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Physics Kinematics + Trigonometry Application (Physics 12u)

Wild ownz al

Member
Nov 11, 2018
30
Show that the maximum range of a projectile launched with Vi from Yi=0 is 45 degrees. (HINT: sin20 = 2sinθcosθ)
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
Show that the maximum range of a projectile launched with Vi from Yi=0 is 45 degrees. (HINT: sin20 = 2sinθcosθ)
Can you show what you have so far?
 

Wild ownz al

Member
Nov 11, 2018
30
Can you show what you have so far?
So far...

Xf = Xi + Vixcosθt

Xf = 0 + Vixcosθt

Xf = Vixcosθt

t = Xf / Vixcosθ

Then I use position equation

Yf = 0 + ViySinθt+1/2ayt^2

Plug in t = Xf / Vixcosθ

Yf = 0 + ViySinθ(Xf/Vixcosθ)+1/2ay(Xf/Vixcosθ)^2

This is what I have so far, however I still don't understand HOW I am suppose to prove the max range with trig, though I do know we are suppose to use the hint given, thanks.
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
I would begin with velocity, with our coordinate axes oriented such that the initial position of the projectile is at the origin, and observe we are neglecting drag:

\(\displaystyle v_x(t)=v_0\cos(\theta)\)

\(\displaystyle v_y(t)=-gt+v_0\sin(\theta)\)

Hence:

\(\displaystyle x(t)=v_0\cos(\theta)t\)

\(\displaystyle y(t)=-\frac{g}{2}t^2+v_0\sin(\theta)t=\frac{t}{2}\left(2v_0\sin(\theta)-gt\right)\)

Now, the range of the projectile is where it returns to the ground, which is the non-zero root of \(y(t)\), which is what value of \(t\)?
 

Wild ownz al

Member
Nov 11, 2018
30
I would begin with velocity, with our coordinate axes oriented such that the initial position of the projectile is at the origin, and observe we are neglecting drag:

\(\displaystyle v_x(t)=v_0\cos(\theta)\)

\(\displaystyle v_y(t)=-gt+v_0\sin(\theta)\)

Hence:

\(\displaystyle x(t)=v_0\cos(\theta)t\)

\(\displaystyle y(t)=-\frac{g}{2}t^2+v_0\sin(\theta)t=\frac{t}{2}\left(2v_0\sin(\theta)-gt\right)\)

Now, the range of the projectile is where it returns to the ground, which is the non-zero root of \(y(t)\), which is what value of \(t\)?
Do I use the quadratic formula to determine that? Or is it 1/2 t?
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
Do I use the quadratic formula to determine that? Or is it 1/2 t?
I factored \(y(t)\) so you could more easily determine the roots. We see that one root is \(t=0\), and that corresponds to the moment when the projectile is launched from the ground. We don't want that root, we want the other one, corresponding to when it lands (when its y-coordinate is also zero), or returns to the ground. So, we equate the other factor to zero to determine when that is:

\(\displaystyle 2v_0\sin(\theta)-gt=0\)

Solving this for \(t\) will tell us when the projectile lands, and then we can use this value of \(t\) in \(x(t)\) to determine how far (horizontally) is traveled, which is by definition, its range.

Can you proceed?
 

Wild ownz al

Member
Nov 11, 2018
30
I factored \(y(t)\) so you could more easily determine the roots. We see that one root is \(t=0\), and that corresponds to the moment when the projectile is launched from the ground. We don't want that root, we want the other one, corresponding to when it lands (when its y-coordinate is also zero), or returns to the ground. So, we equate the other factor to zero to determine when that is:

\(\displaystyle 2v_0\sin(\theta)-gt=0\)

Solving this for \(t\) will tell us when the projectile lands, and then we can use this value of \(t\) in \(x(t)\) to determine how far (horizontally) is traveled, which is by definition, its range.

Can you proceed?
But I'm not looking for the distance traveled, I need to prove why 45 degrees is the max range.
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
But I'm not looking for the distance traveled, I need to prove why 45 degrees is the max range.
What you will obtain is the range in terms of the parameters \(v_0\) and \(\theta\). And then you will need to demonstrate that for some positive value of \(v_0\), which value of \(\theta\) will maximize the range.

In other words, in order to maximize the range, we need to know what that range is, in terms of our parameters. The range is our objective function.
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
Let's find the root we want:

\(\displaystyle 2v_0\sin(\theta)-gt=0\implies t=\frac{2v_0\sin(\theta)}{g}\)

And so the range of the projectile is:

\(\displaystyle x_{\max}=x\left(\frac{2v_0\sin(\theta)}{g}\right)=v_0\cos(\theta)\left(\frac{2v_0\sin(\theta)}{g}\right)=\frac{2v_0^2\sin(\theta)\cos(\theta)}{g}\)

Applying the double-angle identity for sine, we obtain:

\(\displaystyle x_{\max}=\frac{v_0^2\sin(2\theta)}{g}\)

Can you proceed?
 

Wild ownz al

Member
Nov 11, 2018
30
Let's find the root we want:

\(\displaystyle 2v_0\sin(\theta)-gt=0\implies t=\frac{2v_0\sin(\theta)}{g}\)

And so the range of the projectile is:

\(\displaystyle x_{\max}=x\left(\frac{2v_0\sin(\theta)}{g}\right)=v_0\cos(\theta)\left(\frac{2v_0\sin(\theta)}{g}\right)=\frac{2v_0^2\sin(\theta)\cos(\theta)}{g}\)

Applying the double-angle identity for sine, we obtain:

\(\displaystyle x_{\max}=\frac{v_0^2\sin(2\theta)}{g}\)

Can you proceed?
I think I'm starting to see it now, but how do I proceed without knowing theta or Vi?
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
I think I'm starting to see it now, but how do I proceed without knowing theta or Vi?
We will assume that \(\displaystyle \frac{v_0^2}{g}\) is some constant, and only worry about the factor that is a function of \(\theta\). Since it is in the numerator, we want to maximize:

\(\displaystyle \sin(2\theta)\)

In a problem like this, we will typically take:

\(\displaystyle 0^{\circ}\le\theta\le90^{\circ}\)

This implies:

\(\displaystyle 0^{\circ}\le2\theta\le180^{\circ}\)

On this interval, what value of \(2\theta\) maximizes the sine function, and hence, what value of \(\theta\) maximizes the sine function?
 

Wild ownz al

Member
Nov 11, 2018
30
We will assume that \(\displaystyle \frac{v_0^2}{g}\) is some constant, and only worry about the factor that is a function of \(\theta\). Since it is in the numerator, we want to maximize:

\(\displaystyle \sin(2\theta)\)

In a problem like this, we will typically take:

\(\displaystyle 0^{\circ}\le\theta\le90^{\circ}\)

This implies:

\(\displaystyle 0^{\circ}\le2\theta\le180^{\circ}\)

On this interval, what value of \(2\theta\) maximizes the sine function, and hence, what value of \(\theta\) maximizes the sine function?
90 degrees? So theta = 45 degrees
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775

Wild ownz al

Member
Nov 11, 2018
30
I would begin with velocity, with our coordinate axes oriented such that the initial position of the projectile is at the origin, and observe we are neglecting drag:

\(\displaystyle v_x(t)=v_0\cos(\theta)\)

\(\displaystyle v_y(t)=-gt+v_0\sin(\theta)\)

Hence:

\(\displaystyle x(t)=v_0\cos(\theta)t\)

\(\displaystyle y(t)=-\frac{g}{2}t^2+v_0\sin(\theta)t=\frac{t}{2}\left(2v_0\sin(\theta)-gt\right)\)

Now, the range of the projectile is where it returns to the ground, which is the non-zero root of \(y(t)\), which is what value of \(t\)?
How could I have used the quadratic formula to determine the roots? My factoring is that great and it seems odd to do without given coefficients.
 

Wild ownz al

Member
Nov 11, 2018
30
We will assume that \(\displaystyle \frac{v_0^2}{g}\) is some constant, and only worry about the factor that is a function of \(\theta\). Since it is in the numerator, we want to maximize:

\(\displaystyle \sin(2\theta)\)

In a problem like this, we will typically take:

\(\displaystyle 0^{\circ}\le\theta\le90^{\circ}\)

This implies:

\(\displaystyle 0^{\circ}\le2\theta\le180^{\circ}\)

On this interval, what value of \(2\theta\) maximizes the sine function, and hence, what value of \(\theta\) maximizes the sine function?
Also how do you go from Vi^2 / g to 0 degrees < theta < 90 degrees?
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
How could I have used the quadratic formula to determine the roots? My factoring is that great and it seems odd to do without given coefficients.
We could set:

\(\displaystyle y(t)=-\frac{g}{2}t^2+v_0\sin(\theta)t=0\)

Let's multiply by -2:

\(\displaystyle gt^2-2v_0\sin(\theta)t=0\)

Now, for the quadratic formula, we identify:

\(\displaystyle a=g\)

\(\displaystyle b=-2v_0\sin(\theta)\)

\(\displaystyle c=0\)

Hence:

\(\displaystyle y=\frac{-(-2v_0\sin(\theta))\pm\sqrt{(-2v_0\sin(\theta))^2-4(g)(0)}}{2g}=\frac{2v_0\sin(\theta)\pm2v_0\sin(\theta)}{2g}=\frac{v_0\sin(\theta)\pm v_0\sin(\theta)}{g}\)

And so we see our roots are:

\(\displaystyle y\in\left\{0,\frac{2v_0\sin(\theta)}{g}\right\}\)

Factoring is much easier, and I recommend using it whenever you can.

Also how do you go from Vi^2 / g to 0 degrees < theta < 90 degrees?
We are assuming \(\displaystyle \frac{v_0^2}{g}\) is some arbitrary positive constant. Since it remains fixed, we need only concern ourselves with the factor that will vary in accordance with the parameter in which we're interested, which is \(\theta\).
 

Wild ownz al

Member
Nov 11, 2018
30
we could set:

\(\displaystyle y(t)=-\frac{g}{2}t^2+v_0\sin(\theta)t=0\)

let's multiply by -2:

\(\displaystyle gt^2-2v_0\sin(\theta)t=0\)

now, for the quadratic formula, we identify:

\(\displaystyle a=g\)

\(\displaystyle b=-2v_0\sin(\theta)\)

\(\displaystyle c=0\)

hence:

\(\displaystyle y=\frac{-(-2v_0\sin(\theta))\pm\sqrt{(-2v_0\sin(\theta))^2-4(g)(0)}}{2g}=\frac{2v_0\sin(\theta)\pm2v_0\sin(\theta)}{2g}=\frac{v_0\sin(\theta)\pm v_0\sin(\theta)}{g}\)

and so we see our roots are:

\(\displaystyle y\in\left\{0,\frac{2v_0\sin(\theta)}{g}\right\}\)

factoring is much easier, and i recommend using it whenever you can.



We are assuming \(\displaystyle \frac{v_0^2}{g}\) is some arbitrary positive constant. Since it remains fixed, we need only concern ourselves with the factor that will vary in accordance with the parameter in which we're interested, which is \(\theta\).
thank you so much