# relative motion

Mechanics

Nov 14, 2013 (4 years and 8 months ago)

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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

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