Frames of Reference

Caution: Your brain is about to be twisted!

KL phys 09

Science is nothing but perception. Plato

There is no fixed physical reality, no single perception of the world, just

numerous ways of interpreting world views as dictated by one's nervous

system and the specific environment of our planetary existence. Deepak

Chopra

There are children playing in the streets who could solve some of my top problems in physics, because they have modes of sensory perception that I lost long ago. J. Robert Oppenheimer

Consider this…

You are sitting in the passenger seat of your friend’s car. The speedometer reads 30

mph.How fast are you moving?

How fast is your friend moving? You pass a friend standing on the corner.

How fast is he moving?

The reality is that your motion affects what is seen.

Motion is relative!

Your (the passenger) point of view

You are sitting in the passenger seat of your friend’s car. The speedometer reads 30 mph.

How fast are you moving?v=0 mph

How fast is your friend moving?v=0mph

You pass a friend standing on the corner. How fast is he moving? v=-30 mph ( 30 mph backwards)

The driver’s point of view

You are sitting in the passenger seat of your friend’s car. The speedometer reads 30 mph.

How fast are you moving? v=0 mph

How fast is your friend moving? v=0 mph

You pass a friend standing on the corner. How fast is he moving? v = -30 mph (30 mph backwards)

Your friend on the corner’s point of view

You are sitting in the passenger seat of your friend’s car. The speedometer reads 30 mph.

How fast are you moving? v=30 mph

How fast is your friend moving? v=30 mph

You pass a friend standing on the corner. How fast is he moving? v = 0 mph

From the point of view of a car moving along at 20

mph as you pass the slow poke.

You are sitting in the passenger seat of your friend’s car. The speedometer in your friend’s car reads 30 mph.How fast are you moving?

v=0 mph How fast is your friend moving?

v=0 mphHow fast is the car you passed moving ?

v=-10 mphFor the 2 cars moving in the same direction the relative velocity is found by

subtraction and the +/- depends on the reference point.

From the point of view of a car coming head on towards you at 20 mph

You are sitting in the passenger seat of your friend’s car. The speedometer in your friend’s car reads 30 mph.How fast are you moving?

v=0 mph How fast is your friend moving?

v=0 mphHow fast is the car coming towards you moving ?

v= -50 mphFor the 2 cars moving in the opposite directions the relative velocity is found by

addition and the +/- depends on the motion towards or away from a reference point.

Example: Your friend tosses a ball upwards while riding in a car

Does the FBD change if the ball is on its way towards the ceiling?at the top of its trajectory?on its way back down?moving sideways?

Weight

Example: Your friend tosses a ball upwards while riding in a car

What path do you see?If you are at rest (standing on the corner) and the ball moves sideways.

If you are in the car with the ball moving together.

Weight

Frames of Reference

Inertial – Objects obey the law of inertia.Constant velocity

(Constant speed, straight line)

Non-inertial Explanations cause you to create “fictitious” forces because you want to believe Newton’s 1st law.

Accelerated motionWhat you think is true is exactly opposite what is truly happening.