Free-Body Diagrams & Net Force — Drills · Grade 12 Science
Chapter: Forces, Rotation & EquilibriumWhat these drills covers
By the end of Free-Body Diagrams & Net Force you should be able to:
- I can draw a free-body diagram that shows every force acting on a single object as a labeled vector.
- I can compute the net force from several forces, including forces at right angles, and use ΣF = ma to find the resulting acceleration.
- I can recognize when an object is in equilibrium because its forces sum to zero.
6 questions, numeric, multiple choice and written answer, each with a worked explanation.
Vocabulary used: free-body diagram, net force (ΣF), equilibrium, normal force, vector.
Problem 1
Answer and explanation
35 N
Net force = 55 − 20 = 35 N to the right.
Problem 2
Answer and explanation
13 N
Perpendicular forces combine as √(5² + 12²) = √(25 + 144) = √169 = 13 N.
Problem 3
- only the tension, upward
- weight up and tension down
- tension up and weight down, equal in size
- only its weight, downward
Answer and explanation
tension up and weight down, equal in size
A hanging lamp at rest is in equilibrium: the cord's tension (up) exactly balances the weight (down).
Problem 4
- accelerating
- moving in a circle
- at rest and nothing else
- at rest or moving at constant velocity
Answer and explanation
at rest or moving at constant velocity
Zero net force means zero acceleration, so the object either stays at rest or keeps moving at constant velocity.
Problem 5
Answer and explanation
Weight straight down; the normal force from the snow straight up; the rope's tension pointing up and forward along the rope; and kinetic friction pointing backward, opposing the sled's motion. The tension can be split into a forward component (which moves the sled) and an upward component (which slightly reduces the normal force).
Problem 6
Answer and explanation
4 m/s²
Net force = √(24² + 10²) = √(576 + 100) = √676 = 26 N. Then a = ΣF ÷ m = 26 ÷ 6.5 = 4 m/s².