Kinematics Question?

[moomoo345]

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Hey all, I have another question again lol. This time it's physics.

The question from the Kaplan book asks if a bullet is shot into the air with given initial velocity (Vi) and the gravitational acceleration (g), what additional info is needed to determine the horizontal distance covered by the bullet at the instant it reaches its max height.

I wrote out some kinematic equations like: D = Vit + 1/2at^2 and Vf^2 = Vi^2 +2aD. With this, I assumed that I needed to know what t equals so I could figure out what D is.

The solution says that the problem can be solved as is without any additional info. I've been racking my brain on this, and the only conclusion I can think of is that at max height, Vf in the y axis would be 0 since its following a parabolic function. So then, with Vf known, D could be solved with the Vf^2 = Vi^2 +2aD equation? Is this right?

Sorry for all these threads, I'm taking the OAT this Friday so I'm trying to get all the help I can get!

 

[Physio101]

yes, your thinking is correct. since the problem is 2D it is easiest to break it up into the x and y component. using the vf^2 = vi^2 + 2aDy (where vi = 0 and a = g) you can find the bullet's max height. after you have found the max height you can use the following equation to determine how much time it took the bullet to get there (Dy = [vi + vf] x t). after finding the time it takes the bullet to reach its max height you only have one mroe step left, and that is to find the distance travelled by the bullet horizontally. since there is no horizontal accelaration you can use Dx = Vx x t

hope this helps!! good luck with the OAT!! 🙂

 

[Arctic]

The key to this problem (which you already understand, I think) is that velocity is a vector quantity (it contains the directional information). If the problem said you were given the speed of the bullet and g, you could not find the horizontal distance with that info alone. You'd need something else (such as the angle of firing) in order to complete the problem. But since velocity does have a directional component to it, you don't need any additional info.

 

[kiwi5frog]

The key to this problem (which you already understand, I think) is that velocity is a vector quantity (it contains the directional information). If the problem said you were given the speed of the bullet and g, you could not find the horizontal distance with that info alone. You'd need something else (such as the angle of firing) in order to complete the problem. But since velocity does have a directional component to it, you don't need any additional info.

correct.

When I saw this problem when I was using Kaplan, it also caused me some confusion. I consulted the kaplan physics teacher and he said the same thing. Velocity information includes the angle.