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1.4Kinematics

Reference Frames and Relative Motion — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 14 1 quiz

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Lesson summary

1.4 Reference Frames and Relative Motion — AP PHYSICS 1 notes: Describe how position, direction and velocity depend on the reference frame and convert between frames.Reference frame — a coordinate system in which an observer measures position and timeRelative motion — all objects…

  • Measurements
  • Rest and motion
  • Displacement
  • Car's frame
  • Truck's frame
  • Inertial reference frame

Section 1 of 8Reference frames

Reference frame
  • Reference frame a coordinate system in which an observer measures position and time
Relative motion
  • Relative motion all objects move relative to one another;
    • Earth rotates at about 1600 km h-1 and orbits at about 107 000 km h-1
Measurements
  • Measurements position and velocity depend on the frame

Section 2 of 8Direction and motion

Direction
  • Direction two observers on opposite sides of a road see a car moving in opposite directions
Rest and motion
  • Rest and motion a person on a train and one on the platform each consider themselves stationary and see the other moving

Section 3 of 8Converting position

Pattern
  • Pattern car 10 m behind Person A and 10 m ahead of Person B at t = 0
Positions
  • A measures x0 = −10 m and B measures x0 = +10 m
Displacement
  • Displacement magnitude is the same in both frames (5 m), but direction is relative to the frame

Section 4 of 8Converting velocity

Method
  • Method subtract the velocity of the new frame from the object's velocity
Pattern
  • Pattern car 5 m s-1, truck 10 m s-1, Person A at rest
Car's frame
  • Car's frame truck +5 m s-1 and Person A −5 m s-1
Truck's frame
  • Truck's frame car −5 m s-1 and Person A −10 m s-1

Section 5 of 8Inertial frames

Inertial reference frame
  • Inertial reference frame a non-accelerating frame moving at constant velocity
Experience
  • Experience an observer in such a frame cannot feel motion, as in an aircraft at cruising altitude
Acceleration
  • Acceleration every inertial frame measures the same acceleration of an object
AP Physics 1
  • AP Physics 1 only inertial frames are used in conversions

Section 6 of 8More from the specification

Tangent
  • Line touching a curve; its slope gives the instantaneous rate.
Initial velocity
  • Velocity at the start, u.
Final velocity
  • Velocity at the end, v.
Kinematic equation
  • Equation for constant-acceleration motion.
Kinematic equations
  • Equations relating displacement, velocity, acceleration and time for constant acceleration.
Uniform acceleration
  • Acceleration with constant magnitude and direction.

Section 7 of 8Key terms

TermMeaning
Reference frameCoordinate system used by an observer.
Relative velocityVelocity measured from a moving frame.
Inertial frameNon-accelerating reference frame.
Frame of referenceAnother name for reference frame.

Section 8 of 8Exam technique

Avoid these mistakes
Thinking only one observer is correct — each is correct in their own frame.
Adding frame velocity instead of subtracting it when converting to that frame.
Using an accelerating frame in AP Physics 1 conversions.

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