ sto 3 significant figures

Conclusion

The ball passes the ledge twice, once rising and once falling back, so both times answer the question; they sit evenly either side of the top of the flight, about 2.04 s after the throw.

The Ruby

1require_relative "../physics"23question :both_times_a_ball_passes_a_ledge do4  kind :numerical5  describes "A ball is thrown straight up at %{throw_speed} from the foot of a wall with a ledge %{height} up it. " \6            "Taking g as %{gravity}, %{times}, in seconds?"7  given throw_speed: "20 m/s"8  given height: "15 m"9  given gravity: "9.8 m/s2"10  asks times: "at what times after the throw does it pass the ledge"11end

1require_relative "../questions/both_times_a_ball_passes_a_ledge"2require_relative "../science/motion_in_a_straight_line/uniform_acceleration"34solve :both_times_a_ball_passes_a_ledge, using: UniformAcceleration do5  objects :ball6  ball.goes(:flying)78  deduce_from "thrown straight up", "Taking g as", ball.acceleration => -g9  take throw_speed: ball.initial_velocity, gravity: g, height: ball.final_position10  deduce_from "from the foot of a wall", ball.initial_position => "0 m"11  take times: flying.time_taken12  answer find_value(:times, in: "s", figures: 3, root: :all)1314  conclusion "The ball passes the ledge twice, once rising and once falling back, so both times answer the " \15             "question; they sit evenly either side of the top of the flight, about 2.04 s after the throw."16end

1require_relative "../solutions/both_times_a_ball_passes_a_ledge"23play_with :both_times_a_ball_passes_a_ledge do4  title "Both Times a Ball Passes a Ledge"5end

The laws

quantity	written	units	within	from
position	$x$	m	—	Velocity
displacement	$Δ x$	m	—	Velocity
path length	$d$	m	—	Velocity
time taken	$Δ t$	s	0 … 1e+05	Velocity
average velocity	$v_{av}$	m/s	—	Velocity
average speed	$c_{av}$	m/s	—	Velocity
starting position	$x_{0}$	m	—	Velocity
final position	$x$	m	—	Velocity
velocity	$v$	m/s	—	Acceleration
change of velocity	$Δ v$	m/s	—	Acceleration
average acceleration	$a_{av}$	m/s²	—	Acceleration
starting velocity	$v_{0}$	m/s	—	Acceleration
final velocity	$v$	m/s	—	Acceleration
acceleration	$a$	m/s²	—	UniformAcceleration
gravity	$g$	m/s²	—	UniformAcceleration
time	$t$	s	0 … 1e+05	UniformAcceleration
displacement of ball during flying	$Δ x$	m	—
position of ball at end of flying	$x_{1}$	m	—
position of ball at start of flying	$x_{0}$	m	—
average velocity of ball during flying	$v_{av}$	m/s	—
time taken of flying	$Δ t$	s	0 … 1e+05
average speed of ball during flying	$c_{av}$	m/s	—
path length of ball during flying	$d$	m	—
change of velocity of ball during flying	$Δ v$	m/s	—
velocity of ball at end of flying from ground	$v_{1}$	m/s	—
velocity of ball at start of flying from ground	$v_{0}$	m/s	—
average acceleration of ball during flying	$a_{av}$	m/s²	—
acceleration of ball during flying	$a$	m/s²	—

x_{0} \equiv x_{i} (start of motion)

starting position

x \equiv x_{i} (end of motion)

final position

v_{0} \equiv v_{i} (start of motion)

starting velocity

v \equiv v_{i} (end of motion)

final velocity

t \equiv Δ t_{m}

time

d_{i} (whole) = d_{i} (part 1) + d_{i} (part 2)

path in parts

Δ t_{w} = Δ t_{p} + Δ t_{p}

time in parts

a_{i} (part 1) = a_{i} (whole)

steady through its parts

Δ x = x_{1} - x_{0}

change of position(flying)

v_{av} = \frac{Δ x}{Δ t}

displacement a time(flying)

c_{av} = \frac{d}{Δ t}

path a time(flying)

Δ v = v_{1} - v_{0}

velocity gained(flying)

a_{av} = \frac{Δ v}{Δ t}

velocity gained a time(flying)

v_{1} = v_{0} + a \cdot Δ t

velocity after a time(flying)

x_{1} = x_{0} + v_{0} \cdot Δ t + \frac{a \cdot {Δ t}^{2}}{2}

position after a time(flying)

{v_{1}}^{2} = {v_{0}}^{2} + 2 \cdot a \cdot (x_{1} - x_{0})

velocity after a distance(flying)

x_{1} - x_{0} = \frac{(v_{1} + v_{0}) \cdot Δ t}{2}

mean of the velocities(flying)

a = - g

thrown straight up and Taking g as