# How do I solve this projectile motion problem?

lolseeker2
Member warned that the homework template must be used
The volleyball player serves the ball from pt.A w/ an initial Vo at angle theta to the horizontal. If the ball just cleared the net at pt. C and landed on the base at line B.

Determine the value of theta.

Determine the value of Vo.

Determine the time reach to the highest point at the reference from the horizontal surface.

The h from which the ball is thrown is 0.9 m.

Figure is in the link.

https://imgur.com/a/qDkzsbO

I tried solving Vox using Vox = 0.9sin90 by assuming the other angle from Vox to h to be 90 degrees.

I don't know how to get Voy to solve for the theta.

## Answers and Replies

Homework Helper
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We can assume that the distances AB,AC,CB as well as the height of point C from the ground, are all known to us?

lolseeker2
We can assume that the distances AB,AC,CB as well as the height of point C from the ground, are all known to us?
The only variable that has a value is the height relative to the ground from which the ball is thrown by the player which is 0.9m

Homework Helper
Gold Member
Vox = 0.9sin90
The 0.9 (m) is a distance, so that is not going to produce a velocity. I have no idea what
the other angle from Vox to h to be 90 degrees.
means.
As Δ2 points out, you need to know the dimensions of a volleyball court. Look those up.

You should not have deleted the template. What relevant standard equations do you know for motion under constant acceleration?

lolseeker2
The 0.9 (m) is a distance, so that is not going to produce a velocity. I have no idea what

means.
As Δ2 points out, you need to know the dimensions of a volleyball court. Look those up.

You should not have deleted the template. What relevant standard equations do you know for motion under constant acceleration?

Ah yes...my mistake...should have considered that. What equations should i use for solving theta?

Homework Helper
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Something else, when you say "it just clears the net at point C" , do you mean that the ball is at its highest point at point C?

lolseeker2
Something else, when you say "it just clears the net at point C" , do you mean that the ball is at its highest point at point C?
yes

Homework Helper
Gold Member
Something doesn't look quite right... For a volleyball court it is AC=CB right?

or point A is a bit out than the baseline at that side, while point B is at the baseline?

lolseeker2
Something doesn't look quite right... For a volleyball court it is AC=CB right?
yes...i just did a diagram of it...I also assumed the height for the net is for men's.

Homework Helper
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If the ball is at its highest point at point C, and AC=CB then at point B it will be at height h from the ground (its from the symmetry of the situation, if it spends horizontal distance AC to reach the highest point, then will also spend another AC=CB distance to reach at height h which is the height it is initially launched).
It cant land exactly there, at point B, regardless of what we have for ##v_0## and ##\theta##

lolseeker2
Something doesn't look quite right... For a volleyball court it is AC=CB right?

or point A is a bit out than the baseline at that side, while point B is at the baseline?

Point B is at the baseline. Pt. A itself is where the player throws the ball with a height of 0.9 m from the ground.

Homework Helper
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is Point A exactly above the baseline at that side and at height 0.9m?

lolseeker2
The ball was thrown from pt. A via player's hand at a height of 0.9m from the ground. The ball reach max height at pt. C when it cleared the net and then it dropped exactly at pt. B baseline. That's how I pictured out problem.

lolseeker2
is Point A exactly above the baseline at that side and at height 0.9m?
yes

Homework Helper
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The ball was thrown from pt. A via player's hand at a height of 0.9m from the ground. The ball reach max height at pt. C when it cleared the net and then it dropped exactly at pt. B baseline. That's how I pictured out problem.
This scenario isn't possible unless we account for air resistance. Are we to neglect air resistance or not?

lolseeker2
This scenario isn't possible unless we account for air resistance. Are we to neglect air resistance or not?
yes

Homework Helper
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if we neglect air resistance, you scenario just cannot happen.

lolseeker2
Well the teacher who gave the problem stated there is no air resistance involved.

Homework Helper
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@haruspex what do you think, if AC=CB and the ball is at its highest point at C, isn't at point B gonna be also at height h above the ground? (air resistance neglected)

Homework Helper
I know of a way to solve this. We have three points on a parabola: (0, 0.9), (9, 2.43), (18, 0). One can find the equation of the parabola which can lead to the answer. Point C will not be the highest point, it'll peak just before point C.

So if you want to follow this method, start by finding the coefficients of y = ax^2 + bx + c.

Delta2
Homework Helper
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I know of a way to solve this. We have three points on a parabola: (0, 0.9), (9, 2.43), (18, 0). One can find the equation of the parabola which can lead to the answer. Point C will not be the highest point, it'll peak just before point C.

So if you want to follow this method, start by finding the coefficients of y = ax^2 + bx + c.
If the ball peaks before point C then I agree it is solvable. Otherwise we have to take into account air resistance (drag).

Homework Helper
Gold Member
@haruspex what do you think, if AC=CB and the ball is at its highest point at C, isn't at point B gonna be also at height h above the ground? (air resistance neglected)
Should not assume it is at the highest point at the net. That is not implied by "just clears the net".
We have three (x,y) points that we know it passes through. That gives us four equations. Correspondingly, there are four unknowns: the initial velocity components, the time to reach the net, and the time to reach the baseline.

Homework Helper
Ok, I can see Haruspex's method is the one to use (for sure).

Homework Helper
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Ok, I can see Haruspex's method is the one to use (for sure).

(Not sure) , your method involves at start solving a system of 3 linear equations, which i guess wont be that hard, then we can find theta by ##\tan\theta={\frac{dy}{dx}}_{x=0}##. And then we can find the ##x_0## for which we have the peak by solving ##\frac{dy}{dx}=0## (also a linear equation) and then we can find the ##y_0## (highest height) and then get ##v_0## from ##v_0\sin\theta=\sqrt{2gy_0}## (also linear equation) and the time of ascent ##t_0##, by solving another linear equation ##x_0=v_0t_0\cos\theta##.

Haruspex method involves solving at least 2 quadratic equations if I am not mistaken.

Last edited:
Homework Helper
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Haruspex method involves solving at least 2 quadratic equations if I am not mistaken.
It is not necessary to solve any quadratic equations, just two (or three, depending on how you choose to write them) simultaneous equations which are effectively linear.
The way I expressed it in post #23 was not intended to represent the best approach, merely that there is the right number of equations for the unknowns without assuming the highest point is at the net.

Homework Helper
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I still think your methods involves solving a system of two linear and two quadratic equations... What do you mean by the term simultaneous equations?

Homework Helper
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I still think your methods involves solving a system of two linear and two quadratic equations... What do you mean by the term simultaneous equations?
In its simplest form, two linear equations with two unknowns. In this case, it is quadratic in one unknown, but to find the angle you only need to solve for the other one.

Homework Helper
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In its simplest form, two linear equations with two unknowns. In this case, it is quadratic in one unknown, but to find the angle you only need to solve for the other one.
Not sure what you mean here, but I agree if you correctly treat the system , you can solve one quadratic equation and some linear equations and get the angle and the velocities.

Homework Helper
(Not sure) , your method involves at start solving a system of 3 linear equations, which i guess wont be that hard, then we can find theta by ##\tan\theta={\frac{dy}{dx}}_{x=0}##. And then we can find the ##x_0## for which we have the peak by solving ##\frac{dy}{dx}=0## (also a linear equation) and then we can find the ##y_0## (highest height) and then get ##v_0## from ##v_0\sin\theta=\sqrt{2gy_0}## (also linear equation) and the time of ascent ##t_0##, by solving another linear equation ##x_0=v_0t_0\cos\theta##.

Haruspex method involves solving at least 2 quadratic equations if I am not mistaken.

I believe Haruspex's method does not require one to differentiate at all which I think is much nicer.

Homework Helper
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I believe Haruspex's method does not require one to differentiate at all which I think is much nicer.
That's very true, I still like more your method for that exact reason, that it involves derivatives!!!

Homework Helper
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Not sure what you mean here, but I agree if you correctly treat the system , you can solve one quadratic equation and some linear equations and get the angle and the velocities.
You get two linear equations in which the unknowns are vy/vx and 1/vx2, vx and vy being the initial velocity components.

Delta2