Do not know how to projectile velocity at given time.

[karateman1]

*Do not know how to find projectile velocity at given time!*(Seems easy, please help)

1. Please help I wasn't taught this in class...

I have a diagram of a projectile being fired at 40 Degrees at 30m/s. The path of the projectile is an arc/semi-circle, showing it landed. I used a triangle to split the 30m/s into vertical and horizontal.




2. I also used V2=V1+AT to find hang time of 3.92 seconds and horizontal distance was found to be 90.1 m. How do I find velocity at the given time of 1.2 seconds? And max Height, thank you, I am a noob.

The Attempt at a Solution

 

[karateman1]

Can anyone please give me a hint, at least, please, i think that i supplied enough info.

 

[Dougggggg]

One little error you may want to look at is the way you used the formula to find the hangtime. You solved for time when the velocity (on the y-axis) was equal to zero. So that will give you the time it takes for the object to reach the point when it is no longer going upwards, and it is about to start falling. Try to think about just the vertical for a moment, when your starting y and final y are the same number (perferably zero for easier math) then the object has hit the ground.

You did a good job seperating x and y velocities. Now write down what you know. (starting point for both x and y, ending point for y, intial velocity for both x and y). Post whatever you get in a second.

 

[karateman1]

One little error you may want to look at is the way you used the formula to find the hangtime. You solved for time when the velocity (on the y-axis) was equal to zero. So that will give you the time it takes for the object to reach the point when it is no longer going upwards, and it is about to start falling. Try to think about just the vertical for a moment, when your starting y and final y are the same number (perferably zero for easier math) then the object has hit the ground.

You did a good job seperating x and y velocities. Now write down what you know. (starting point for both x and y, ending point for y, intial velocity for both x and y). Post whatever you get in a second.

Thank you so much for replying, though I don't quite understand my error, the velocity I entered was zero (for it being in the middle of the arc) no vertical velocity, I then found the time at halfway, then multiplied by 2 to get the full hang time. ?
thats what he showed us to do, calculate the time that it takes for the full motion to complete.

 

[Dougggggg]

That will work, I haven't checked the math on your work, but finding the time to the middle of the arc would be a possible way to do it.

Now to go any further, take that list of formula's you have been given and split them all into x and y categories. Don't plug in numbers right away. Then based on what you want to know, you can then decide what values you want to put where.

 

[karateman1]

That will work, I haven't checked the math on your work, but finding the time to the middle of the arc would be a possible way to do it.

Now to go any further, take that list of formula's you have been given and split them all into x and y categories. Don't plug in numbers right away. Then based on what you want to know, you can then decide what values you want to put where.

I want to use the formula D=1/2(V2+V1)T because I don't know acceleration, to find the velocity at 1.2 s what do i do? How do i split this one, and how do i put it back together, man I am confused...

Thanks soo much, I have a lot of homework that i don't know how to finish... Could you please lead me through how you woul do it, and speak to me like i am stupid? thanks

 

[Dougggggg]

Now let's think about projectile motion here for just a moment. There is one beautiful thing about it that makes our lives and the math we do a lot easier. The x and y velocities and acceleration can be completely separated. Now if I throw something through the air (ignoring air resistance), after it leaves my hand, would it accelerate? Nothing else would be causing it to accelerate, gravity is acting down, not left and right. So actually, using the formulas with acceleration in them is easier, because you get to drop certain terms.

I hate to do this, but I am going to have to leave you man. There are many other helpers who can continue to help, plus I will check this in the morning. It is nearly 4 AM here and I wake up to go to the airport at 5:30 and I still haven't packed.

 

[karateman1]

Now let's think about projectile motion here for just a moment. There is one beautiful thing about it that makes our lives and the math we do a lot easier. The x and y velocities and acceleration can be completely separated. Now if I throw something through the air (ignoring air resistance), after it leaves my hand, would it accelerate? Nothing else would be causing it to accelerate, gravity is acting down, not left and right. So actually, using the formulas with acceleration in them is easier, because you get to drop certain terms.

I hate to do this, but I am going to have to leave you man. There are many other helpers who can continue to help, plus I will check this in the morning. It is nearly 4 AM here and I wake up to go to the airport at 5:30 and I still haven't packed.

What an amazing dude, I had no idea there were dedicated, helpful people out there like you, i don't even care about the question anymore...WOW 4am... THANKS

 

[zgozvrm]

I want to use the formula D=1/2(V2+V1)T because I don't know acceleration, to find the velocity at 1.2 s what do i do? How do i split this one, and how do i put it back together, man I am confused...

But you DO know the acceleration!

Think about it...
What is the acceleration in the vertical direction?
What is the acceleration in the horizontal direction?

 

[joshmdmd]

This is an outdated question. It would be simple to separate velocity and then work on vertical velocity at 1.2s. Then you can add the components back together to get the velocity.