How to calculate the Work (Joules) of lifting a book

[Alyssa Eiger]

1. I'm trying to calculate work it takes to lift 4 books, and stack them on top of each other in a 20 cm high stack. The problem reports:
- 5 books are lying on the ground. They are each: 2kg, 4cm thick.

Using the formula for Potential Energy (mgh), my calculations reflect it takes 8J:
- Book 1 is already on the ground.
- Book 2 on Book 1: (2kg)*(10m/s^2)*(.04m) = 8 J
- Book 3 on Book 2: 1.6 J
- Book 4 on Book 3: 2.4 J
- Book 5 on Book 4: 3.2 J

Thus, I get 8J - however the answer key to the problem reports it's 80J. Am I missing a critical component?

Any input is appreciated.

 

[Alyssa Eiger]

UPDATE: this should read: "- Book 2 on Book 1: (2kg)*(10m/s^2)*(.04m) = .8 J"

 

[gneill]

Hi Alyssa Eiger,

Welcome to Physics Forums!

In future, please retain formatting template in your post. Don't delete or write over it. Thanks.

Your work looks good and the result fine. Perhaps the answer key has a typo and should read "8.0 J".

 

[Alyssa Eiger]

Hi Alyssa Eiger,

Welcome to Physics Forums!

In future, please retain formatting template in your post. Don't delete or write over it. Thanks.

Your work looks good and the result fine. Perhaps the answer key has a typo and should read "8.0 J".

Thank you GNeill. I will be sure to keep formatting in the future.
AE

 

[Dr.D]

I have not checked your arithmetic, but one potential difficulty is in the problem statement itself. It uses mixed units. Centimeters are not SI units; this data needs to be converted to meters to have any prospect of a correct result. This is a flaw in the way the problem is posed, not in the OP solution.