Table of Contents
What Is Normal Force?
The normal force is the contact force exerted by a surface perpendicular to the surface on an object resting on it. It is a reaction force that prevents objects from passing through surfaces. On a horizontal surface with no other vertical forces, the normal force equals the object's weight. On inclined surfaces or when additional forces are applied, the normal force changes accordingly.
The normal force is essential for calculating friction (f = μN), determining whether an object will slide on an inclined plane, and analyzing contact mechanics. It appears in virtually every mechanics problem involving surfaces and is a direct consequence of Newton's third law.
Formulas
Inclined Planes
| Angle | Normal Force (% of weight) |
|---|---|
| 0° (flat) | 100% |
| 15° | 96.6% |
| 30° | 86.6% |
| 45° | 70.7% |
| 60° | 50% |
| 90° (vertical) | 0% |
Frequently Asked Questions
Can the normal force be greater than weight?
Yes. If you push down on an object on a surface, the normal force increases to balance both the weight and the applied downward force. In an elevator accelerating upward, the normal force on you exceeds your weight, which is why you feel heavier.
Can the normal force be zero?
Yes. The normal force is zero when the object loses contact with the surface, such as at the top of a loop-the-loop if speed is high enough, or in free-fall (weightlessness). The normal force cannot be negative since surfaces can only push, not pull.