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**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.

- 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.

- 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.