Chapter 3 3-4 Relative Motion

Objectives • Describe situations in terms of frame of

reference.

• Solve problems involving relative velocity.

Relative Motion • What is relative motion?• The laws of physics which apply when you are at

rest on the earth also apply when you are in any reference frame which is moving at a constant velocity with respect to the earth. For example, you can toss and catch a ball in a moving bus if the motion is in a straight line at constant speed.

• The motion may have a different appearance as viewed from a different reference frame, but this can be explained by including the relative velocity of the reference frame in the description of the motion.

Relative Velocity• What is relative velocity?• One must take into account relative velocities to

describe the motion of an airplane in the wind or a boat in a current. Assessing velocities involves vector addition and a useful approach to such relative velocity problems is to think of one reference frame as an "intermediate" reference frame in the form:

Relative velocity• Put into words, the velocity of A with respect to C

is equal to the velocity of A with respect to B plus the velocity of B with respect to C. Reference frame B is the intermediate reference frame. This approach can be used with the airplane or boat examples.

Relative motion and velocity

• Velocity of the moving objects with respect to other moving or stationary object is called “relative velocity” and this motion is called “relative motion”.

Example#1• A boat heading north crosses a wide river with a

velocity of 10km/h relative to the water.• The river has a uniform velocity of 50 km/h due

east .Determine the boat’s velocity with respect to an observer on shore.

Example#2• A river flows due east at 1.50 m/s. a boat crosses

the river from the south shore to the north shore by maintaining a constant velocity of 10 m/s due north relative to the water.

• A. What is the velocity of the boat as viewed by an observer on shore?

• B. if the river is 325 m wide , how far downstream the boat when it reaches the north shore?

Example#3• The pilot of an aircraft whishes to fly due west in

a 50 km/h wind blowing toward the south.• The speed of the aircraft in the absence of wind

is 205 km/h.• In what direction should the aircraft head?• What should its speed relative to the ground be?

Example#4 • A swimmer can swim in still water at a speed of

9.50 m/s . He intended to swim directly across a river that has a downstream current of 3.75 m/s

• A. What is the swimmer’s direction?• B. What is his velocity relative to the bank?

Student guided practice

• Lets work on worksheet problems

Homework!!!• Do problems 1-4 from page 103

closure• Today we saw about relative motion and relative

speed and how we can solve for them.• Next class we are going to start chapter 4