MHB [ASK] How long must he wait until the ball fall to the ground if g = 10m/s^2

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To determine how long it takes for the ball to fall to the ground after reaching a maximum height of 10 m, the discussion outlines the use of kinematic equations. The ball's vertical motion is influenced solely by gravity, with an acceleration of -g. The time to reach maximum height is calculated using the initial vertical velocity, which can be derived from the maximum height formula. By applying the symmetry of projectile motion, the total time for the ball to return to the ground is found to be 2√(2y_max/g), leading to a final result of approximately √2 seconds for the fall from the apex. Thus, the total time until the ball hits the ground is √2 seconds.
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In a soccer match Putu kicked the ball with the elevation angle $$\alpha$$ so that it reached the maximum height 10 m. How long must he wait until the ball fall to the ground if g = $$10m/s^2$$?

I don't know how to do it if the initial velocity isn't known. Can someone help me?
 
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Let's let $y$ be our vertical axis of motion, where up is in the positive direction. Neglecting drag, the only force operating on the ball after being kicked is gravity, and so the acceleration $a$ of the ball is:

$$a=-g$$

And so the velocity $v$ along the vertical axis is:

$$v=-gt+v_0\sin(\alpha)$$

And the height $y$ is:

$$y=-\frac{g}{2}t^2+v_0\sin(\alpha)t=t\left(-\frac{g}{2}t+v_0\sin(\alpha)\right)$$

When the ball reaches the maximum height, we know $$v=0\implies t=\frac{v_0\sin(\alpha)}{g}$$ and so:

$$y_{\max}=y\left(\frac{v_0\sin(\alpha)}{g}\right)=\frac{v_0\sin(\alpha)}{g}\left(-\frac{g}{2}\cdot\frac{v_0\sin(\alpha)}{g}+v_0\sin(\alpha)\right)=\frac{v_0^2\sin^2(\alpha)}{2g}\implies v_0\sin(\alpha)=\sqrt{2gy_{\max}}$$

What do we know about $y$ when the ball returns to the ground?
 
y = 0?
Where do I substitute that?
 
In a soccer match Putu kicked the ball with the elevation angle [FONT=MathJax_Math]α so that it reached the maximum height 10 m. How long must he wait until the ball fall to the ground if g = [FONT=MathJax_Main]10[FONT=MathJax_Math]m[FONT=MathJax_Main]/[FONT=MathJax_Math]s[FONT=MathJax_Main]2?

at max height $v_y = 0$

from the top of the trajectory to the ground ...

$\Delta y = v_y \cdot t - \dfrac{1}{2}gt^2$

$-10 = 0 - 5t^2$

solve for $t$ ...
 
Monoxdifly said:
y = 0?
Where do I substitute that?

Well, we now have:

$$y=t\left(-\frac{g}{2}t+\sqrt{2gy_{\max}}\right)$$

So, set $y=0$ and find the non-zero root...
 
$$t=\sqrt2$$?
 
Monoxdifly said:
$$t=\sqrt2$$?

The non-zero root is:

$$t=2\sqrt{\frac{2y_{\max}}{g}}$$

To find the time $t_F$ the ball falls from the apex of its trajectory, we need to subtract the time it took to get there:

$$t_F=2\sqrt{\frac{2y_{\max}}{g}}-\sqrt{\frac{2y_{\max}}{g}}=\sqrt{\frac{2y_{\max}}{g}}$$

We should expect this, as by the symmetry of motion, the ball takes the same anount of time to rise as it does to fall.

Plugging in the data, we find:

$$t_F=\sqrt{\frac{2(10\text{ m})}{10\dfrac{\text{m}}{\text{s}^2}}}=\sqrt{2}\text{ s}\quad\checkmark$$
 
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