An object is in equilibrium under the action of many common point forces, such as 5N, 7n, 9N, 12n, 20n, etc. now the 9N force is reversed for balance How much force and direction should be added?

18N
In order to restore balance, first eliminate the reversed 9N force, add a 9N force opposite to the current 9N force, and then restore the 9N force in the original direction
In this way, the resultant force of two forces of 9N in common direction applied to restore equilibrium is 18N

If four forces act on the same object and their sizes are 6N, 7n, 8N and 9N respectively, the resultant force may be ()
A. 1NB. 10NC. 19ND. 35N

The resultant force range of 6N and 7n is greater than or equal to 1n, less than or equal to 13N, and the resultant force range of 8N and 9N is greater than 1n, less than or equal to 17N. It is known that the resultant force of the four forces may be zero. Then the minimum value of the resultant force of the four forces is 0n. When the four forces are in the same direction, the maximum value of the resultant force is 30n, so the resultant force range of the four forces is 0n ≤ f ≤ 30n

The magnitude of three forces acting on an object is 7n, 8N and 9N respectively, and the resultant force may be ()
A. 0B. 7NC. 15ND. 25N

According to the theme, when three forces act on an object in the same direction, the resultant force is the largest, the maximum is Fmax = 7n + 8N + 9N = 24N; the resultant force of 7n and 8N ranges from 1n to 15N, then 9N can be balanced with the resultant force of 7n and 8N, so the resultant force of three forces can be zero, so the resultant force of three forces ranges from 0 ≤ f ≤ 24N, so ABC is correct, D is wrong ：ABC．

An object is in equilibrium under the action of many common point forces, such as 5N, 7n, 9N, 12n, 20n, etc. now the 9N force is reversed for balance How much force and direction should be added?