# Falling Objects

1. Aug 4, 2005

### mystry4

I have just started a Physics class after having been away from algebra and equations for quite awhile. This is probably a very silly, elemenetary question, but here goes:
With falling objects, I understand that g=10m/s2
and I know the formula that is used, however, I am working with only one number (the height of 300m) to find how long and how fast,and I can't figure out how to plug one number into the formula.
I think that the t=30.0s and v=300m/s and if that is correct I don't know how to do the work to get there.
I know this is easy, but I am missing something.
Can anyone help?
Thank you.

2. Aug 4, 2005

### gravenewworld

Does the equation y=y_0 +v_0t+1/2at^2 ring a bell? You are probably supposed to make some assumptions here. Assume you are starting off with zero initial velocity. Choose a coordinate system for the problem and assign the appropriate values for the initial and final positions. From that find time, after finding time find velocity

3. Aug 4, 2005

### Jelfish

Perhaps it would help if you wrote out the complete question for us.

Generally, to solve these sort of constant acceleration problems, there are five different variables that can come into play. These are acceleration, initial velocity, final velocity, elapsed time, and elapsed distance. If you are learning about kinematics, you should have learned several equations that relate at some of the variables together.

Fill out the following chart with the information given. The first thing you should do is figure out what the question is asking. For example, for "how long?" you are looking for elapsed time. So put a question mark (?) next to the delta t. Then fill in the information that you know. For example, since this is free fall, you would put 10m/s^2 next to "a". Do you know how fast the object was moving when it first starts (v-initial)? Do you know how fast the object was moving at the end (v-final)? Do you know how much distance elapsed (delta x)?

$$a=$$

$$v_i=$$

$$v_f=$$

$$\Delta t=$$

$$\Delta x=$$

Once you've filled in as much information as possible, you can put an X next to the variables to which you have no information. For example, if you're solving for the time but you have no information on how fast it was when it reached the ground, then put an X next to v_f.

Then go through your equations and find the one that has every variable except the one that you put an X through. You should have several equations to choose from, each with one of the five variables missing. Then take that equation and fill in your known information and solve for the one variable.

Good luck!

4. Aug 4, 2005

### mystry4

Thank you! That helped me solve the problem!