Projectile Motion: Finding Time

[degrass1fan]

Homework Statement

Ok, so I'm having a lot of trouble finding time when it is not given in a projectile motion problem. Here's what I have set up:

X
A=O
Vi=100.0
VF=100.0
D=?
T=?

Y
A=-9.8
Vi=O
Vf=?
D=8848m
T=?

Homework Equations

Distance(d=1/2at2=vit)

The Attempt at a Solution

Ok, here's where my problem is, I can never get the time right. I have failed 2 tests and numerous homework assigments because something is wrong. Here's my attempt at it:

8848=1/2(-9.8)t2

I divide both sides by 8848 then square root it.


I ALWAYS get the wrong time and in turn that gives me the wrong distance and in turn I fail the test.

 

[Hells_Kitchen]

The general idea to the projectile motion problems is:

1. Find time using the relationship of displacement in the y-direction.
2. FInd displacement in the x-direction x= v*t keeping in mind that in the x direction the velocity will be constant.

Now in this particular case i believe that you only have to worry about the first step.

y = .5 gt^2 => t = sqrt(2y/g)

 

[ogb p]

How do I fix this?

I understand your frustration with finding time in projectile motion problems. It can be a tricky concept to grasp, but with practice and understanding of the equations involved, you can improve your accuracy in solving these types of problems.

First, it is important to note that there are two types of projectile motion problems: those with a known initial velocity (Vi) and those with a known final velocity (Vf). In the first problem you presented, Vi=100.0 m/s, but in the second problem, Vi=0 m/s. This makes a significant difference in how you approach the problem.

In the first problem, you can use the equation d=Vit+1/2at^2 to solve for time (t). Rearranging the equation, you get t=(d-Vit)/1/2a. Plugging in the given values, you get t=(0-100.0t)/1/2(-9.8). Simplifying, you get t=20.41 seconds.

In the second problem, you can use the equation Vf=Vi+at to solve for time (t). Again, rearranging the equation, you get t=(Vf-Vi)/a. Plugging in the given values, you get t=(0-100.0)/-9.8. Simplifying, you get t=10.20 seconds.

It is important to pay attention to the given values and use the appropriate equation to solve for time. In addition, it is always helpful to double check your calculations and make sure your units are consistent.

In summary, to improve your accuracy in finding time in projectile motion problems, make sure you are using the correct equation and double check your calculations. With practice and understanding of the concepts, you will be able to solve these types of problems with confidence.