## Projectile Motion Kinematics

GloryCob
Posts: 4
Joined: Sun Sep 09, 2018 8:39 pm

### Projectile Motion Kinematics

The problem statement is

In laboratory situations, a projectile’s range can be used to determine its speed. To see how this is done, suppose a ball rolls off a horizontal table and lands 1.2 m out from the edge of the table.
If the tabletop is 75cm above the floor, determine the time the ball is in the air.

I drew a picture. TheViy is 0. The Vix and Vfx are unknown. The acceleration in the x-direction is zero. In the y-direction its -9.8m/s^2. The Δy is 75cm and the Δx is 1.2m. The t is unknown. Am I supposed to find t from the Δy equation? I tried but got negative square roots.

jeff
Posts: 38
Joined: Mon Mar 05, 2018 11:54 pm

### Re: Projectile Motion Kinematics

Hi GloryCob,
GloryCob wrote:
Mon Sep 10, 2018 11:15 am
The problem statement is

In laboratory situations, a projectile’s range can be used to determine its speed. To see how this is done, suppose a ball rolls off a horizontal table and lands 1.2 m out from the edge of the table.
If the tabletop is 75cm above the floor, determine the time the ball is in the air.

I drew a picture. TheViy is 0. The Vix and Vfx are unknown. The acceleration in the x-direction is zero. In the y-direction its -9.8m/s^2. The Δy is 75cm and the Δx is 1.2m. The t is unknown. Am I supposed to find t from the Δy equation? I tried but got negative square roots.
Yes, that's the right approach. With this problem, you need to find the time first by using the vertical motion. It looks like what is going wrong is that you said that delta y was 75 cm, but since the vertical motion (displacement) is downwards, that needs to be -75 cm (since you are saying that downwards is a negative direction).

Once you change that in the main equation:

$\large&space;\Delta&space;y&space;=&space;v_{iy}t&space;+\frac{1}{2}a_yt^2$

it should give you the correct answer for t.

Does that make it work out correctly? If not, please show me what you're doing and we can figure it out.

GloryCob
Posts: 4
Joined: Sun Sep 09, 2018 8:39 pm

### Re: Projectile Motion Kinematics

that was the problem. I got the right answer. thank you

GloryCob
Posts: 4
Joined: Sun Sep 09, 2018 8:39 pm

### Re: Projectile Motion Kinematics

But can't I call down positive if I want to? why do I have to make it negative here?

GloryCob
Posts: 4
Joined: Sun Sep 09, 2018 8:39 pm

### Re: Projectile Motion Kinematics

nm, if I call down positive than the acceleration is positive so there is still no negative square root.