Which bis the diameter of one sphere?

From the diagram, the four spheres span from the $0 \text{ cm}$ mark to the $8 \text{ cm}$ mark on the rule.

Since there are four identical spheres, the diameter of one sphere is the total length divided by the number of spheres.

The diameter of one sphere is $\boxed{\text{2.0 cm}}$.

From the graph, the speed increases linearly from $0 \text{ m/s}$ at $t=0 \text{ s}$ to $20 \text{ m/s}$ at $t=40 \text{ s}$.

The distance traveled is represented by the area under the speed-time graph. In this interval, the shape is a triangle.

The base of the triangle is the time interval: $40 \text{ s} - 0 \text{ s} = 40 \text{ s}$.
The height of the triangle is the maximum speed reached: $20 \text{ m/s}$.

The distance traveled while the speed is increasing is $\boxed{\text{400 m}}$.

Let's assume forces to the right are positive and forces to the left are negative.
Forces acting:

• $6 \text{ N}$ to the left ($-6 \text{ N}$)
• $2 \text{ N}$ to the right ($+2 \text{ N}$)
• $5 \text{ N}$ to the right ($+5 \text{ N}$)

The net force is $1 \text{ N}$ to the right.

Since the net force is to the right, the acceleration and the direction of movement will also be to the right.
The acceleration is $\boxed{\text{5.0 m/s}^2}$ and the direction of movement is $\boxed{\text{Right}}$.