Types of Forces in Physics

Why This Matters

Forces are the foundation of everything you'll study in AP Physics 1—they're not just one topic, they're the topic. Every problem involving motion, equilibrium, energy transfer, or momentum ultimately comes back to identifying forces and understanding how they interact. The College Board expects you to draw accurate free-body diagrams, apply Newton's laws, and connect forces to concepts like work, torque, impulse, and conservation laws. If you can't identify and characterize forces correctly, you'll struggle with nearly every unit in the course.

Here's the key insight: forces aren't random categories to memorize. They fall into patterns based on what causes them and how they behave mathematically. Some forces act at a distance (like gravity), others require contact (like friction and normal forces), and some follow specific mathematical laws (like spring force). Don't just memorize "friction opposes motion"—understand why static friction can vary while kinetic friction stays constant, or why tension is the same throughout an ideal rope. That conceptual understanding is what FRQs actually test.

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Contact Forces: When Objects Touch

Contact forces arise from direct physical interaction between surfaces or objects. These forces are essential for analyzing everyday scenarios like blocks on ramps, objects being pushed, and systems connected by ropes. At the microscopic level, contact forces result from electromagnetic interactions between atoms at surfaces.

Normal Force

• Perpendicular to the surface—always acts at 90° to the contact surface, regardless of the surface's orientation
• Adjusts to prevent interpenetration—the normal force is a response force that takes whatever value is needed to keep objects from passing through each other
• Changes on inclines—on a surface tilted at angle $\theta$, the normal force equals $N = mg\cos\theta$, not $mg$

Friction Force

• Opposes relative motion or attempted motion—friction acts parallel to the surface, in the direction that resists sliding
• Two types with different behaviors—static friction ($f_s \leq \mu_s N$) can vary from zero up to a maximum; kinetic friction ($f_k = \mu_k N$) is constant once sliding begins
• Depends on normal force, not surface area—doubling the weight doubles friction; spreading the same weight over more area doesn't change it

Tension Force

• Pulls along the length of the rope or string—tension can only pull, never push, and acts in the direction of the rope
• Equal throughout an ideal (massless) rope—if the rope has negligible mass and no friction at pulleys, tension is the same at every point
• Critical for connected systems—tension transmits forces between objects in pulley problems and Atwood machines

Applied Force

• Any push or pull from an external agent—this is the "catch-all" category for forces applied by hands, engines, or other sources
• Direction and magnitude are specified in the problem—unlike friction or normal force, applied forces don't follow automatic rules
• Often the "known" force in Newton's second law problems—you'll use applied force to find acceleration or to determine other unknown forces

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Field Forces: Action at a Distance

Field forces act without physical contact—objects interact through invisible fields that permeate space. These forces follow mathematical laws that depend on properties like mass or charge and the distance between objects.

Gravitational Force

• Attracts any two masses—governed by Newton's Law of Universal Gravitation: $F_g = \frac{Gm_1m_2}{r^2}$, where $G$ is the universal gravitational constant
• Weight is gravitational force near Earth's surface—we use $F_g = mg$ where $g \approx 10 \text{ m/s}^2$ when the object is close enough that $r$ doesn't change significantly
• Creates apparent weightlessness in free fall—when gravity is the only force acting, there's no normal force, so apparent weight is zero (think astronauts in orbit)

Electrostatic Force

• Acts between charged objects—described by Coulomb's Law: $F_e = \frac{kq_1q_2}{r^2}$, with the same inverse-square structure as gravity
• Can attract or repel—unlike gravity (always attractive), electrostatic force is attractive for opposite charges and repulsive for like charges
• Beyond AP Physics 1 scope for calculations—but understanding the conceptual parallel to gravitational force helps with inverse-square reasoning

Magnetic Force

• Acts on moving charges or magnetic materials—stationary charges don't experience magnetic force; motion is required
• Direction perpendicular to both velocity and field—this makes magnetic force unique and is described by the Lorentz force law
• Beyond AP Physics 1 scope—you won't calculate magnetic forces, but recognizing that they exist helps distinguish them from electrostatic forces

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Restoring Forces: Back to Equilibrium

Restoring forces push or pull objects back toward an equilibrium position. The key feature is that the force magnitude depends on displacement—the farther from equilibrium, the stronger the force pulling back.

Spring Force

• Follows Hooke's Law—$F_s = -kx$, where $k$ is the spring constant and $x$ is displacement from equilibrium; the negative sign indicates the force opposes displacement
• Proportional to displacement—double the stretch, double the force; this linear relationship is what makes springs useful for measuring forces
• Stores elastic potential energy—the work done compressing or stretching a spring becomes $PE_s = \frac{1}{2}kx^2$, which connects to Unit 3 energy concepts

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Resistive Forces: Opposing Motion

Resistive forces always act opposite to an object's velocity, removing kinetic energy from the system. These forces depend on motion itself—no motion means no resistive force.

Air Resistance (Drag)

• Opposes motion through a fluid—acts in the direction opposite to velocity, slowing objects down
• Increases with speed—at low speeds, drag is often proportional to velocity; at high speeds, it's proportional to velocity squared
• Often neglected in AP Physics 1—most problems assume "negligible air resistance," but understanding when it matters (terminal velocity, projectile motion accuracy) shows deeper thinking

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Centripetal Force: A Role, Not a Type

This is a critical conceptual distinction that trips up many students. Centripetal force isn't a new kind of force—it's a label for whatever force(s) point toward the center of a circular path.

Centripetal Force

• Directed toward the center of the circle—any object in circular motion experiences a net force toward the center; this is what "centripetal" means
• Provided by other forces—tension in a string, gravity for orbits, friction on a curved road, or normal force on a loop-the-loop; identify the actual force, not just "centripetal"
• Magnitude given by $F_c = \frac{mv^2}{r}$—this tells you how much center-directed force is required for circular motion at speed $v$ and radius $r$

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Quick Reference Table

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Self-Check Questions

1. Which two forces both act at surfaces but in perpendicular directions, and how would you draw them on a free-body diagram for a block on a ramp?

2. A book sits motionless on a tilted surface. What force prevents it from sliding, and why isn't this force equal to $\mu_s N$ in this case?

3. Compare gravitational force and spring force: which one depends on position, and how does this affect the type of motion each produces?

4. A car rounds a flat curve without slipping. What force provides the centripetal acceleration, and what would happen if this force weren't large enough?

5. An astronaut in the International Space Station appears weightless. Is the gravitational force on the astronaut zero? Explain using the concept of apparent weight and free fall.