then reload this page.
The scatter plot to the left shows $y$, the
approximate height in meters of a soccer ball, $x$ seconds after it has been
kicked into the air.
Slide the values of $a$, $h$, and $k$, and find an equation in the form
$y=a(x-h)^2+k$ that matches the data.
(Use the location of the vertex to find $h$ and $k$.
Use the shape of the parabola to find the value of $a$. The error to
the right of the grid will be 0.0 — and it will disappear — when your parabola matches the
Click or tap inside the grid to the left and drag to the left or
right. To the right of the grid you will see the $y$-value of the point on
the graph with the $x$-value you have selected.
You can also use the left and right arrow keys to move in small increments.
Example: Find an equation for the parabola without using the sliders.
Now all that is left to do is find the value of $a$. To do this, pick another point on the
graph, say $(0,0.4)$. Because this point is on the graph, you can substitute 0 for
$x$ and 0.4 for $y$ in $y=a(x-2)^2+20$:
So, an equation for the parabola is: $y=-4.9(x-2)^2+20$
The main cables of the Golden Gate Bridge have the shape of part of a parabola.
Each tower of the Golden Gate Bridge rises 152 meters above the roadbed. The length of the
main span is 1280 meters. We wish to find an equation for a parabola that could model the
Golden Gate’s main cables. The diagram below shows a graph of the main cables of the Golden Gate
The vertex of the parabola is assumed to be at the origin. Since it
is halfway between the two towers, the distance between the vertex and the towers is
$$1280/2$$ meters or 640 meters. Because the towers are 152 meters tall, this
means the points $(-640, 152)$ and $(640, 152)$ must be on the parabola.
Use the sliders to find an equation in the form $y=a(x-h)^2+k$
that approximates the main cables of the Golden Gate Bridge to within about 0.5 meters.
Use the method shown in the example below findHeight to
find an equation for the main cables of the Golden Gate Bridge.
Solve for $a$, by using the fact that $(640,152)$ satisfies that equation. Write your final
answer as a decimal, rounded to as many decimal places as you can fit in the answer box. (You
can use a calculator to do the arithmetic.)
One of the most important discoveries in science was the description of how gravity affects
objects rising from or falling to the earth’s surface. In 1638 Galileo claimed that the height
of such an object is a quadratic function of its time in the air.
Suppose that a baseball player hits the ball straight above home plate. If
the bat meets the ball 0.49 meters above the ground and sends it up at a rate of
30.42 meters per second, then the height of the ball, in meters, $t$ seconds later is
predicted by the rule:
Click or tap inside the grid to the left and drag to the left or right. To the right of the
grid you will see the $h$-value of the point on the graph with the
$t$-value you have selected. You can also enter a value for $t$ directly into its input box.
Use the graph and the equation to answer the following questions.
What do you think an equation for the height of the ball would be if the bat gave it a
vertical velocity of 27 meters per second?
A bakery sells more loaves of bread when it reduces its price, but if the
price is too low, then the profits are very low. The equation
models the bakery’s daily profits in dollars, where $x$ is the price of a loaf of bread in
dollars. To the left is a graph of this equation.