Force is an interaction between two objects that influences one of them. Vector quantity: magnitude and direction. SI unit: newton (N), equivalent to kg \cdot m/s^2.
Four fundamental forces
All forces trace to one of four fundamental interactions:
- Gravity — between all masses (weakest but longest-range)
- Electromagnetism — between electric charges
- Strong nuclear — binds protons and neutrons in nuclei
- Weak nuclear — radioactive decay
In everyday physics we deal with the first two. Friction, normal force, rope tension, spring force — all rooted in electromagnetism (between atoms).
Contact vs non-contact forces
- Contact: normal (ground on foot), friction, rope tension, fluid pressure
- Non-contact (field): gravity, electromagnetism
Note: quantum mechanically there is no fundamental distinction — all forces mediate through particle exchange. But for classical physics, the split is useful.
Net force
If multiple forces act on an object, the net force is the vector sum:
\[ \vec F_\text{net} = \sum_i \vec F_i \]
Second law (§5.3): \vec F_\text{net} = m\vec a. Only the net matters — a thousand forces that cancel are collectively invisible.
Typical force magnitudes
| Force | Magnitude |
|---|---|
| Moon's tidal pull (per unit area at coast) | ~10⁻⁵ N/m² |
| Light wind pressure | ~1 N/m² |
| Human weight | ~700 N |
| Car engine | ~5000 N |
| Shuttle rocket at launch | ~30 MN (30 million) |
A few notes
1. Force is an interaction between two objects. No object has force by itself. Always two parties. Foundation of Newton's third law.
2. Force is a vector.
Direction matters. 10 N right and 10 N left cancel.
3. Force relates to acceleration, not velocity. A car cruising at constant speed on a flat road has zero net force (if friction balances drive). High speed, zero acceleration → zero net force.
What you should be able to do
- Define force as a vector interaction
- Name the four fundamental forces
- Distinguish contact vs non-contact
- Compute net force by vector addition
- Recognize the newton and its relation to
kg \cdot m/s^2
Preview of §5.2
Newton's first law: if the net force is zero, the object has constant velocity (or is at rest). Introduces "inertia".
📚 See also: Halliday Vol 1, Ch 5, §5.1.
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