Net Force Calculation Using Velocity Graph

[aligass2004]

Homework Statement

http://i241.photobucket.com/albums/ff4/alg5045/Ex5-07.gif

That figure shows the velocity graph of a 5.5 kg object as it moves along the x-axis. What is the net force acting on this object at the given times? a.) 3s b.) 4s and c.) 7s

Homework Equations

F=ma

The Attempt at a Solution

For 3 seconds, I used F= m (change in velocity/change in time), and I got 22N, which was the right answer.

For 4 seconds, I got zero.

I am stuck on how to find the 7 second one.

 

[learningphysics]

What is the acceleration at 7s?

 

[aligass2004]

By looking at the graph, I assumed the velocity was 6m/s. I got the acceleration to be .857 m/s^2. Then I did the same thing that I did for part a, and I got 4.714, but that was wrong.

 

[learningphysics]

By looking at the graph, I assumed the velocity was 6m/s. I got the acceleration to be .857 m/s^2. Then I did the same thing that I did for part a, and I got 4.714, but that was wrong.

What is the slope of the v-t graph at t = 7s? The slope is the same all the way from 6 to 8s.

 

[aligass2004]

Yeah, about that slope business. I haven't had a math class using slope for well over three years. I don't remember. I just remember that it's rise over run

 

[learningphysics]

Yeah, about that slope business. I haven't had a math class using slope for well over three years. I don't remember. I just remember that it's rise over run

Yeah, you can do the first part like that... slope = rise/run = change in velocity/change in time.

Just do it the same way you did the first part... the acceleration is the same all the way from 6s to 8s... what is the change in velocity from 6s to 8s... what is the change in time?

 

[aligass2004]

The change in velocity is 12 m/s and the change in time is 2 s. So the acceleration is 6m/s^2. Then the force would be 33. I tried that, but it was wrong.

 

[learningphysics]

The change in velocity is 12 m/s and the change in time is 2 s. So the acceleration is 6m/s^2. Then the force would be 33. I tried that, but it was wrong.

The change is -12m/s.

 

[aligass2004]

Ohhhhh...thank you for pointing that out.