5.6 Common Forces - University Physics Volume 1 | OpenStax

Learning Objectives

• Define normal and tension forces
• Distinguish between real and fictitious forces
• Apply Newton's laws of motion to solve problems involving a variety of forces

Introduction to Forces

• Forces are often described as push, pull, thrust, and weight.
• Forces are grouped into categories relating to their source, transmission, or effects.

Catalog of Forces

Normal Force

• Normal Force: Perpendicular force exerted by a surface to support the weight of an object resting on it.
• Acts upwards to counteract gravity.
• Illustrated by objects on horizontal surfaces or inclines.
• Formula (horizontal surface): ( N = mg )
• On an incline, normal force is less than the object's weight.

Example: Weight on an Incline

• Skier's mass: 60.0 kg
• Acceleration calculations without and with friction (45.0 N)
• On a frictionless incline: ( a_x = g \sin(\theta) )
• With friction: reduced acceleration due to opposing force

Tension

• Tension: Force exerted along the length of a medium like a rope or cable.
• Acts parallel to the connector and is a pulling force.
• Example: A person holding a mass on a rope, tension equals the mass's weight.

Example: Tightrope Tension

• Tightrope walker's weight causes wire tension.
• Wire tension calculated using trigonometry.

Other Forces

Friction

• Opposes motion or its tendency.
• Acts parallel to surfaces in contact.
• To be discussed in detail in the next chapter.

Spring Force

• Spring Force: Restoring force proportional to displacement.
• Described by Hooke's Law: ( F = -kx )
• Acts in the opposite direction to displacement.

Real Forces vs. Fictitious Forces

• Real forces have physical origins (e.g., gravity).
• Fictitious forces arise in non-inertial frames (e.g., rotational effects).
• Example: Coriolis effect observed in rotating frames like Earth.
• Inertial frames: All forces are real and align with Newton's laws.

Conclusion

• Discussed forces are real but not exhaustive.
• Some forces are more fundamental or manifestations of the same force.
• More on forces and physics will be covered in later sections.