Chapter 2 Displacement and Velocity Frame of Reference - In order to measure the distance of an object we must use a frame of reference. Point A -------->

Chapter 2 Displacement

andVelocity

Frame of Reference

- In order to measure the distance of an object we must use a frame of reference.

Point A --------> Point B

- These are usually distinct points on the Earth's surface or distinguishing points in the sky.

The perception of motion depends on the observer’s frame of reference

• Describe the motion observed by one of the boys in the drawing, how does the motion appear to be different to the other boy?

The perception of motion depends on the observer’s frame of reference

• Imagine you are the girl observing the bus, describe the motion of each object that you can see

Distance is a scalar quantity and tells you only the magnitude

(number sum only) of a path taken.

Displacement is the shortest distance between the starting

point and the end point.

Distance vs. Displacement

• You drive the path, and your odometer goes up by 8 miles (your distance).

• Your displacement is the shorter directed distance from start to stop (yellow arrow).

• What if you drove in a circle?start

stop

Constant speedConstant speed

A moving object that doesn’t change A moving object that doesn’t change it’s speed travels at constant speedit’s speed travels at constant speed

Constant speed means equal Constant speed means equal distances are covered in an equal distances are covered in an equal amount of timeamount of time

Calculating Speed

• Speed = Distance Time

• If a runner travels 100 m in 10 seconds what was his average speed?

• Probably not constant• Can solve for the other pieces too• Distance = speed x time• Time = Distance

Speed

• instantaneous velocity - the velocity that something has at any one instance

– (The terms instantaneous speed and avg. speed may also be used)

interval timentdisplaceme = velocity average

t

dv

_

The average velocity for a trip might be 53

miles/hour.

However, during this trip your instantaneous

speed might have been 0 miles/hour at a stoplight or 70

miles/hour on the open road.