1. A spring exerts a force of 12 Newton's when it is stretched 2 meters beyond its natural length. What is the work done if this spring is stretched from its natural length to this point?

2. A crane is hauling up an object weighing 20 Newton's. The top of the crane is 10 meters above the ground. The cable weighs 2 Newton's/meter. How much work is done in hauling the object from the ground up to a height of 10 meters?

3. If two oppositely charged electric particles are separated by a distance of x meters, the force of attraction between them is given by: F = kq₁q₂/x²

where k, q₁ and q₂ are constants. If we start with a distance of a meters between the charges, calculate the work done if we move the charges apart until they are a distance of 2a meters apart.

4. The pressure P of a gas is related to its volume V by the formula P = k/v where k is a constant. A quantity of gas with an initial volume of 1 cubic meter and a pressure of 2.5 Newton's per square meter expands to a volume of 16 cubic meters. Find the work done by the gas.