CHAPTER 3: TWO DIMENSIONAL MOTION & VECTORS Hanan Anabusi

3-4 Relative Motion Objectives: Describe situations in terms of frame of reference. Solve problems involving relative velocity.

3-4 Vocabulary Frame of reference. Relative velocity.

WHAT IS RELATIVE VELOCITY? Sometimes it appears that objects are moving faster or slower than they actually are because of a “frame of reference”. Since velocity is a vector (magnitude & direction), we need to consider these when determining relative velocity. We use vector addition or subtraction for these problems. Read the subscripts carefully – it matters! vac is read, velocity of “a” relative to “c”.

Sample Problem F A boat heading north crosses a wide river with a velocity of 10.00 km/h relative to the water. The river has a uniform velocity of 5.00 km/h due east. Determine the boat’s velocity with respect to an observer on shore.

E W N S y θ x East of North

Classwork & Homework Class work: Practice F on page 105 questions 1 & 3 Homework: Chapter 3 review, page 110, question 41

a passenger at the rear of a train travelling at 15 m/s relative to Earth throws a baseball with a speed of 15 m/s in the direction opposite the motion of the train. What is the velocity of the baseball relative to Earth as it leaves the thrower’s hand? x y

A ferry is crossing a river A ferry is crossing a river. If the ferry is headed due north with a speed of 2.5 m/s relative to the water and the river’s velocity is 3.0 m/s to the east, what will the boat’s velocity relative to Earth be? (Hint: Remember to include the direction in describing the velocity.) x y