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

flyparnape
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

Homework Helper
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The fact that the projectile was in the air for ##4s## must tell you something.

Mentor
I don't know the distance
What does "horizontal length" mean?

Hint: Consider the vertical component of motion.

flyparnape
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

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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?

flyparnape
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

Mentor
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?

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The horizontal length comes from the position of the rocket before launch
Yes, the POSITION of the rocket, not the length of the rocket.

Mentor
Also, gravity is obviously 9.8m/s^2
(fixed your units for you) Homework Helper
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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
What aspect of the launch parameters determines the time in the air (for a flight that starts and finishes at the same height)?

flyparnape
horizontal length, and gravity

Homework Helper
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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.

Mentor
Please post the full problem statement. That way we'll know exactly what you're given and what you're asked to find.

Staff Emeritus
Homework Helper
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?

flyparnape
Here's is a picture of the object

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• Screen Shot 2021-04-07 at 1.21.47 PM.png
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Mentor
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.

Gold Member
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.

• Doc Al
Homework Helper
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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.

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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?

Mentor
If your objective is just to calculate the initial velocity, you have all the information you need. (And some that you don't!)

flyparnape
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