Finding initial speed given two angles and a distance

  • #1

Homework Statement



A child runs down a 11 degree hill and then suddenly jumps upward at a 16degree angle above the horizontal and lands 1.2 m down the hill as measured along the hill

What was the child's initial speed?

Homework Equations



Projectile motion

x = Vx*t

y = yo + Vyo*t - 1/2gt^2

The Attempt at a Solution



tried solving for time t from the vertical motion equation y = yo+ Vyo*t - 1/2gt^2 and plug it in the
x = vx*t..but it doesnt work
 
Last edited:

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
8
Hi heyhowsitgoin
Welcome to PF.
If you draw the figure of inclined plane and the landing of the child, you can see that
y = 1.2*sin11.
t = x/vx = 1.2*cos11/vo*cos16
vyo = vo*sin16.
Substitute these values in the equation and solve for vo.
 
  • #3
Hi thanks for the reply. Which equation are you referring to solve for vo?
 
  • #4
rl.bhat
Homework Helper
4,433
8
Hi thanks for the reply. Which equation are you referring to solve for vo?

y = yo+ Vyo*t - 1/2gt^2
 
  • #5
hrmm ok

so i end up with 0.229 = 0 + Vo*sin16 (1.2*cos11 / Vo*cos16) - 1/2(9.8)(1.2*cos11/Vo*cos16)^2

is this right? not sure how to solve for Vo at this point.
 
  • #6
rl.bhat
Homework Helper
4,433
8
It is right. Now simplify everything. And solve for Vo.
 
  • #7
Hello,

I was looking at this question and was not able to isolate Vo in the final equation. I ended up with a negative number under a square root...
Any suggestions on how to correctly isolate Vo?
 
  • #8
rl.bhat
Homework Helper
4,433
8
hrmm ok

so i end up with 0.229 = 0 + Vo*sin16 (1.2*cos11 / Vo*cos16) - 1/2(9.8)(1.2*cos11/Vo*cos16)^2

is this right? not sure how to solve for Vo at this point.
In the equation final position y is zero, and initial position yo is 0.229
 

Related Threads on Finding initial speed given two angles and a distance

Replies
12
Views
10K
Replies
1
Views
10K
Replies
3
Views
50K
  • Last Post
Replies
11
Views
2K
Replies
3
Views
6K
Replies
8
Views
8K
Replies
3
Views
10K
Replies
11
Views
33K
Top