# Determining Net Force Based on Motion Diagram

## Homework Statement

It's webbased, so I just took a screenshot: http://img27.yfrog.com/img27/2237/pyquestion.jpg [Broken]

F = ma

## The Attempt at a Solution

Well, the way I figured it was that we could determine the force by calculating the average velocity between two time periods and subtracting to see how much it changed. Doing this I came up with that the acceleration is -75 cm/s. Converting to m/s and plugging in to F = ma (along with the 4 kg) I got an answer that is apparently wrong. I tried some other ways as well to no avail.

Help!

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berkeman
Mentor

## Homework Statement

It's webbased, so I just took a screenshot: http://img27.yfrog.com/img27/2237/pyquestion.jpg [Broken]

F = ma

## The Attempt at a Solution

Well, the way I figured it was that we could determine the force by calculating the average velocity between two time periods and subtracting to see how much it changed. Doing this I came up with that the acceleration is -75 cm/s. Converting to m/s and plugging in to F = ma (along with the 4 kg) I got an answer that is apparently wrong. I tried some other ways as well to no avail.

Help!
They give you position as a function of time, so you need to calculate v(t) and a(t) from that. Can you post what you calculated for each of the snapshot positions shown? Is the a(t) really constant (it may be, but it's not obvious to me from looking).

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They give you position as a function of time, so you need to calculate v(t) and a(t) from that. Can you post what you calculated for each of the snapshot positions shown? Is the a(t) really constant (it may be, but it's not obvious to me from looking).
I'm not actually sure what you mean. By v(t) do you mean velocity * time? If that's the case, I'm not sure how to find velocity other than by subtracting the position at two points and dividing by the difference in time at those two points.

For that, I got (9-0)/(2-0) for the first two points.

berkeman
Mentor
Subtracting and dividing is exactly what I mean. Now do it for the other points, and list the v(t) and delta t values. Then use those to calculate the acceleration value(s).

Subtracting and dividing is exactly what I mean. Now do it for the other points, and list the v(t) and delta t values. Then use those to calculate the acceleration value(s).
(16-9)/2

(21-16)/2

(24-21)/2

(25-24)/2

Taking two of the values to determine acceleration:

[(1/2)-(3/2)]/(2) = -0.5 units/sec

Since 1 unit = 75 cm, this comes to -37.5 cm/s^2. Converting to m/s we get 0.375 m/s.

Plugging in to F = ma along with 4 kg we get a value of -150 N.

This value is incorrect.

Bump

I know I asked late, but this is due in 30 minutes, so if anyone could help that'd be wonderful!

berkeman
Mentor
Bump

I know I asked late, but this is due in 30 minutes, so if anyone could help that'd be wonderful!
Bumps after one hour are not allowed. Sounds like you should have worked on your homework a bit earlier. I'll try to help tomorrow, but that sounds like it will be too late for your assignment.