Topic 1.4: Reference Frames and Relative Velocity

Name: _______________________________ Date: _______________

Before We Start: Yesterday's Recap

Yesterday was review day. Without looking back: what does the SLOPE of a velocity-time graph tell you?

Today's New Concepts

AP CED Alignment: Unit 1, Topic 1.4

Reference Frame

• The viewpoint you're measuring motion from — "moving" only makes sense relative to something
• Inside a frame moving at constant velocity, everything behaves exactly like it would if the frame were sitting still — you can't feel steady motion, only acceleration

Cruise Ship Examples

• Dropping a ball on a moving ship: lands at your feet — gravity is all that matters inside the ship's frame
• Mini golf on a moving ship: the ball rolls normally — everything's moving together at the ship's speed
• Watching another ship pass: can look faster than either ship's actual speed — that's relative velocity

Relative Velocity

• Combine an object's velocity with the velocity of the frame it's in
• Same direction → add. Opposite direction → subtract (pick a positive direction first, keep it consistent)

Worked Example — Walking on a Moving Ship:

• Ship cruises at 20 mph. Walk toward the bow at 3 mph (same direction) → $20+3=$ 23 mph relative to the water
• Walk toward the stern instead (opposite direction) → $20-3=$ 17 mph relative to the water

Worked Example — Two Ships Passing:

• Ship A: +20 mph. Ship B: −20 mph. Relative velocity of B as seen from A: $-20-(+20)=$ −40 mph
• This is why passing ships (or oncoming trains/cars) look like they're moving much faster than either one actually is

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Keep This Sheet!

These sheets build into your semester study guide. Keep them in order in a binder or folder — you'll want to flip back through them before quizzes, unit tests, and when AP exam review starts in the spring.