My First Problem about Physics

Suppose you are a basketball player with a height of 2 meters, currently practicing in a basketball court. You want to shoot the ball into the ring, which is 5 meters high and 6 meters away from you, with initial speed 5 m/s 5\text{ m/s} at an angle of θ \theta degrees with the horizontal.

Is there any possible value of θ \theta that makes the ball enter the ring?


Details and Assumptions:

  • You are shooting the ball in a parabolic way (a parabola that opens downward).
  • You don't jump while shooting the ball (it's like doing a free throw shot).
  • Use g = 9.8 m/s 2 . g=-9.8 \text{ m/s}^2.
Yes No Maybe

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Michael Friese
Nov 18, 2017

The highest the basketball player can throw the ball will be if he threw it at a 90 degree angle (straight up) With an initial velocity of 5 m/s, the highest the ball will go is 1.27 meters above his initial height of 2 m. Therefore, the maximum height of the ball, above ground, is 3.27 meters, which is lower than the hoop height of 5 meters. So, the answer is no.

Christian Daang
Oct 18, 2017

Let's identify the given:

I used where the basketball player is standing as the reference point (origin).

r i = 2 j ^ r f = 6 i ^ + 5 j ^ V = 5 m/s a = 9.8 j ^ \textbf{r}_i = 2 \hat{j} \\ \textbf{r}_f = 6 \hat{i} + 5 \hat{j} \\ |\textbf{V}| = 5 \ \text{m/s} \\ \textbf{a} = -9.8 \hat{j}

Then, we will use this 2 formulas:

V = V o x i ^ + V o y j ^ \textbf{V} = V_{ox}\hat{i} + V_{oy} \hat{j}

and

r f = r i + V t + 0.5 a t 2 \textbf{r}_f = \textbf{r}_i + \textbf{V}t + 0.5\textbf{a} t^2


V = V o x i ^ + V o y j ^ = 5 cos θ i ^ + 5 sin θ j ^ \begin{aligned} \textbf{V} &= V_{ox}\hat{i} + V_{oy} \hat{j} \\ &= 5\cos\theta \hat{i} + 5\sin\theta \hat{j} \end{aligned}


r f = r i + V t + 0.5 a t 2 6 i ^ + 5 j ^ = 2 j ^ + ( 5 cos θ i ^ + 5 sin θ j ^ ) t 4.9 t 2 j ^ \begin{aligned} \textbf{r}_f &= \textbf{r}_i + \textbf{V}t + 0.5\textbf{a} t^2 \\ \implies 6 \hat{i} + 5 \hat{j} &= 2 \hat{j} + \left( 5\cos\theta \hat{i} + 5\sin\theta \hat{j}\right)t - 4.9t^2 \hat{j} \end{aligned}

By analogy,

6 = 5 t cos θ t = 6 5 cos θ 6 = 5t\cos \theta \implies t = \dfrac{6}{5\cos \theta }

and

5 = 2 + 5 t sin θ 4.9 t 2 3 = 5 ( 6 5 cos θ ) ( sin θ ) 4.9 ( 6 5 cos θ ) 2 75 cos 2 θ = 150 sin θ cos θ 4.9 ( 36 ) 75 cos 2 θ = 150 ( 1 cos 2 θ ) cos θ 4.9 ( 36 ) ( 75 cos 2 θ + 4.9 ( 36 ) ) 2 = 15 0 2 ( 1 cos 2 θ ) ( cos 2 θ ) 28125 a 2 + 3960 a + 31117 = 0 a = cos 2 θ ; a 1 \begin{aligned} 5 &= 2 + 5t\sin\theta - 4.9t^2 \\ 3 &= 5\left(\dfrac{6}{5\cos \theta }\right)(\sin \theta) - 4.9 \left( \dfrac{6}{5\cos \theta } \right)^2 \\ 75\cos ^2 \theta &= 150 \sin\theta\cos\theta - 4.9(36) \\ 75\cos ^2 \theta &= 150\left( \sqrt{1 - \cos ^2 \theta} \right) \cos\theta - 4.9(36) \\ \left( 75\cos ^2 \theta + 4.9(36)\right)^2 &= 150^2 (1 - \cos ^2 \theta)(\cos ^2 \theta) \\ 28125 a^2 + 3960 a + 31117 &= 0 \hspace{15mm} \color{#3D99F6}{a = \cos ^2 \theta \ ; \ a \le 1} \end{aligned}

If you get the discriminant of that quadratic equation, you'll see that a a is imaginary, hence, there is NO \textbf{NO} such θ \theta applicable.

would you, please, be able to use any drawings or graphics to show the cos and sin? I am sort of confused about how the sum of the initial velocities for the basis vectors is 5 cos(theta)i + 5 sin(theta)j

Matthew Agona - 3 years, 6 months ago

Log in to reply

I will try my best to add picture on my solution as soon as possible. I can't do it right now cause I am just a little bit busy. XD

Christian Daang - 3 years, 5 months ago

Log in to reply

that would be really cool. I would be interested to see it.

Thanks

Matthew Agona - 3 years, 5 months ago

Log in to reply

@Matthew Agona I think, illustrations might not help so, I'll just explain it:

The vector V \textbf{V} ( velocity ) can be express as V = V o x i ^ + V o y j ^ \textbf{V} = V_{ox} \hat{i} + V_{oy} \hat{j} .

It is by definition/convenience that V o x V_{ox} (x - component of the velocity) is equal to V cos θ | \textbf{V} | \cos \theta

Why cos \cos ? Remember that in trigonometry, usually, cosines are interconnected to the value of abscissa.

Meanwhile, V o y = V sin θ V_{oy} = | \textbf{V} | \sin \theta . Meanwhile, sines are interconnected to the value of ordinate.

I hope this explanation somehow enlighten you. I can't even explain this things properly. :( Sorry about that.

Christian Daang - 3 years, 5 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...