Relative Motion - Chapter 3.4


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


Relative Motion

Chapter 3 Section 4

Frames of Reference

If your in a car going down the road at
60mi/hr due north and a car passes you
going 70mi/hr due north, it seems as if the
car passing you is going 10mi/hr.

Someone standing on the side of the road
would view the faster car going 70mi/hr,
not the 10mi/hr which you see it as.

Frames of Reference

If you are riding your bicycle along the road
at 10mi/hr due east and another bicycle
passes you going 15 mi/hr due west, it
looks as if the faster moving bicycle is going

To someone standing along the road, it
looks as if the bicycle is only going 15 mi/hr.

Frames of Reference

The examples are good explanations of how
velocities change when viewed in different
reference frames.

Frame of Reference
: The motion of an
object is determined by the reference point
of the observer and can change from one
point to the next.

Relative Velocity

There is no general equation used to
determine the relative velocity of an object
given different reference points, but vector
addition is used to solve most problems
dealing with relative velocity.

Relative Velocity Example

A boat heading north crosses a wide river
with a velocity of 10.00 km/hr relative to
the water. The river has a uniform velocity
of 5.00 km/hr due east. Determine the
velocity with respect to an observer on

Pg 108

sample problem 3F

Example Problem #1

A passenger at the rear of a train traveling
at 15m/s relative to earth throws a
baseball with a speed of 15m/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?

Example Problem #1 Answer

Example Problem #2

A spy runs from the front to the back of an
carrier with
a speed of 3.5m/s. If
the aircraft carrier is moving forward at
18m/s, how fast does the spy appear to be
running when viewed by an observer on a
near by submarine?

Example Problem #2 Answer

Example Problem #3

A ferry is crossing a river. If the ferry is
heading due north with a speed 2.5m/s
relative to the water and the river’s velocity
is 3.0m/s to the east, what will the boat’s
velocity be relative to Earth?

Example Problem #3 Answer