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.
