Section 2.4: Normal Forces
This section explains the normal force, the perpendicular contact force exerted by a surface on an object. Normal forces are critical for understanding friction and motion on surfaces.
Key Points:
- Normal force (F_N) acts perpendicular to the surface of contact.
- On a horizontal surface with no vertical acceleration: F_N = mg.
- On an inclined plane: F_N = mg cosθ.
- Normal force adjusts to balance perpendicular forces; it does not always equal weight.
- Normal forces are used to calculate friction: f_s ≤ μ_s F_N, f_k = μ_k F_N.
Example 1
A 12 kg box rests on a horizontal floor. Find the normal force.
F_N = mg = 12 * 9.8 = 117.6 N
Example 2
A 10 kg box rests on a 30° incline. Find the normal force.
F_N = mg cosθ = 10*9.8*cos30° ≈ 84.9 N
Practice Problems
- Block of 15 kg on horizontal table. Find normal force.
- Box of 8 kg on 25° incline. Compute normal force.
- Block of 5 kg in elevator accelerating upward at 2 m/s². Find normal force.
- Block of 12 kg on 30° incline with friction. Find normal force to calculate friction.
- Object of 20 kg on a horizontal surface, additional downward force of 50 N applied. Find normal force.