# Homework Help: Velocity of a Projectile

1. Feb 3, 2013

### mgerman63016

I would like to better understand how to work Velocity of a Projectile object. I have homework due tonight but I'm going to turn it in late to have a better understanding.

Question: A projectile is launched with an initial velocity of 39.3 m/s at 56.2° above the horizontal. Calculate the magnitude and direction of its velocity at each of the following times.

a.) 2.05 seconds after launch.
b.) 5.10 seconds after launch.
c.) Calculate the magnitude of it's velocity at 5.10 seconds
d.) What is the direction of the projectile's velocity vector 5.10 seconds after launch?

I've tried to google videos and tried to get help from the internet and nothing was coming out right. I still got everything wrong.

some guidance would be greatly appreciated PLEASE.

2. Feb 3, 2013

### tms

The first step is to write down the relevant equations.

3. Feb 3, 2013

### FeynmanIsCool

Last edited by a moderator: Sep 25, 2014
4. Feb 3, 2013

### mgerman63016

relevant equations like:

the vertical (y) picture for this setup
which I came up with

Y =
Voy = 39.3 m/s
a = -9.81 m/s2
t =
Y =
Vy =

But that's all I got.

5. Feb 3, 2013

### tms

All you've done is write down one of the initial conditions and the acceleration due to gravity. What equations describe motion in a line? There are several equations that relate position, velocity, acceleration, and time.

6. Feb 3, 2013

### mgerman63016

I'm sorry I don't know I'm thinking the equations we could use for the problem would be

Vy=Vox+at
y-y0=1/2(Vy+Voy)t
2a(y-y0) = Vy2-Voy2
y-y0=Voyt+1/2at2
x-x0=Voxt

these are the equations I'm suppose to use

7. Feb 3, 2013

### FeynmanIsCool

pssst.. maybe displacement X=Vot(cosθ) displacement Y= Vot(sinθ)-1/2(gt2)

8. Feb 3, 2013

### mgerman63016

I've never seen those before.

9. Feb 3, 2013

### haruspex

No, that's mixing vertical and horizontal velocities.
No, |Vo| = 39.3 m/s. Given the launch angle, how do you determine Voy and Vox ?
Once you know Voy, which equation will you use to find Vy at time t?

10. Feb 3, 2013

### voko

You need equations that

1. Relate velocity components (horizontal and vertical) with the magnitude of velocity (speed) and its direction (angle to the ground), and vice versa.

2. Describe how the vertical velocity component is affected by acceleration due to gravity.

11. Feb 3, 2013

### FeynmanIsCool

Remember, your question has an angle involved, but none of the formulas that you stated included θ as a variable....

12. Feb 3, 2013

### FeynmanIsCool

also remember, horizontal (movement along x) and vertical (movement along y) do not effect each other in ANY way. So, they must be calculate separately. In order to find direction at time (t) you must calculate x and y movement independent of each other

13. Feb 3, 2013

### mgerman63016

to determine Voy and Vox and since I have a angle could I use the cos sin and tan and the pythagorean theorem to determine the distance traveled?

14. Feb 4, 2013

### haruspex

You use sin/cos/Pythagoras to obtain Voy and Vox. Thereafter, treat the vertical and horizontal motions separately to find later velocities and or distances. In this question, you'll need to recombine the velocities later.

15. Feb 4, 2013

### mgerman63016

Vy= 39.3sin(56.2)
Vy= 32.66 m/s

Vx=39.3 m/s cos(56.2)
Vx=21.86

this is what I got so far. I'm trying to work on the Pythagoras theorem right now. But I'm not sure what I'm suppose to use

16. Feb 4, 2013

### haruspex

Those are the initial vertical and horizontal velocities. Now you need to use your kinematic equations to find the velocities at the give times.

17. Feb 4, 2013