Answer:
12 J
Explanation:
The amount of energy stored in a spring stretched by a distance x is:
E = 1/2 k x², where k is the stiffness of the spring.
So the spring has 4J of energy when it is stretched 10 cm:
4 J = 1/2 k (0.10 m)²
k = 800 N/m
When the stretched to 20 cm, the amount of energy is:
E = 1/2 (800 N/m) (0.20 m)²
E = 16 J
So it takes an additional 12 J to stretch the spring an additional 10 cm.
