A 5 kg block rests on an inclined ramp. A student's free-body diagram includes four vectors: gravity, the normal force, friction, and the block's acceleration down the ramp. What is wrong with this diagram?
AThe normal force should point straight up, not perpendicular to the ramp
BAcceleration is not a force and should not appear in the free-body diagram
CFriction should not be included because the block is at rest
DGravity should be split into components before drawing the diagram
A free-body diagram shows only real forces — acceleration is the result of the net force, not a force itself. Including 'ma' as a vector in the FBD is one of the most common errors in dynamics. Friction is correctly included (static friction keeps a resting block from sliding), and gravity appears as a single downward vector (components are extracted during the algebra, not on the FBD itself).
Question 2 True / False
The normal force on an object generally points straight up, opposing gravity.
TTrue
FFalse
Answer: False
The normal force is always perpendicular to the contact surface, not necessarily straight up. On a horizontal floor it does point straight up, but on an inclined ramp it points perpendicular to the ramp surface — at an angle to the vertical. Confusing 'perpendicular to surface' with 'opposite to gravity' leads to incorrect component calculations in inclined-plane problems.
Question 3 Short Answer
Why should a free-body diagram be drawn before writing any equations in a dynamics problem?
Think about your answer, then reveal below.
Model answer: A free-body diagram makes all forces and their directions explicit, ensuring that Newton's second law (ΣF = ma) is applied with the correct signs and components in each direction — preventing errors from omitting forces or misidentifying directions.
The FBD is a systematic accounting tool: it forces you to identify every force before writing ΣF = ma. Without it, students frequently forget forces (especially the normal force on inclined planes) or assign wrong signs to components. Drawing the FBD first separates the physical analysis from the algebraic calculation, reducing both types of error.