Work done by a Variable Force

  • Thread starter mbrmbrg
  • Start date
  • #1
mbrmbrg
493
2
A single force acts on a 7.0 kg particle-like object in such a way that the position of the object as a function of time is given by x = 3.0t - 4.0t^2 + 1.0t^3, with x in meters and t in seconds. Find the work done on the object by the force from t = 0 to t = 3.0 s.


I want to use the equation [tex]W = \int F(x)dx = \int madx[/tex]
To find acceleration, I took the second derivative of the position function:
[tex]x(t) = 3t-4t^2+t^3[/tex]
[tex]v=x'(t) = 3-8t+3t^2[/tex]
[tex]a=x''(t) = -8+6t[/tex]

When I go to plug values into my work integral, I get [tex]W=\int m(6t-8)dx[/tex]
I would then take mass out of the integral, but having t and dx in the integral is evil. Do I say that dx=x'(t) and so substitute my velocity function (with dt tacked onto the end) for dx?
 
Last edited:

Answers and Replies

  • #2
quasar987
Science Advisor
Homework Helper
Gold Member
4,793
21
"Yes". The change of variable theorem states that,

[tex]\int_{x(t=0)}^{x(t=3)}F(x)dx = \int_0^3F(x(t))\dot{x}(t)dt[/tex]

But as you cleverly noted, F(x(t))=ma(t).
 
  • #3
PhanthomJay
Science Advisor
Homework Helper
Gold Member
7,179
513
mbrmbrg said:
A single force acts on a 7.0 kg particle-like object in such a way that the position of the object as a function of time is given by x = 3.0t - 4.0t^2 + 1.0t^3, with x in meters and t in seconds. Find the work done on the object by the force from t = 0 to t = 3.0 s.


I want to use the equation [tex]W = \int F(x)dx = \int madx[/tex]
To find acceleration, I took the second derivative of the position function:
[tex]x(t) = 3t-4t^2+t^3[/tex]
[tex]v=x'(t) = 3-8t+3t^2[/tex]
[tex]a=x''(t) = -8+6t[/tex]

When I go to plug values into my work integral, I get [tex]W=\int m(6t-8)dx[/tex]
I would then take mass out of the integral, but having t and dx in the integral is evil. Do I say that dx=x'(t) and so substitute my velocity function (with dt tacked onto the end) for dx?
That looks like it will take away the evil and give you the correct solution!l
 
  • #4
mbrmbrg
493
2
Thank you very kindly!
 

Suggested for: Work done by a Variable Force

  • Last Post
2
Replies
59
Views
670
  • Last Post
Replies
6
Views
427
Replies
8
Views
741
  • Last Post
Replies
7
Views
634
  • Last Post
Replies
7
Views
617
Replies
2
Views
693
Replies
6
Views
310
  • Last Post
Replies
4
Views
453
Top