# Calculating work done

1. Aug 27, 2014

### cyberjupiter

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

W = mgh

3. 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?

2. Aug 27, 2014

### Staff: Mentor

The formula for work involves the dot product of two vectors.

W = F . s

Does this give you a hint?

Last edited: Aug 27, 2014
3. Aug 27, 2014

### HallsofIvy

Staff Emeritus
In order to do work, you must move an object with mass in the direction of a force vector.

4. Aug 27, 2014

### cyberjupiter

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. Aug 27, 2014

### Staff: Mentor

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. Aug 27, 2014

### cyberjupiter

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. Aug 27, 2014

### Staff: Mentor

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°?