MotionMotion

Definition

Motion is a change in an object’s position relative to a reference

point.

Motion is Relative

• Relative – it is described compared to a REFERENCE POINT

• Example: As a car drives away from school its position is changing relative to school.

Describing motion

• We describe motion in words, with graphs and using mathematical quantities.

• The mathematical quantities we use to describe motion are distance, time, speed, velocity and acceleration.

Describing motion

• These mathematical quantities have certain SI units.

Quantity Unit

Distance Meter

Time Second

Speed/Velocity Meter per Second

Acceleration Meter per second squared

Speed: We are concerned with 3 different types of speed.

• Speed is the rate of change in position.

• We will talk about average speed, instantaneous speed, and constant speed.

Speed: Average Speed

– Total distance traveled divided by the time interval during which the motion occurred.

• s= d / ts= d / t

• t = d / st = d / s

• d = s x td = s x t

Example: If you travel 200 milesIn 4 hours, your average speed is50 mph.

SpeedInstantaneous Speed

• Speed at any instant

Example: In your trip, you might be going 60 mph one moment andSlowing down and stopping for a break a few moments later.

SpeedConstant Speed

• Speed that does not change– Instantaneous speed that does not change– When an object has a constant speed, its

average speed and instantaneous speed will be the same.

Velocity• Speed AND direction

• This is important for a full description of motion

• Pilots need to know wind speed and direction.

• Calculations of velocity are different than speed.

• v= change in position / timev= change in position / time

Velocity: Direction matters

• Imagine that two birds leave the nest at the same time. The both fly at 10 km/h for 5 min, 12 km/h for 8 min, and 5 km/h for 10 min. Why don’t they end up at the same place?

Problem

A swimmer swims 50 m in 40 s. What is the swimmer’s average

speed?

A swimmer swims 20 m in 40 s. What is the swimmer’s average speed?

A swimmer swims 20 m in 40 s. What is the swimmer’s average speed?

S = D/T

S = 50m/40s

S = 1.25m/s

Problem

What is the average speed (mph) for a car journey of 120 miles that began at 09:30am

and finished at 12:00 noon?

Problem

What is the average speed (mph) for a car journey of 120 miles that began at

09:30 and finished at 12:00 noon?S = D/T

D = 120

T = 2.5 hours (from 9:30 – 12:00)

S = 120m / 2.5h

S = 48 mph

How far does a bicycle travel at a speed of 6

m/s for 15 seconds?

How far does a bicycle travel at a speed of 6

m/s for 15 seconds?

How far does a bicycle travel at a speed of 6 m/s for 15 seconds?

How far does a bicycle travel at a speed of 6 m/s for 15 seconds?

s=d/t

6m/s = d/15s

15s (6m/s) = d/15s (15s)

90 m = d