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Work done

  • #1

Homework Statement


A 70kg mass is in a space and moving at 2.5 m/s. You push it with 200N for 4 m. You then move to the front of the mass and push it backwards with 170N for 3 m.

What was the total work done?
What was the final velocity?


Homework Equations


W = rest energy + F1*r - F2*r


The Attempt at a Solution


rest energy = 1/2*70*(2.5)2 = 218.75
F1*r = 200*4 = 800
F2*r = 170*3 = 510
W = 218.75 + 800 - 510 = 508.75 J

How would i get the final velocity?
 

Answers and Replies

  • #2
LowlyPion
Homework Helper
3,090
4
How would i get the final velocity?
Initial KE + Work = Final KE

Initial ½mv² + W = Final ½mv²
 
  • #3
so its 218.75 + 800 + 510?
 
  • #4
LowlyPion
Homework Helper
3,090
4
so its 218.75 + 800 + 510?
Perhaps I should rephrase that since you want to plug in numbers literally without thinking about the signs:

Initial ½mv² + ΔW = Final ½mv²

Btw your initial calculation incorrectly includes the initial KE in the ΔW they asked you to find.
 

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