Force Types

Why This Matters

Forces are the foundation of everything in physics—they explain why objects move, stop, change direction, or stay perfectly still. When you study force types, you're really learning about Newton's laws in action, and that's exactly what you'll be tested on. Every force problem on your exam comes down to identifying which forces are present, understanding their directions, and applying the right equations to predict motion.

The key concepts you need to master include contact vs. non-contact forces, action-reaction pairs, equilibrium conditions, and force equations. Don't just memorize that friction opposes motion—know why it matters for calculating net force. Don't just recall that gravity pulls things down—understand how it relates to weight, orbits, and free fall. You're being tested on your ability to analyze force diagrams, compare how different forces behave, and apply the correct mathematical relationships.

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Contact Forces: When Objects Touch

Contact forces only act when two objects are physically touching. These forces arise from electromagnetic interactions at the atomic level between surfaces in contact.

Friction Force

• Opposes relative motion between surfaces—static friction prevents motion from starting, kinetic friction acts during sliding
• Depends on surface type and normal force—calculated as $f = \mu N$, where $\mu$ is the coefficient of friction
• Essential for traction—without friction, walking, driving, and braking would be impossible

Normal Force

• Acts perpendicular to the contact surface—always pushes away from the surface, never parallel to it
• Equals weight only on flat surfaces—on inclined planes, $N = mg\cos\theta$, which is less than the object's full weight
• Determines friction magnitude—since $f = \mu N$, changing the normal force directly changes friction

Applied Force

• Any push or pull from an external source—you, a machine, or another object providing the force
• Directly relates to acceleration through Newton's second law: $F = ma$
• Can act in any direction—unlike gravity or normal force, applied forces have no fixed orientation

Tension Force

• Transmitted through ropes, strings, or cables—always pulls, never pushes
• Acts along the length of the connector—tension direction follows the rope's path toward the pulling source
• Constant throughout an ideal rope—in massless rope problems, tension is the same at both ends

Spring Force

• Follows Hooke's Law: $F = -kx$, where $k$ is the spring constant and $x$ is displacement from equilibrium
• Restoring force—the negative sign indicates the force always opposes displacement, pushing back toward equilibrium
• Proportional to stretch or compression—double the displacement, double the force (within elastic limits)

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Non-Contact Forces: Action at a Distance

Non-contact forces act between objects that aren't touching. These forces are mediated by fields—gravitational, electric, or magnetic—that extend through space.

Gravitational Force

• Attracts any two masses—always attractive, never repulsive, and acts along the line connecting the centers
• Governed by Newton's universal gravitation: $F = G\frac{m_1 m_2}{r^2}$, where $G = 6.67 \times 10^{-11} \text{ N·m}^2/\text{kg}^2$
• Creates weight near Earth's surface—simplifies to $F = mg$ when one mass is a planet and distance is approximately constant

Electrostatic Force

• Acts between charged objects—like charges repel, opposite charges attract
• Follows Coulomb's Law: $F = k\frac{q_1 q_2}{r^2}$, where $k = 8.99 \times 10^9 \text{ N·m}^2/\text{C}^2$
• Much stronger than gravity—at atomic scales, electrostatic force dominates and governs chemical bonding and atomic structure

Magnetic Force

• Acts on moving charges or magnetic materials—stationary charges don't experience magnetic force
• Depends on field strength and particle velocity—for a moving charge: $F = qvB\sin\theta$
• Powers modern technology—electric motors, generators, MRI machines, and data storage all rely on magnetic forces

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Quick Reference Table

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Self-Check Questions

1. Which two forces follow an inverse-square relationship with distance, and what key difference determines whether they attract or repel?

2. An object rests on a 30° inclined plane. How does the normal force compare to the object's weight, and why does this affect the friction force?

3. Compare tension force and spring force: both transmit force through a physical connector, but how does each force's magnitude behave differently along that connector?

4. A charged particle moves through a region with both electric and magnetic fields. Which force acts on the particle if it's stationary? Which acts if it's moving? Explain why.

5. FRQ-style: A box is pushed across a rough floor at constant velocity. Identify all forces acting on the box, explain why the net force must be zero, and describe what would happen to the friction force if the applied force increased.