Types of Forces
Forces are pushes or pulls that can cause an object to accelerate, slow down, remain in place, or change direction. Understanding the different types of forces helps us analyze how objects interact in the physical world.
Fundamental Force Types
Common Types of Forces You Should Know
- Gravitational Force: The attractive force between two masses. It gives objects weight.
- Normal Force: The support force exerted by a surface perpendicular to the object resting on it.
- Frictional Force: The force opposing motion between two surfaces in contact. Comes in static and kinetic forms.
- Tension Force: The pulling force transmitted through a string, rope, cable, or wire when it is pulled tight.
- Applied Force: Any force applied to an object by a person or another object.
- Air Resistance (Drag): A special type of frictional force that acts opposite to motion through air.
- Spring Force: The force exerted by a compressed or stretched spring upon any object attached to it.
Strong Nuclear Force (not covered in AP Phyiscs C but interesting to know)
The Strong Nuclear Force is one of the four fundamental forces of nature. It acts at the subatomic level, holding protons and neutrons together in an atom's nucleus despite protons repelling each other.
Although it operates over a very short range (about the size of an atomic nucleus), it is the strongest force known in nature.
How Forces Interact
Forces can act alone or in combination. The net force on an object determines how it moves according to Newton's Second Law. When multiple forces act on an object, they combine vectorially to produce the net force.
Common Force Formulas in Dynamics
Here are the formulas you need to know for dynamics (don't worry too much about drag force, they will most likely give you the formula on the test)
- Gravitational Force: Fgravity = mg
Weight of an object near Earth's surface. Depends on mass and gravitational field strength. - Normal Force: Varies depending on surface orientation. On a flat surface with no acceleration up or down: Fnormal = mg (if no other vertical forces)
- Friction Force:
Ffriction = μFN
Resists motion between two surfaces. μ is the coefficient of friction, and FN is the normal force. - Drag Force (Linear): Fdrag = -bv
Often used at low speeds. b is a drag coefficient, v is velocity. - Drag Force (Quadratic): Fdrag = -cv²
Dominates at higher speeds. c is a different drag coefficient. - Tension Force: Varies with context, but often just equals the force applied through a taut string, rope, or cable. Often solved with Newton's second law.
- Spring Force (Hooke's law, covered next topic): Fs = -kx
The force provided by a spring is equal to -kx where k is the spring constant and x is the displacement from its equilibrium point. - Net Force: Fnet = ma
Newton's Second Law – the sum of all forces determines the object's acceleration.
Force Comparison Table
| Force | Formula | Variables | When It Applies |
|---|---|---|---|
| Gravitational | F = mg | m: mass, g: 9.8 m/s² | Object near Earth's surface |
| Normal | FN = Fg (flat surface) | m: mass, g: 9.8 m/s² | Support force from a surface |
| Friction | F = μFN | μ: coefficient, N: normal force | Between contacting surfaces |
| Drag (linear) | F = -bv | b: drag constant, v: velocity | Slow speeds in fluid (air/water) |
| Drag (quadratic) | F = -cv² | c: drag constant, v: velocity | High-speed motion in fluids |
| Tension | Depends on net force | Depends on system | Pull through strings/ropes |
| Spring Force | Fs = -kx | k: spring constant, x: displacement from equilibrium | Through springs and usually applies to any elastic object |
| Net Force | F = ma | m: mass, a: net acceleration | Any system with an acceleration |
Real-World Examples
Gravitational Force: The Earth pulls you downward, giving you weight.
Normal Force: A book resting on a table experiences an upward normal force balancing gravity.
Friction: When you slide a box across the floor, friction resists its motion.
Tension: A rope holding a hanging swing experiences tension force.
Air Resistance: A skydiver feels air resistance slowing their fall.
Strong Nuclear Force: Protons and neutrons stick together in the nucleus because of this force.
Interactive Demo: Select a Force Type
Click the buttons below to learn more about each force type.
Fun Force Matching Game
Match each Force Name with its correct Description. Click one from the left, then one from the right. Try to match all!