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Molodets [167]
3 years ago
9

A little boy is standing at the edge of a cliff 1000 m high. He throws a ball straight downward at an initial speed of 20 m/s, a

nd it falls straight down to the ground below. At a time of 6 seconds after it was thrown, how far above the ground is it? The acceleration due to gravity is 10 m/s2 .
Physics
1 answer:
marin [14]3 years ago
6 0

Answer:

The ball will be at 700 m above the ground.

Explanation:

We can use the following kinematic equation

y(t) = \ y_0 \ + \ v_0 \ t \ + \frac{1}{2} \ a \ t^2.

where y(t) represent the height from the ground. For our problem, the initial height will be:

y_0 \ = \ 1000 m.

The initial velocity:

v_0 = - 20 \frac{m}{s},

take into consideration the minus sign, that appears cause the ball its thrown down.  The same minus appears for the acceleration:

a=-10\frac{m}{s}

So, the equation for our problem its:

y(t) = \ 1000 m \ - \ 20 \ \frac{m}{s} \ t \ - \frac{1}{2} \ 10 \frac{m}{s^2} \ t^2.

Taking t=6 s:

y(6 \ s) = \ 1000 m \ - \ 20 \ \frac{m}{s} \ * \ 6 \ s \ - \frac{1}{2} \ 10 \frac{m}{s^2} \ * \ (6 s)^2.

y(6 \ s) = \ 1000 m \ - 120 m - \frac{1}{2} \ 10 \frac{m}{s^2} \ * \ 36 s^2.

y(6 \ s) = \ 1000 m \ - 120 m - 180 m.

y(6 \ s) = \ 1000 m \ - 300 m.

y(6 \ s) = \ 700 m.

So this its the height of the ball 6 seconds after being thrown.

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Explanation:

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2 years ago
A balloon is rising vertically upwards at a velocity of 10m/s. When it is at a height of 45m from the ground, a parachute bails
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(a) 30.9 m

Let's analyze the motion of the parachutist. Its vertical position above the ground is given by

y=h+ut+\frac{1}{2}gt^2

where

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u = 10 m/s is the initial velocity (upward)

t is the time

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Substituting t=3 s , we find the height of the parachutist when it opens the parachute:

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Here we have to find first the height of the balloon 3 seconds after the parachutist has jumped off from it. The vertical position of the balloon is given by

y = h + ut

where

h = 45 m is the initial height

u = 10 m/s is the initial velocity (upward)

t is the time

Substituting t = 3 s, we find

y = 45 m + (10 m/s)(3 s) = 75 m

So the distance between the balloon and the parachutist after 3 s is

d = 75 m - 30.9 m = 44.1 m

(c) 8.2 m/s downward

The velocity of the parachutist at the moment he opens the parachute is:

v = u +gt

where

u = 10 m/s is the initial velocity (upward)

t is the time

g = -9.8 m/s^2 is the acceleration of gravity (downward)

Substituting t = 3 s,

v = 10 m/s + (-9.8 m/s^2)(3 s)= -19.4 m/s

where the negative sign means it is downward

After t=3 s, the parachutist open the parachute and it starts moving with a deceleration of

a =+5 m/s^2

where we put a positive sign since this time the acceleration is upward.

The total distance he still has to cover till the ground is

d = 30.9 m

So we can find the final velocity by using

v^2-u^2 = 2ad

where this time we have u = 19.4 m/s as initial velocity. Taking the downward direction as positive, the deceleration must be considered as negative:

a = -5 m/s^2

Solving for v,

v=\sqrt{u^2 +2ad}=\sqrt{(19.4 m/s)^2+2(-5 m/s^2)(30.9 m)}=8.2 m/s

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We can find the duration of the second part of the motion of the parachutist (after he has opened the parachute) by using

a=\frac{v-u}{t}

where

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u = 19.4 m/s is the initial velocity

t is the time

Solving for t, we find

t=\frac{v-u}{a}=\frac{8.2 m/s-19.4 m/s}{-5 m/s^2}=2.24 s

And added to the 3 seconds between the instant of the jump and the moment he opens the parachute, the total time is

t = 3 s + 2.24 s = 5.24 s

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