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2.2.1Force and Translational Dynamics

Free Body Diagrams — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 36 1 quiz

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Lesson summary

2.2.1 Free Body Diagrams — AP PHYSICS 1 notes: Draw and use free-body diagrams with coordinate axes, including on inclined planes.Purpose — identify forces in each plane and find the resultant forceArrows — one arrow per force on the object, starting at its center of massScale — …

  • Suspended object
  • Object on a rough surface
  • Accelerating car
  • Weight components
  • Normal force
  • Without friction

Section 1 of 8Free-body diagrams

Purpose
  • Purpose identify forces in each plane and find the resultant force
Arrows
  • Arrows one arrow per force on the object, starting at its center of mass
Scale
  • Scale length proportional to magnitude and direction shows the line of action
Labels
  • Labels name, symbol or description of the interaction, e.g. gravitational pull of Earth on the ball

Section 2 of 8Examples

Examples
Suspended object
  • Suspended object tension upward and weight downward
Object on a rough surface
  • Object on a rough surface applied force, friction opposing, weight and normal force
Accelerating car
  • Accelerating car thrust greater than friction, with weight equal to the normal force

Section 3 of 8Coordinate systems

Coordinate systems
Structure
  • Structure two perpendicular axes with a positive and negative direction
Choice
  • Choice put one axis parallel to the acceleration
Angles
  • Angles can be measured from either axis

Section 4 of 8Inclined planes

Inclined planes
Axes
  • Axes parallel and perpendicular to the slope
Weight components
  • Fg cos θ perpendicular and Fg sin θ parallel (down the slope)
Normal force
  • Normal force perpendicular to the slope; FN = Fg cos θ when no motion perpendicular to the slope
Without friction
  • Fg sin θ accelerates the object down the slope

Section 5 of 8Checklist

1
Include all forces on the object.
2
Draw each as a vector from the center of mass.
3
Make arrow lengths proportional to magnitudes.
4
Draw equal-length arrows for balanced directions, such as weight and upthrust on a boat at rest.

Section 6 of 8More from the specification

More from the specification
Frictional contact forces and terminal speed
  • Frictional contact forces and terminal speed microscopic collisions between contacting surface atoms generate opposing frictional drag as speed increases.
Constant velocity
  • Zero acceleration.
Equilibrium
  • State with zero net force.
Electron
  • Negatively charged particle in an atom.
Stationary
  • Not moving.
Constant
  • Not changing.
Translational equilibrium
  • Net force zero.
Coordinate system
  • Perpendicular axes used to resolve forces.
Reaction force
  • Force from a support pushing up on an object.
Force triangle
  • Closed tip-to-tail vector triangle of balanced forces.

Section 7 of 8Key terms

TermMeaning
Free-body diagramVector diagram of forces on an object.
Coordinate systemPerpendicular axes used to resolve forces.
Inclined planeFlat surface tilted at an angle.
WeightFg = mg, the gravitational force.

Section 8 of 8Exam technique

Avoid these mistakes
Leaving out friction or normal force.
Drawing the weight perpendicular to the slope.
Using forces exerted by the object itself.

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