# Projectile motion problem

m_scott

## Homework Statement

A tennis player standing 12.6m from the net hits the ball at 3 degrees above the horizontal. To clear the net, the ball must rise at least .330m. If the ball just clears the net at the apex of its trajectory, how fast was the ball moving when it left the racquet?

## Homework Equations

I tried using v2=v02+2a(x-x0)

I have no clue

## Answers and Replies

Homework Helper
welcome to pf!

hi m_scott! welcome to pf! (try using the X2 icon just above the Reply box )

you need two equations, one for the x and one for the y direction …

then eliminate t …

what do you get? m_scott
I still don't completely understand. What formulas am i supposed to use? And Im not sure what to do with the .330m

I set it up as: 0(final velocity)= v02+2(-9.8)(12.6) and got 15.7 but i know thats wrong...

Karlx
Hi m_scott.
You must see the motion of the ball as a composition of two kinds of motion: one in the horinzontal axis and another in the vertical axis.
Which kind of motion on the x axis?
Which kind of motion on the y axis?
Try to write the equations of motion for each axis.
And from it, up to you...

Homework Helper
uhh? forget the final velocity …

nobody asked for it!! m_scott
what equation should i use to find motion for the x and y axis?

Karlx
What kind of motion do you think that has the ball on the y direction ?

Homework Helper
well, you're only given s and vi and a (a = 0 for the x direction, = -g for the y direction), and you want to find t for both directions and then eliminate it …

so which of the standard constant acceleration equations do you think you should use? m_scott
s=v0t+(1/2)at2??

And if so, do i plug in -9.8 for a and 0 for v0?

i got 1.6s Is that for the x or y axis?

Homework Helper
s=v0t+(1/2)at2??

yes And if so, do i plug in … 0 for v0?

no, v0 will be the initial component of velocity in the y direction

m_scott
Ok. How do I find the initial component of velocity in the y direction? Btw, sorry for all the newb questions. This is my 3rd day in physics and im kinda lost...

Homework Helper
total velocity times sin3°

(and the initial x component is total velocity times cos3°)

haven't you done components? m_scott
But the problem doesnt give me the total velocity

so call that v, and solve 