Determining Net Force Based on Motion Diagram

In summary, the conversation is about a homework problem that involves calculating force using the equation F = ma. The person discussing their attempt at a solution and receiving help from others to determine the correct values for velocity and acceleration. They also mention a mistake in their final calculation and express concern over submitting the assignment late.
  • #1
Anirak
7
0

Homework Statement



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

Homework Equations



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 into 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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  • #2
Anirak said:

Homework Statement



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

Homework Equations



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 into 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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  • #3
berkeman said:
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.
 
  • #4
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).
 
  • #5
berkeman said:
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 into F = ma along with 4 kg we get a value of -150 N.

This value is incorrect.
 
  • #6
Bump

I know I asked late, but this is due in 30 minutes, so if anyone could help that'd be wonderful!
 
  • #7
Anirak said:
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.
 

1. What is a motion diagram?

A motion diagram is a visual representation of the motion of an object over a period of time. It shows the position of the object at different points in time and the direction and magnitude of its velocity.

2. How can I determine the net force of an object based on a motion diagram?

To determine the net force of an object, you can look at the changes in the object's velocity and acceleration over time on the motion diagram. The net force is equal to the mass of the object multiplied by its acceleration.

3. Can a motion diagram show the direction of the net force?

Yes, a motion diagram can show the direction of the net force by indicating the direction of the object's acceleration. If the object is accelerating in a certain direction, the net force is acting in that same direction.

4. What information do I need to create a motion diagram?

To create a motion diagram, you will need to know the position of the object at different points in time and the direction and magnitude of its velocity. This information can be obtained through experiments or by using equations of motion.

5. Can a motion diagram be used to predict future motion?

Yes, a motion diagram can be used to predict future motion by analyzing the object's velocity and acceleration at different points in time. This can help in understanding how the net force is affecting the object's motion and make predictions about its future path.

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