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2.1.1Force and Translational Dynamics

Describing Systems — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 36 1 quiz

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Lesson summary

2.1.1 Describing Systems — AP PHYSICS 1 notes: Define systems and decide whether they are open or closed and whether they can be treated as single objects.System — a collection of objects analysed together to understand, model or predict their behaviourSurroundings — all objects …

  • Conditions
  • Properties
  • Characteristics
  • Conservation
  • Block only
  • Block-spring

Part 1 of 7Defining a system

Defining a system
System
  • System a collection of objects analysed together to understand, model or predict their behaviour
Surroundings
  • Surroundings all objects not part of the system
Boundary
  • Boundary drawing a circle round the system helps identify external forces

Part 2 of 7Single-object systems

Single-object systems
Conditions
  • Conditions internal properties of the parts do not matter for the overall behaviour
Properties
  • Properties mass is the total mass and all points move in the same direction
Example
  • Example a car under a net external force treated as one object

Part 3 of 7Open systems

Open systems
Characteristics
  • Characteristics a net external force acts and energy or mass is exchanged
Conservation
  • Conservation total mechanical energy and momentum are not conserved
Example
  • Example a mass on a rough surface loses energy as heat through friction

Part 4 of 7Closed systems

Closed systems
Characteristics
  • Characteristics no net external force and no exchange of energy or mass
Conservation
  • Conservation total mechanical energy and momentum are conserved
Example
  • Example a mass projected by a spring up a smooth incline

Part 5 of 7Systems with internal structure

SystemInternal forcesResult
Object-EarthGravitational forcesNo net external force; no acceleration of the system
Two colliding objectsForces between objectsTotal momentum constant
People-rope-cartRope tensionCart does not accelerate
Block-spring-EarthSpring and gravityClosed system

Part 6 of 7Pattern: block on a vertical spring

Block only
  • Block only spring force and weight are external, so the system is open
Block-spring
  • Block-spring weight is external, so open
Block-Earth
  • Block-Earth spring force is external, so open
Block-spring-Earth
  • Block-spring-Earth no external forces, so closed

Part 7 of 7More from the specification

  • Position
    • Location relative to a reference point.

Key terms

TermMeaning
SystemObjects chosen for analysis.
Internal forceForce between objects inside the system.
External forceForce from the surroundings.
Isolated systemSystem with no external interaction.

Avoid these mistakes

Avoid these mistakes
Assuming a system is closed without checking for external forces.
Counting internal forces in the net force on the system.
Failing to define the system boundary.

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