Calculate Initial Speed of Ball Thrown Vertically 14m in 3s

AI Thread Summary
To determine the initial speed of a ball thrown vertically that reaches a height of 14 meters in 3 seconds, the equation d = Vi.t + 1/2 a.t^2 is appropriate, with acceleration due to gravity set at -9.78 m/s². The correct approach involves applying a consistent sign convention, where upward motion is positive and downward motion is negative. After substituting the values into the equation and solving, the initial speed can be calculated. It's crucial to ensure that the signs for acceleration are correctly assigned to avoid errors in the final answer. Using these principles will lead to the correct initial speed of the ball.
patelneel1994
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Homework Statement


A ball thrown vertically from ground level is caught 3.0 s later by a person on a balcony which is 14 m above the ground. Determine the initial speed of the ball.

t = 3.0 s, d = 14 m, Vi = ?

Homework Equations


d = Vi.t + 1/2 a.t^2


The Attempt at a Solution


I have no idea how to get this problem. I don't know if the equation is right or not?


Thank You
 
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You can certainly use that equation. What would you use for the acceleration? (Careful with signs.)
 
How do I find acceleration?
 
patelneel1994 said:
How do I find acceleration?
It's a projectile. You should know the acceleration.
 
Is it 3 ms-2?
 
patelneel1994 said:
Is it 3 ms-2?
No. Where in the world did that come from?
 
I coudn't think of any. I thought time and acceleration some how are related, which are not.
Could we use gravity, which is 9.79 m/s2?
 
patelneel1994 said:
I thought time and acceleration some how are related, which are not.
The acceleration is constant.

Could we use gravity, which is 9.79 m/s2?
Of course you should use the acceleration due to gravity!
 
So if we solve using acceleration, then a = 0 right!
How can I solve using gravity? Is there any different formula?

Thank you
 
  • #10
patelneel1994 said:
So if we solve using acceleration, then a = 0 right!
No. Why would you think that?

How can I solve using gravity? Is there any different formula?
You have the right formula. For the acceleration, use the acceleration due to gravity.
 
  • #11
Sorry a = 9.78 m/s^2
I've tried that didn't ge the right answer.
14 = Vi . 3 s + [4.9 m/s^2 . 9 s^2]
14 - 44.1 / 3 s = Vi
 
  • #12
patelneel1994 said:
Sorry a = 9.78 m/s^2
What's the sign of the acceleration? Which way does gravity act?
 
  • #13
Gravity pull an object down towards the ground. a = 9.78 ms-2!
 
  • #14
patelneel1994 said:
Gravity pull an object down towards the ground. a = 9.78 ms-2!
Use a proper sign convention in your equation. Let 'up' be positive and 'down' be negative.
 
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  • #15
Tha gives me the right answer. So whenever its down - and up +
a = -9.78 ms-2
Thank you
 

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