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2.2Force and Translational Dynamics

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

5 min read 9 parts Lesson 5 of 36 1 quiz

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Lesson summary

2.2 Forces and Free-Body Diagrams — AP PHYSICS 1 notes: Represent forces with free-body diagrams, describe contact forces and find net force and equilibrium.Purpose — identify forces in each plane and find the resultant forceArrows — one arrow per force on the object, starting at…

  • Non-contact forces
  • Definition
  • Consequence
  • Graphical test
  • Normal force
  • Translational equilibrium

Part 1 of 7Free-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

Part 2 of 7Coordinate systems and inclines

Coordinate systems and inclines
Axes
  • Axes two perpendicular axes with one parallel to the acceleration
Incline
  • Incline take one axis along the slope
  • Fg = mg splits into Fg cos θ perpendicular and Fg sin θ parallel to the slope.
  • Without perpendicular acceleration, FN = Fg cos θ.

Part 3 of 7Forces as interactions

Force
  • Force a push or pull from an interaction with another object; it can change speed, direction, shape or size
Contact forces
  • Contact forces normal, tension, friction, thrust and upthrust
Non-contact forces
  • Non-contact forces gravitational, electrostatic and magnetic

Part 4 of 7Normal force

Direction
  • Direction perpendicular to the surface and away from it
Origin
  • Origin repulsion between electrons in the surfaces
Magnitude
  • Magnitude equals the weight component perpendicular to the surface when there is no other perpendicular force

Part 5 of 7Net force

Net force
  • Net force the vector sum of all forces; an object cannot exert a net force on itself
Balanced
  • Balanced net force zero; unbalanced — net force non-zero
Example
  • Example tug-of-war with 80 N and 100 N gives 20 N toward the stronger side

Part 6 of 7Translational equilibrium

1Definition
  • Definition a configuration of forces with zero net force: ΣFi = 0
2Consequence
  • Consequence the object is at rest or moves at constant velocity
3Graphical test
  • Graphical test force vectors drawn tip to tail form a closed triangle

Part 7 of 7More from the specification

More from the specification
Contact force origin between surface atoms
  • Contact force origin between surface atoms macroscopic normal reaction forces originate from microscopic electrostatic repulsion between outermost electron clouds of contacting atoms.
Reaction force
  • Force from a support pushing up on an object.
Stationary
  • Not moving.
Translational equilibrium
  • Net force zero.
Coordinate system
  • Perpendicular axes used to resolve forces.

Key terms

TermMeaning
Free-body diagramDiagram of the forces acting on one object.
Net forceVector sum of forces.
Normal forcePerpendicular contact force from a surface.
Translational equilibriumNet force zero.
Contact forceForce requiring touching objects.

Avoid these mistakes

1
Including forces the object exerts on others.
2
Drawing the normal force vertical on an incline.
3
Drawing arrows not proportional to the magnitudes.
4
Thinking motion requires a net force.

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