2.5 Thermodynamics and Contact Forces

2.5 Thermodynamics and Contact Forces

Contact Forces Overview

Contact forces arise from the physical interaction between two objects and result from interatomic electric forces. These include:

• Tension: Forces exerted through ropes, strings, or chains.

• Friction: Resistance between surfaces in relative motion.

• Normal Force: Perpendicular reaction force exerted by a surface.

• Spring Force: Forces from elastic materials compressed or stretched.

• Buoyancy: Upward force exerted by a fluid on a submerged or floating object.

Key Concepts from Physics 1

Contact Forces

Contact forces occur when objects are in direct interaction:

• Tension: Points along the direction of the rope, string, or chain.

• Friction:

  • Static friction opposes the initiation of motion.

  • Kinetic friction opposes ongoing relative motion.

• Normal Force: Always acts perpendicular to the contact surface.

• Spring Force: Acts in the opposite direction of compression or extension.

Hooke’s Law

The stretching or compression of a spring is directly proportional to the applied force:

Where:

• : Force exerted by the spring (N).

• : Spring constant (N/m).

• : Displacement from the spring’s equilibrium length (m).

Friction

Friction resists relative motion between two surfaces. The force of friction is calculated as:

Where:

• : Force of friction.

• : Coefficient of friction (static or kinetic).

• : Normal force.

Indicators of Friction:

• “Rough” surfaces or explicit mention of .

• Static friction prevents motion; kinetic friction resists ongoing motion.

Buoyancy

Buoyancy is the upward contact force exerted by a fluid. According to Archimedes’ Principle:

Where:

• : Buoyant force (N).

• : Fluid density (kg/m).

• : Volume of fluid displaced (m).

• : Acceleration due to gravity (m/s).

An object will:

• Sink if its density is greater than the fluid.

• Float if its density is less than the fluid.

Conservative vs. Non-Conservative Forces

• Conservative Forces: Store energy in a recoverable form (e.g., tension, normal force).

• Non-Conservative Forces: Dissipate energy (e.g., friction, air resistance).

Practice Problems

Example Problem #1: Friction

Problem: A 20 kg box sits on a horizontal surface. A 60 N horizontal force is applied. Calculate the force of friction acting on the box.

Solution:

Assuming static friction prevents motion, the friction force matches the applied force:

Example Problem #2: Tension

Problem: A 10 kg pulley is held by a rope with 200 N of tension. Calculate the weight of the pulley.

Solution: Weight:

Example Problem #3: Buoyancy

Problem: A wooden block (density = 600 kg/m, volume = 0.02 m) is placed in water (density = 1000 kg/m). Calculate the buoyant force.

Solution:

Summary

Understanding contact forces, their origins, and their calculations is essential for analyzing thermodynamic systems. Use these foundational principles and problem-solving techniques to master this section and excel in AP Physics 2.