When the load shown in the figure is hung from the spring, it stretches. The spring is an elastic material. a) What is meant by an elastic material? b) The elastic property of a material is explained by Hooke's law. What does the letter x stands for?

The graph shows the velocity decreasing from $20 \text{ m/s}$ at $t=0 \text{ s}$ to $0 \text{ m/s}$ at $t=4 \text{ s}$.

$$\text{Acceleration } (a) = \frac{\text{Change in velocity}}{\text{Change in time}}$$ $$a = \frac{v_f - v_i}{t_f - t_i}$$ $$a = \frac{0 \text{ m/s} - 20 \text{ m/s}}{4 \text{ s} - 0 \text{ s}}$$ $$a = \frac{-20 \text{ m/s}}{4 \text{ s}}$$ $$a = -5 \text{ m/s}^2$$

The acceleration is $\boxed{-5 \text{ m/s}^2}$.

The area under the velocity-time graph is a triangle.

$$\text{Distance} = \text{Area of triangle} = \frac{1}{2} \times \text{base} \times \text{height}$$ $$\text{Distance} = \frac{1}{2} \times 4 \text{ s} \times 20 \text{ m/s}$$ $$\text{Distance} = 40 \text{ m}$$

The distance covered is $\boxed{40 \text{ m}}$.

2. When the load shown in the figure is hung from the spring, it stretches. The spring is an elastic material.

a) What is meant by an elastic material?
An elastic material is a material that can return to its original shape and size after the deforming force or stress has been removed.

b) The elastic property of a material is explained by Hooke's law. What does the letter x stands for?
In Hooke's Law, which is often expressed as $F = kx$, the letter $x$ stands for the extension or compression of the spring (or material) from its equilibrium position.