• Support PF! Buy your school textbooks, materials and every day products Here!

Calculating work done

  • #1

Homework Statement



A carpenter is holding two tool boxes of mass 19kg and 21kg respectively. Calculate the work done to carry the tool boxes through a horizontal distance of 16m.

Homework Equations



W = mgh

The Attempt at a Solution



m = 21kg + 19kg
g = 9.81ms^-2
h = 0

W = (21 + 19)(9.81)(0)
W = 0J

Is this correct? And if this is correct, would you explain why it is 0J?
 

Answers and Replies

  • #2
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,072
The formula for work involves the dot product of two vectors.

W = F . s

Does this give you a hint?

(Yes, your answer of 0J is correct.)
 
Last edited:
  • #3
HallsofIvy
Science Advisor
Homework Helper
41,833
955
In order to do work, you must move an object with mass in the direction of a force vector.
 
  • #4
So if I use W = F . s, then it would be

W = (19 + 21)(9.81)(16) ?
Or is it,
W = (19 + 21)(9.81)(16) cos 90 ?
 
  • #5
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,072
So if I use W = F . s, then it would be

W = (19 + 21)(9.81)(16) ?
Or is it,
W = (19 + 21)(9.81)(16) cos 90 ?
this ^^^

Instead of writing the vector formula, it is often expressed as W = F s cosθ
and that's probably the best one to memorise.
 
  • #6
NascentOxygen, would you tell me why the angle is 90? Shouldnt it be 0? And how about if I use sin 0? I will still get 0J.
 
  • #7
NascentOxygen
Staff Emeritus
Science Advisor
9,244
1,072
NascentOxygen, would you tell me why the angle is 90? Shouldnt it be 0? And how about if I use sin 0? I will still get 0J.
The question just tests how easily confused you are.

The only force mentioned is weight, and it acts downwards ⇓
while the motion described is horizontal ⇒
and the angle between those two vector quantities is ..... ?

You want to invent your own formula using sin 0° ? What angle in this picture is 0°?
 

Related Threads on Calculating work done

  • Last Post
Replies
3
Views
926
  • Last Post
Replies
11
Views
7K
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
17
Views
2K
  • Last Post
Replies
2
Views
5K
  • Last Post
Replies
2
Views
4K
  • Last Post
Replies
4
Views
3K
  • Last Post
Replies
1
Views
426
  • Last Post
2
Replies
25
Views
694
Replies
2
Views
7K
Top