The Metric, and the US Standard systems. :)
Fist you would put them in order and then cross one off from each side so like 4kg and 13kg and the last two that you have you would add the 4kg and the 6kg and divide by 2 to get the aswer of 5kg.
4kg+6kg=10kg
10kg/2kg=5kg
You need to do spring constant x extension
It's acceleration a = 20 / 7 = 2.857 ms^-2.
Using the equation of motion under constant acceleration:
Distance travelled = 1/2 * a * t^2
= 1/2 * 2.857 * 7^2
= 70.00 m (to the nearest hundredth).
Downward force of magnitude 5 N is exerted on the book by the force of <em>gravity</em>. We call that the "weight" of the book.
At the same time, upward force of 5 N is exerted on the book by the table. This one is called the "normal force".
Since the vertical forces on the book are 'balanced' (add up to zero), the book just lays there on the table, and does not accelerate.