Calculating Final Velocity: Need Help!

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Homework Help Overview

The discussion revolves around a projectile motion problem where an object is thrown through a window 3.25 m high, located 8 m away, at an angle of 75 degrees above the horizontal. The initial launch speed is given as 13.3 m/s, and the participants are trying to determine the final speed of the object as it passes through the window.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants are attempting to use kinematic equations to find the time of flight and the final velocity. There is discussion about separating the vertical and horizontal components of motion and how to correctly apply the equations for each component. Questions arise regarding the correctness of the time calculated and the final velocity obtained.

Discussion Status

There is ongoing exploration of the problem with various attempts to calculate the final velocity. Some participants are questioning their calculations and the assumptions made regarding the time of flight and the components of velocity. Guidance has been offered regarding the use of different equations and the treatment of horizontal and vertical components.

Contextual Notes

Participants express confusion about the calculations and the application of kinematic equations. There is a noted discrepancy in the time calculated using different components, which may affect the final results. The discussion reflects a learning process with multiple interpretations being explored.

theoronhalde
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final velocity...need help!

Homework Statement



An object is to be thrown through a window 3.25 m high a distance 8 m away. It is launched at an angle 75 degree above the horizontal.

The initial launch speed at the specified angle which will allow the projectile to pass through the window is 13.3 m/s.

What is the final speed of the object (that is, the speed at which it flies through the window)?
Y= height
Yo= initial height
Voy= initial speed
Vf= final velocity
g= gravity
t= time

Homework Equations


1)Y= Yo+Voy*t-(1/2)gt^2
2)Vf= Vo+at


3. The Attempt at a Solution

By using the y= yo+Voy*t-(1/2)gt^2 formula,
= 3.25+ (13.3sin75)t-(1/2)(-9.81)t^2
I found the time t=2.85 s

But then the answer i got by using the formula Vf= Vo+at
= 13.3+ (-9.81)(2.85)
= -14.7 m/s
IS wrong!What is my mistake??
 
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Welcome to PF!

Hi theoronhalde ! Welcome to PF! :smile:

(try using the X2 and X2 tags just above the Reply box :wink:)
theoronhalde said:
An object is to be thrown through a window 3.25 m high a distance 8 m away. It is launched at an angle 75 degree above the horizontal.

The initial launch speed at the specified angle which will allow the projectile to pass through the window is 13.3 m/s.

What is the final speed of the object (that is, the speed at which it flies through the window)?
…
But then the answer i got by using the formula Vf = Vo+at
= 13.3+ (-9.81)(2.85)
= -14.7 m/s
IS wrong!What is my mistake??

Vfy = Voy + at :wink:
 


hi! thanks a lot!

Not sure i understand your answer? Do i have to use the formula you wrote instead of the one i was using to fnd the final velocty? I'm a bit confuse...
 
Last edited:
Yes … you have to deal with the y-components separately. :smile:

(the x-component of velocity, of course is constant :wink:)
 


ok s it's going to be:

Vfy= Voy+at
= 13.3sin75+ (-9.81)(2.85)
= -15.1 m/s

But how come the horizontal velocity be constant? And am I suppose to use
V=Sqrt((Vox^2)+(Vfy^2)) to find that final speed?
 
The horizontal acceleration is zero, so the horizontal velocity must be constant.

And yes, use Pythagoras. :smile:
 


oh ok...i undestand what you are saying but i still end up with a wrong answer...here is what i did:

Vfx= Vox + at (a=0)
=13.3cos75

Vfy= Voy+at
= 13.3sin75 + (9.81)(2.85)
= -15.1


Vf= SQRT( Vfy^2 + Vfx^2)
= 15.5 m/s

The applet says that the answer is wrong :S ??
 
I haven't checked how you got t using Vy, but when I do it using Vx, I don't get 2.85.
 


oh...i'm so lost once again. I tought we should get the time By using:
y= yo+Voy*t-(1/2)gt^2 formula,
= 3.25+ (13.3sin75)t-(1/2)(-9.81)t^2
If not can you explain me how to find the time and how to get the components Vfy and Vfx.
 

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