The Clown's Interceptor Pie

Algebra Level 2

Two clowns, Twinkle and Jingle, are throwing pies at each other. Twinkle throws a pie toward Jingle from 500 500 centimeters away. Its flight path is given by parametric equations { x = 100 t y = 80 t 16 t 2 \begin{cases} x &=& 100t \\ y &=& 80t - 16t^2 \end{cases} where t t is time in seconds.

Two seconds later Jingle launches an interceptor pie from his location with the flight path { x = 500 500 ( t 2 ) y = K ( t 2 ) 16 ( t 2 ) 2 \begin{cases} x &=& 500 - 500(t-2) \\ y &=& K(t-2) - 16(t-2)^2 \end{cases} Find the value of K K which will guarantee that the interceptor pie will hit its target (the pie thrown by Twinkle).


The answer is 208.

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7 solutions

Julio Reyes
Oct 13, 2013

For the two pies to collied the x and y position have to be equal for the same time t .

First, find the time when the x positions are equal.

100 t = 500 500 ( t 2 ) 100 t = 500 500 t + 1000 100 t = 1500 500 t 600 t = 1500 t = 5 2 100t = 500 - 500(t-2) \\ 100t = 500 - 500t + 1000 \\ 100t = 1500 - 500t \\ 600t = 1500 \\ t = {5 \over 2}

The y positions have to be equal for this t value, so we can replace t and solve for K .

80 t 16 t 2 = K ( t 2 ) 16 ( t 2 ) 2 80 5 2 16 ( 5 2 ) 2 = K ( 5 2 2 ) 16 ( 5 2 2 ) 2 100 = 1 2 K 4 104 = 1 2 K K = 208 80t - 16t^2 = K(t-2) - 16( t - 2)^2 \\ 80\cdot {5 \over 2} - 16 \cdot ({5 \over 2})^2 = K({5 \over 2} - 2) - 16({5 \over 2} - 2)^2 \\ 100 = {1 \over 2}K - 4 \\ 104 = {1 \over 2}K \\ K = \fbox{208}

I do not understand this at all

alder fulton - 7 years, 8 months ago

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The problem or solution?

They gave us paramatric equations for the position of each pie with respect to time. With exception of the equation containing K, choose any value for t and it will give you the (x, y) values representing the postion of the pie at that point in time.

For the pies to collied, their postion (x, y) not only have to be the same, but it has to happen at the same value for t .

Knowing this, you can set the equations for the x position of each pie and solve for t, to get the time when the pies collied.

The y postions have to also be equal at this point in time. So we set the equations for y equal to each other, and since we know the value for t we can substitule it into our expression.

After we do that, the only variable left is K, so we just solve the equality. The value that we get for K is the value that produces the y positions to be equal that point in time.

Julio Reyes - 7 years, 8 months ago

nice

basit azhar - 7 years, 7 months ago

Does Twinkle's pie going 2 seconds ahead make any differences?

Bản Vũ Linh - 7 years, 7 months ago

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Yes it does. If you notice they gave us the parametric equations for Jingle's pie as (t-2). Essentially whenever we plug in a t value into the equations we are getting the position 2 seconds late. So if we plug in t = 2 => 2 - 2 = 0, and that ends up giving us the position at 0 seconds. which means the pie hasn't left Jingle's hand yet.

Julio Reyes - 7 years, 7 months ago
Adrabi Abderrahim
Oct 14, 2013

for can interceptor pie hit the pie thrown by Twinkle, need the two pies have same position in same time, so: { 100 t = 500 500 ( t 2 ) 80 t 16 t 2 = K ( t 2 ) 16 ( t 2 ) 2 \left\{\begin{matrix} 100t = 500 - 500(t - 2) \\ 80t - 16t^2 = K(t-2) - 16(t-2)^2 \end{matrix}\right.

{ t = 5 / 2 80 ( 5 / 2 ) 16 ( 5 / 2 ) 2 = K ( ( 5 / 2 ) 2 ) 16 ( ( 5 / 2 ) 2 ) 2 \left\{\begin{matrix} t = 5/2 \\ 80(5/2) - 16(5/2)^2 = K((5/2)-2) - 16((5/2)-2)^2 \end{matrix}\right.

so K = 208 K = 208

William Cui
Oct 14, 2013

Let us take the two equations considering x x . We have x = 100 t x=100t and x = 500 500 ( t 2 ) x=500-500(t-2) .

Substituting for x, we have 500 500 ( t 2 ) = 100 t 500-500(t-2)=100t . We can then solve for t = 5 2 t=\frac{5}{2} .

Now, we look at the two equations considering y. Plugging in t = 5 2 t=\frac{5}{2} , we have

80 ( 5 2 ) 16 ( 5 2 ) 2 = K ( 5 2 2 ) 16 ( 5 2 2 ) 2 80(\frac{5}{2})-16(\frac{5}{2})^2=K(\frac{5}{2}-2)-16(\frac{5}{2}-2)^2 . 200 100 = K ( 1 2 ) 16 ( 1 4 ) 200-100=K(\frac{1}{2})-16(\frac{1}{4}) 100 = K 2 4 100=\frac{K}{2}-4

We can then simplify this down to 104 = K 2 104=\frac{K}{2} , so we solve for K = 208 \boxed{K=208} .

Great job!

The question can appear intimidating and scary, and I'm glad that you were able to approach it well.

Calvin Lin Staff - 7 years, 7 months ago

awesome

Rohan Ishwarkar - 7 years, 7 months ago
Ajit Athle
Oct 13, 2013

For the two pies to meet at a point, the x & y co-ordinates must match after say t seconds. Hence, 100t = 500t - 500(t-2) giving t = 2.5 secs. Further, 80t - 16t^2 = K(t-2) -16(t-2)^2 where we plug in t = 2.5 to obtain K = 208

for make it more simple, jst compare the value of y from perametric eqn and thn solve for t..put value of t in eqn to get value of k..

haris hussain - 7 years, 7 months ago
Vincent Huang
Oct 13, 2013

Setting the two expressions for the x values equal gives us t=2.5 Now we can plug that into the expression for y to get y= K (0.5) -16 (0.25) which also equals $80t - 16t^2$, and we can solve for k.

Hammad Ahmed
Oct 14, 2013

the first equation is supposed as "A" the second the third and the fourth equation as supposed as B, C, D. now simulatneously solving the A & C eqaution gives the value of t which is = 5/2. now simulataneously solving equation B & D gives the value of k when we put the value of t from the above equation in it as 208. which is the required answer

Sirajuddin Jalil
Oct 17, 2013

the Solution x = x 100 t = 500 - 500(t-2) 600 t = 1500 t=2,5 s

y=y 0,5k= 100+4 k=104/0,5 k=208

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