Projectile Motion Question (physics 101)

[cheese89]

Homework Statement

A projectile is shot from the edge of a cliff 125 m above ground level with an initial speed of 65.0 m/s at an angle of 37 degrrees with the horizontal. Find the horizontal and vertical components of its velocity at the instant just before the projectile hits point P (the ground).

Homework Equations

Vxo = Vo cos 37
Vyo = Vo sin 37
Vy = Vyo - gt ...?

The Attempt at a Solution

The answer is given in the back of the book. Plugging in the initial velocity in the first equation gives the correct answer (which I expected), but the answer I achieve for Vyo should be negative and I haven't yet acheived that. I tried to plug the answer I got for Vyo (the positive one) into the equation Vy = Vyo - gt, but still did not get the right answer. I'm truly grasping at straws here.

 

[edziura]

You can use Vy = Vyo - gt to find the vertical component, but must first find the time at which the projectile reaches P. To do this, you use the equation for the y-coordinate as a function of time.

 

[tiny-tim]

Vxo = Vo cos 37
Vyo = Vo sin 37
Vy = Vyo - gt ...?
…
Plugging in the initial velocity in the first equation gives the correct answer (which I expected), but the answer I achieve for Vyo should be negative and I haven't yet acheived that. I tried to plug the answer I got for Vyo (the positive one) into the equation Vy = Vyo - gt, but still did not get the right answer. I'm truly grasping at straws here.

Hi cheese89!

I don't understand how you got any result for V_y from Vy = Vyo - gt …

you don't know what t is.

Hint: you haven't used y = 125 yet, have you?

 

[ritwik06]

Homework Statement

A projectile is shot from the edge of a cliff 125 m above ground level with an initial speed of 65.0 m/s at an angle of 37 degrrees with the horizontal. Find the horizontal and vertical components of its velocity at the instant just before the projectile hits point P (the ground).

Homework Equations

Vxo = Vo cos 37
Vyo = Vo sin 37
Vy = Vyo - gt ...?

The Attempt at a Solution

The answer is given in the back of the book. Plugging in the initial velocity in the first equation gives the correct answer (which I expected), but the answer I achieve for Vyo should be negative and I haven't yet acheived that. I tried to plug the answer I got for Vyo (the positive one) into the equation Vy = Vyo - gt, but still did not get the right answer. I'm truly grasping at straws here.

Its an oral question dear. Just think will the boy acclerate in x direction. If it does, vertical component will change otherwise not. For the y direction, whatever you throw come down with same speed.

 

[jmckennon]

For this problem you should check your constant acceleration formulas. The Acceleration in this case is a constant -9.8m/s^2(gravity). And there is no acceleration in the x direction. The velocity would be v=Vo+at. where v= final velocity, Vo= initial velocity, a=acceleration, and t= time. Look at the information given to you in the problem and consult some of the other formulas. You have a displacement of 125m, a Vo, the acceleration, the angle. all you need is the time it took to get to the ground and you will have your answer =)

 

[tiny-tim]

whatever you throw come down with same speed.

Hi ritwik06!

That only applies when we throw from the ground … this is from a 125 m cliff.

 

[kmikias]

you have to find time first using the formula Y = vt + 1/2 g t^2

 

[ritwik06]

Hi ritwik06!

That only applies when we throw from the ground … this is from a 125 m cliff.

I know. I thought cheese was working for that interval as well.