Question

if f < 210 n, the σf acting on the object is
if f = 210 n, the σf acting on the object is
do 4 problems

Explanation:

Step1: Determine the direction of forces

Assume the right - hand direction is positive. The force of \(300N\) is in the positive direction, and the forces of \(90N\) and \(F\) are in the negative direction.

Step2: Calculate the net force when \(F < 210N\)

The formula for the net force \(\sum\vec{F}\) is \(\sum\vec{F}=300-(90 + F)\).
When \(F < 210N\), \(\sum\vec{F}=300-(90 + F)=210 - F>0\) (since \(F < 210\)), so \(\sum\vec{F}>0\) (net force is in the positive direction).

Step3: Calculate the net force when \(F = 210N\)

Substitute \(F = 210N\) into the formula \(\sum\vec{F}=300-(90 + F)\).
\(\sum\vec{F}=300-(90 + 210)=300 - 300=0\)

Answer:

When \(F < 210N\), \(\sum\vec{F}>0\); when \(F = 210N\), \(\sum\vec{F} = 0\)