# Homework Help: Force and distance/velocity/acceleration

1. Oct 30, 2008

### jammhawk

1. The problem statement, all variables and given/known data
A 2kg fan-cart is in linear motion (straight line on a very long track); it experiences 4N forward force due to the fan. Find the following assuming it is at rest initially:

a. The acceleration:
b. Its velocity 3 seconds after it starts
c. Its velocity 5 seconds after it starts
d. Its displacement between 3s and 5s
e. Its displacement between 0s and 5s
f. Its average velocity using the result from 4 for displacement

2. Relevant equations

3. The attempt at a solution

I know it sounds like a lot and I need A LOT of help. Our professor explained nothing and just handed this to us and said go. We don't even have a text book, so help is really really needed. I am no where near being smart in physics.

2. Oct 30, 2008

### jammhawk

Attempt
a.

a = F/m
a = 4N/2kg
a=4kg*m/ss/2kg

a=2m/s/s

Is that right?

3. Oct 30, 2008

### risk2112

acceleration = net force / mass
acceleration = (final velocity - initial velocity) / timespan

displacement = (final V + initial V) \ 2) x timespan

With displacement, your initial velocity for "Its displacement between 3s and 5s" will not be 0.

*also acceleration is 2 m / s^2, and you need that to plug into all the equations i listed, and interpret what the questions r asking you to do

4. Oct 30, 2008

### jammhawk

Can someone help me with a velocity formula? I keep looking for formulas but I can't really understand it.

5. Oct 30, 2008

### tiny-tim

Welcome to PF!

Hi jammhawk! Welcome to PF!
Yes, that's fine.

In words, that means that the acceleration is constant.

You know some formulas for constant acceleration, don't you?

6. Oct 30, 2008

### jammhawk

I have no clue what I am doing! I'm confused about all of this.

Ok.

I got
b. 6 m/s
c. 10 m/s
but i have no clue how i got it. and its probably wrong

7. Oct 30, 2008

### tiny-tim

Hi jammhawk!

Always show us how you got it (or how you think you got it)!

Hint: what formulas do you know that involve constant acceleration?

8. Oct 30, 2008

### jammhawk

Basically in my head i said.
Aceleration = 2 m/s/s
Velocity = x m/s
so maybe V= at

9. Oct 30, 2008

### jammhawk

And if those are true...

displacement = .5(a)(t^2) + vinitial(t) + dinitial

D = .5(2)(2^2) + 6(2) + 0
D = 4 + 12 + 0
D = 16

d. 16?

10. Oct 30, 2008

### tiny-tim

No maybe about it!

General formula v = u + at (and in this case u = 0).
Also fine … excpet that you can scrub "+ dinitial" form the first line (though personally, I'd use vinitial = 0, and do two equations).

11. Oct 30, 2008

### jammhawk

ok so all answers are good so far.

now how do i use the displacement of answer d (16) to find the average velocity?

12. Oct 30, 2008

### tiny-tim

Definitions:

Speed = instantaneous distance-per-time

Average speed = total distance per total time.

13. Oct 30, 2008

### jammhawk

so displacement = 16 meters
total time = 2 seconds

average velocity = 16 m/2s
8 m/s

right?

14. Oct 30, 2008

### tiny-tim

Yup!

btw:
but your professor did tell you all the equations you've just used, didn't he?

Moral: physics is equations … just learn the equations, and practise choosing the right one to use.

15. Oct 30, 2008

### jammhawk

actually i found all these online through yahoo. our professor gives us an assignment just sets us free. i wish he was more hands on

16. Oct 30, 2008

### tiny-tim

ah … then I sympathise

web research is not a reliable way of learning physics.

17. Oct 30, 2008

### jammhawk

i know... and I would like to thank you for the best help available!