Chapter 9 Section 2

Speed and Velocity

◦Speed is the distance traveled by an object divided by the time taken to travel that distance.

Speed Depends on Distance and Time

The SI unit for speed is meters per second ( M/ Sec )

Most objects do not travel at a constant speed, therefore you calculate average speed: ( Speed = Total Distance / Time)

Speed can be shown on a graph as distance versus time

◦On a graph, actual distance varies because the speed is not constant therefore the actual speed line is not straight. A constant speed line on the graph will be a straight diagonal line due to average constant velocity.

What is your average speed if you take 0.5 hours to jog 4,000 M?

If the average speed of a car is 110KM/Hr, how long will it take the car to travel 715KM?

Time Time = Total Distance x --------

Distance

Kira jogs to a store 72M away in a time of 36Sec, what is her avg. speed?

If you travel 7.5KM by walking for 1.5hrs. What is your speed?

An airplane traveling from San Fran to Chicago travels 1,260Km in 3Hrs, what is its speed?

A. Imagine that 2 birds leave the same tree at the same time. They both fly 10KM/Hr for 5Min, 12Km/Hr for 8Min, and 5KM/Hr for 10Min. Why don’t they end up at the same place?

Velocity: Direction Matters

1. Their speeds were the same but they flew in different directions

2. Velocity is the speed and the direction of an object.

Speed and Velocity ARE NOT the same thing. Velocity MUST have a REFERENCE DIRECTION!!

(eg) The car’s velocity is 60KM/Hr South

You can think of velocity as the rate of change of an object’s position

An object’s velocity is constant only if it’s speed and direction do not change, therefore constant velocity is always along a straight line.

An object’s velocity changes if EITHER it’s speed or direction change.(EG) (a)If a bus goes 15KM/hr then speeds up to 20KM/hr. (b) The bus goes from heading North to East. ( c) The bus speeds up and makes a left turn.

When you combine two velocities that are in the same direction, add them together to find the resultant velocity.

Your on the bus going 15M/Sec East and you stand up and walk 1M/Sec East to the front of the bus.

15 + 1 = 16M/Sec East

Combining Velocities

When you combine two velocities that are in opposite directions, subtract the smaller velocity from the larger velocity to find the

resultant velocity. The resultant velocity is in the direction of the larger velocity. Your on the bus going 15M/Sec East and you

stand up and walk 1M/Sec WEST to the back of the bus.

15 – 1 = 14M/sec East