Relative velocity

A waiter walks toward the front of a dining car at 1.0 m/s while the train travels at 24 m/s. Which velocity does a seated passenger in the dining car measure for the waiter?

The seated train passenger of the last lesson measures the walker at 1 m/s whether the train rolls at 20 m/s or at 30 m/s. That steady number, the one measured from the train, deserves its own name.

The velocity an object appears to have when measured from another object is called its relative velocity with respect to that object.

To a seated train passenger, a walker heading up the aisle at 1 m/s has a relative velocity of 1 m/s forward, whatever the train's speed.

Measured from the train, the walker heading up the aisle does 1 m/s forward.

The train speeds up; the measurement made from the train does not change.

The phrase names both objects. 'The walker's relative velocity with respect to the train' says whose motion is measured, and from where.

A parade float rolls down Main Street, and a performer walks toward the front of the float. A rider on the float measures 1.5 m/s; a spectator on the sidewalk measures 9.5 m/s. Which measurement is the performer's relative velocity with respect to the float?

A relative velocity is still a velocity: it has a magnitude and a direction, and it can point forward, backward, or at any angle.

The same train; the walker turns toward the rear. A relative velocity has direction: 1 m/s backward.

A tour boat moves up a river at 4 m/s. A duck paddles beside it, holding station level with the front of the boat. What is the duck's relative velocity with respect to the boat?

Measured from the boat, the duck neither gains ground nor falls behind.

An object that holds station has no velocity in the measuring frame.

The duck's relative velocity with respect to the boat is 0 m/s.

A jogger runs along the deck of a cruise ship at 2 m/s. Nika says the jogger's relative velocity with respect to the ship is 2 m/s whether the ship sails at 5 m/s or at 15 m/s. Is she correct?

Your turn

Which statement about an object's relative velocity with respect to another object is true?

A gull flies past a moving fishing boat. Measured from the boat, the gull does 3 m/s; measured from the harbor wall, it does 10 m/s. What is the gull's relative velocity with respect to the boat?

A passenger on a moving city bus measures a cyclist's velocity from the bus. What is this measurement called?

One item remains: the name itself, from memory.

How fast and in which direction an object appears to move when measured from another object is called its ___. Type the two-word name.