Displacement & velocity

Divisions of classical physics

• Kinematics: the study of motion w/o regard to cause.

Have you ever seen the root word “kine” before? Where?

• Dynamics: the study of the forces which cause & change motion.

• We are going to begin our study of classical physics with a study of kinematics

distance & displacement

• distance – scalar (d)

• displacement – vector (d)

• Distance is the magnitude of displacement.

• Dimensions of d (or d) = length m (MKS) or cm (CGS).

speed & velocity

• Dimensions of t (time)? MKS? CGS?

• Dimensions of v or (v)? MKS? CGS?

• By dimensional analysis – what must be the formula for d?

• d = vt or d = vt

• What is the relationship b/w the direction of d and the direction of v?

• How do you know?

• Given the formula, d = vt, derive the formula for velocity.

• d = vt d/t = vt/t v = d/t

• Now check the formula to see if it is dimensionally correct.

• m/sec = m/sec

Simplest motion

• The simplest type of motion: motion with constant speed in a straight line.

• Does constant v always have constant v? • Give an example when an object with

constant speed does not have constant velocity.

Principle of Relativity

• All motion is relative. The velocity of an object depends on the “frame of reference” from which one observes the event.

• Frame of reference: where you view an event. Point of view.

• What is the horizontal velocity of a ball throw straight up on a bus if the bus is traveling 50 mi./hr. E?

• Vbe or Vbb? Which is correct?• What is your velocity right now? • Motion w/ constant velocity (constant mag. &

dir.) cannot be proven.• Ex. Car @ a stoplight.• If one were in an airplane that is moving w/

constant velocity it would be impossible from inside the airplane to prove whether the plane or the earth is moving!

Playing catch in a train

Determining relative velocity

• Vector addition or subtraction. You must learn which. This is not to be confused w/ + or – direction.

Subscript system

• Velocity of what – 1st subscript

• Relative to what – 2nd subscript

• second subscript same subtract

• If 2nd subscripts are different -> add

Sample relative velocity problem

• A train is moving E @ 60 km/hr., a waiter is walking twd the rear of the train @ 2 km/hr., and a fly is crawling across the waiters tray @ 3 km/hr N. What is the velocity of the fly relative to earth?

vte

vwt

vfwvfeVector diagram