# Homework Help: Basic Motion Graphs (velocity-time)

1. Sep 24, 2009

### PolyFX

Momentum & Net Force Question

Okay, so I got the previous question I asked and I had another new question to ask so rather than make a new topic and seem like I'm spamming the forums, I just decided to edit my original post. I just hope no one replies to my old post while I'm writing this one!

1. The problem statement, all variables and given/known data
A ping-pong ball whose mass is 0.0027 kg (2.7 g) is acted upon by the Earth and the air (there is air resistance, and there is also a strong wind). This table gives the position of the ball at several times:
Time Position

12.35 s < 3.17, 2.54, -9.38 > m
12.39 s < 3.25, 2.50, -9.40 > m
... ...
14.35 s < 10.17, -1.16, -10.40 > m
14.39 s < 10.37, -1.27, -10.42 > m

In the time interval from 12.35 s to 14.35 s, what was the average net force acting on the ball?

2. Relevant equations
Fnet = P/t where p is momentum and t is time interval
Fnet = -mg

3. The attempt at a solution

First I get the displacement by subracting the final position vector - initial position vector. I then divide that vector by the time interval (2 seconds) to get the average velocity. I use the formula p = mv to find the momentum.

I rearrange the formula p = fnet( t ) to isolate for fnet
so fnet = p/t.

However, with this method I do not get the right answer.

Last edited: Sep 24, 2009
2. Sep 24, 2009

### PolyFX

Okay, so I got the previous question I asked and I had another new question to ask so rather than make a new topic and seem like I'm spamming the forums, I just decided to edit my original post. I just hope no one replies to my old post while I'm writing this one!