Finding the Velocity of a Projectile knowing only Launch Angle and Flight Time

AI Thread Summary
To find the velocity of a projectile given only the launch angle and flight time, the discussion emphasizes the importance of understanding the vertical component of motion and the role of gravity. The time of flight, which is 4 seconds, is crucial for determining the initial vertical velocity, while the horizontal length of the object is deemed irrelevant. Participants express confusion over the term "horizontal length," clarifying it should refer to the distance traveled horizontally rather than the object's physical length. The conversation highlights that the launch angle and initial speed are the key parameters that influence the projectile's trajectory. Ultimately, the information provided is sufficient to calculate the initial velocity of the projectile.
flyparnape
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Homework Statement
Quick question, how would I find the velocity of Projectile Motion without knowing the total distance
Relevant Equations
I'm given the object mass: 13.874g, Angle of Launch: 40˚, the horizontal length of 60cm and the time it lands on the ground: 4s. Also, gravity is obviously 9.8m/s
I don't know the distance or the horizontal velocity so I can't find any logical solution
 
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The fact that the projectile was in the air for ##4s## must tell you something.
 
flyparnape said:
I don't know the distance
What does "horizontal length" mean?

Hint: Consider the vertical component of motion.
 
So horizontal length is the length of the object when placed at the angle of launch. Am I supposed to find the distance from the ground
 
flyparnape said:
So horizontal length is the length of the object when placed at the angle of launch. Am I supposed to find the distance from the ground
Where did the horizontal length come from? Is that mentioned in the problem statement?
 
PeroK said:
Where did the horizontal length come from? Is that mentioned in the problem statement?
The horizontal length comes from the position of the rocket before launch
 
flyparnape said:
The horizontal length comes from the position of the rocket before launch
Not sure how that is relevant. Could you please post the full problem statement so we can see what's being asked for in context?
 
flyparnape said:
The horizontal length comes from the position of the rocket before launch
Yes, the POSITION of the rocket, not the length of the rocket.
 
flyparnape said:
Also, gravity is obviously 9.8m/s^2
(fixed your units for you) :smile:
 
  • #10
flyparnape said:
Homework Statement:: Quick question, how would I find the velocity of Projectile Motion without knowing the total distance
Relevant Equations:: I'm given the object mass: 13.874g, Angle of Launch: 40˚, the horizontal length of 60cm and the time it lands on the ground: 4s. Also, gravity is obviously 9.8m/s

I don't know the distance or the horizontal velocity so I can't find any logical solution
What aspect of the launch parameters determines the time in the air (for a flight that starts and finishes at the same height)?
 
  • #11
horizontal length, and gravity
 
  • #12
flyparnape said:
horizontal length, and gravity
I don't think that's the answer we were expecting. Normally, in projectile motion problems the horizontal length of an object is not relevant. In any case, the length of an object tells you nothing about launch speed and angle.

There a hint there about the correct answer.
 
  • #13
Please post the full problem statement. That way we'll know exactly what you're given and what you're asked to find.
 
  • #14
flyparnape said:
So horizontal length is the length of the object when placed at the angle of launch.
I sense a language issue. By "horizontal length", do you mean how far the projectile moves horizontally between its launch and when it lands?
 
  • #15
Here's is a picture of the object
 

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  • #16
Interesting. Now (as previously advised) see if you can use the given information (that 4s, for instance) to figure out the vertical component of the initial velocity.
 
  • #17
So, not actually a rocket. That takes care of one pointless mystery. Another two or three pages and we might actually figure out what the homework problem is.
 
  • Haha
Likes Doc Al
  • #18
If you are trying to determine the initial velocity of the projectile from the time of flight, then you should shoot it straight up in the air, i.e. make the "known angle" 90o.
 
  • #19
flyparnape said:
horizontal length, and gravity
Neither of those are "launch parameters".
When you set up to launch an object on some trajectory, what aspects do you control?
 
  • #20
If your objective is just to calculate the initial velocity, you have all the information you need. (And some that you don't!)
 
  • #21
haruspex said:
Neither of those are "launch parameters".
When you set up to launch an object on some trajectory, what aspects do you control?
You control the angle of launch
 
  • #22
flyparnape said:
You control the angle of launch
And the speed.
Or to look at it another way, the parameters are the horizontal speed and the vertical speed.
Of those four, is there just one that determines the time in the air, assuming the landing height is the same as the launch height?
 
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