How to find initial velocity given distance and angle

  • Thread starter tsukuba
  • Start date
  • #1
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Homework Statement


A projectile is fired with initial velocity Vo at an angle of 20° with the horizontal. Determine the required value of Vo if the projectile is to hit point b.
point b is 3 km away.


Homework Equations


y=(Vy)o t + 1/2 a t^2


The Attempt at a Solution


i tried isolating time(t) from the above formula which gave me square root of 2y/a but i cant take the square root of a negative number that bring the acceleration
 

Answers and Replies

  • #2
326
3
Some hints: Draw the diagram and then use the formula. What do you know about the x-distance and y-distance?
 
  • #3
47
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I have to multiply it by cos20 and sin 20
 
  • #4
20,844
4,543
Hi Tsukuba. Welcome to Physics Forums.

What is the value of y when the projectile reaches the ground again? What is the acceleration "a" in your equation for y? Can you use your equation for y to figure out how much time the projectile is in the air?

Chet
 
  • #5
47
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hello and thank you.

a=-9.81 m/s^2
I can use the equation because i would have 2 unknowns, that being the initial velocity and time. Like i said I tried isolating time and the formula i get is square root of 2x/a and i cant take the square root of a negative number.
 
  • #6
20,844
4,543
hello and thank you.

a=-9.81 m/s^2
I can use the equation because i would have 2 unknowns, that being the initial velocity and time. Like i said I tried isolating time and the formula i get is square root of 2x/a and i cant take the square root of a negative number.
As I see it, your equation for the y direction is going to be:
[tex]0=v_0\sin(20)t-\frac{9.8}{2}t^2[/tex]
Is this in agreement with your assessment? If so, you can solve this equation for t by factoring the right hand side, and discarding the root at t = 0. In terms of v0, what do you get for t?

Chet
 
  • #7
47
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instead of the 0 wouldn't i have the distance of 3km?
 
  • #8
47
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here is the question and the diagram
mec.png
 
  • #9
20,844
4,543
instead of the 0 wouldn't i have the distance of 3km?
We are talking about the y direction here, not the x direction. When the projectile hits the ground, y = 0.
 

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