3-4 Relative Motion

• Describe situations in terms of frame of reference.
• Solve problems involving relative velocity.

• 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.

• 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:

• 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.

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

• 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.

• 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?

• 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?

• 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?

• Lets work on worksheet problems

• Do problems 1-4 from page 103

• 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