A box sits on a flat table with no one touching it. How many forces should appear on its free-body diagram?
AOne — gravity only
BTwo — gravity pulling down and the normal force pushing up
CThree — gravity, normal force, and friction
DZero — it is not moving
Gravity pulls the box downward and the table's normal force pushes it upward. Since the box is not being pushed sideways and is not moving, there is no friction force. Two forces, balanced.
Question 2 True / False
In a free-body diagram, the arrow for a stronger force should be drawn longer than the arrow for a weaker force.
TTrue
FFalse
Answer: True
Arrow length in a free-body diagram represents the magnitude (strength) of the force. A longer arrow indicates a greater force.
Question 3 Short Answer
Why should a free-body diagram only include forces acting on the object, not forces the object exerts?
Think about your answer, then reveal below.
Model answer: Because Newton's Second Law (F = ma) uses the net force on the object to determine its acceleration. Forces the object exerts on other things affect those other objects, not this one.
A free-body diagram isolates one object so you can sum the forces on it and apply Newton's Second Law. Including forces it exerts on others would mix up the analysis.