AP Physics C: Mechanics Flashcards: Reference Frames and Relative Motion
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
How does the choice of reference frame affect measurements of an object's motion?
The choice of reference frame will determine the direction and magnitude of quantities, such as velocity, that are measured by an observer in that frame.
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How does the choice of reference frame affect measurements of an object's motion?
The choice of reference frame will determine the direction and magnitude of quantities, such as velocity, that are measured by an observer in that frame.
A person walks at 2 m/s toward the front of a train that is moving at 30 m/s. What is the person's velocity as measured by an observer standing on the ground?
The person's observed velocity is 32 m/s in the direction of the train's motion, as their velocity combines with the velocity of the train's reference frame.
A boat moves north at 5 m/s across a river flowing east at 2 m/s. From the perspective of an observer on the riverbank, is the boat's speed 5 m/s?
No, the observer on the bank (a different reference frame) will measure a resultant velocity that combines the boat's motion and the river's motion, resulting in a different speed and direction.
What is a reference frame?
A reference frame is the perspective of a given observer from which the direction and magnitude of an object's motion are measured.
What two factors combine to determine the observed velocity of an object?
The observed velocity of an object results from the combination of the object's own velocity and the velocity of the observer's reference frame.
Car A moves east at 60 mph. Car B moves east at 40 mph. From the reference frame of the driver in Car A, what is the motion of Car B?
From Car A's reference frame, Car B appears to be moving backward (west) at 20 mph, which is the difference in their velocities.
Can measurements of motion from one reference frame be used to find the measurements in a different reference frame?
Yes, measurements from a given reference frame may be converted to measurements from another reference frame by accounting for the relative motion between the frames.
Why would two observers in different inertial reference frames (e.g., one on a train, one on the ground) measure different velocities for the same moving object?
They measure different velocities because the motion they observe is a combination of the object's velocity and the velocity of their own unique reference frame.
Define relative motion.
Relative motion is the motion of an object as measured by an observer in a specific, potentially moving, reference frame.
What is an inertial reference frame?
An inertial reference frame is a frame where an object's motion does not change velocity unless a force is applied; it is a non-accelerating frame of reference.