Forces in Two Dimensions - Mission F2D3 Detailed Help

If an object is at equilibrium, then which of the following MUST be true? List all that apply ...


 
Definition of Equilibrium:
Equilibrium is the condition in which all the individual forces acting upon an object are balanced.


 
Perhaps the most important word in this question is the word MUST. The presence of this word makes all the difference between which answers you choose and which you eliminate. Because of the word MUST, any answer that you pick must be an answer which is always true of an object at equilibrium. If any choice is true sometimes but not under all conditions, then it is not a valid answer to this MUST be true question.


 
If an object is at equilibrium, then the individual forces acting upon the object are balanced. Thus, there is no unbalanced force - the net force is 0 Newton. Given this as the definition, one can apply Newton's second law of motion and confidently state that the object is definitely not accelerating. If not accelerating, the velocity is definitely constant. While we do not know for certain what the velocity value is, we do know that it is definitely a constant, unchanging value. It could be 0 m/s in which case the object is at rest and staying at rest. But the velocity doesn't have to be 0 m/s. It could be 4 m/s, south in which case the object is moving and will continue moving with this speed (4 m/s) and in this direction (south).


 
Some students have the improper habit of thinking that an object at equilibrium is an object upon which all forces are equal. After all, the word equilibrium sounds like the word equal. But don't be fooled! Equilibrium has to do with a balance of forces. While it is possible that all the forces might be equal, they don't have to be equal to be balanced. For instance, an object with an up force of 20 N and a down force of 20 N and a right force of 8 N and a left force of 8 N is at equilibrium. Are all the forces balanced? Yes! Are all the forces equal? No! The 20 N is not equal to the 8 N.