• Support PF! Buy your school textbooks, materials and every day products Here!

Kinematics-acceleration in two dimensions

  • Thread starter PEZenfuego
  • Start date
  • #1
48
0

Homework Statement



A tennis player standing 8.0 meters from the net hits a ball 1.5 meters above the ground toward her opponent. The ball leaves her racquet with a speed of 25.0m/s at an angle of 14.0 degrees above the horizontal. The net is 1.0 meters high. The baseline is 12 meters back from the net; a ball striking the ground beyond the baseline is out of play

Homework Equations


x=V.costheta(t)
y=V.sintheta(t)-0.5g(t^2)


The Attempt at a Solution



I attempted to solve for t using the quadratic equation in conjunction with the above formula for y. I ended up with two solutions, neither was extraneous from what I could tell. I plugged both in for x and ended up with two answers. One was slightly more than 8 (meaning that the ball landed in), the other was a little over 21 (meaning that the ball landed out). I am lost, which is it and how do I know?
 

Answers and Replies

  • #2
gneill
Mentor
20,793
2,773
You need to include the initial height of the ball (where its launched) when you work with the vertical component of the position.
 
  • #3
48
0
You need to include the initial height of the ball (where its launched) when you work with the vertical component of the position.
Right, and I did. I got the equation deltaY=V.sintheta(t)-0.5gt^2

Rearranged and solve the quadratic equation. I ended up with two answers. One was 0.89 and the other was 0.3437. How am I supposed to determine which is correct and which is not?
 
  • #4
gneill
Mentor
20,793
2,773
Right, and I did. I got the equation deltaY=V.sintheta(t)-0.5gt^2

Rearranged and solve the quadratic equation. I ended up with two answers. One was 0.89 and the other was 0.3437. How am I supposed to determine which is correct and which is not?
I'll bet you've set your Δy to a positive value, right :wink: But doesn't it FALL from its initial position to hit the ground?
 
  • #5
48
0
:facepalm: I appreciate the help. I remember running into this same problem in high school. Maybe after learning from the same mistake twice, I won't make it again. Fingers crossed.
 

Related Threads on Kinematics-acceleration in two dimensions

Replies
4
Views
3K
  • Last Post
Replies
7
Views
591
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
4
Views
15K
  • Last Post
Replies
1
Views
4K
  • Last Post
Replies
1
Views
807
  • Last Post
Replies
5
Views
2K
  • Last Post
2
Replies
33
Views
2K
  • Last Post
Replies
6
Views
2K
Top