2.1speed And Velocity

Speed and Velocity

• Speed is a measure of how fast something is moving. • It is the rate at which a distance is covered.

eFootage Play Clip.mov

Distance, Speed and Time

Speed = distance (in metres) time (in seconds)

EQUATIO N

v

d =

t

v = speed d = distance t = time

If it takes you one hour to move the 10m from the couch and your wii remote to your PC to complete the physical science Unit on motion, what is your speed? (see next slide)

ANSWER v

d

d=1m

=

t = 1 hr

t 1m

v = 1hr

V = 1m/hr

AN EASY SHORTCUT METHOD FOR SOLVING EQUATIONS

v

d

d =

t

v

t

In a three variable equation like this one, move the variable in the numerator to the top of the triangle. The remaining two variable go below. Now you need only to look at the triangle to solve for the variable. If I want to solve for distance then it is v x t (note: they are side by side). If I want to solve for speed (v) then it is d/t (note: d is over t in the triangle. Get it?

• • • • •

Practice solving speed equations Dave walks 200 metres in 40 seconds. What is his speed? Laura covers 2km in 1,000 seconds. What is her speed? How long would it take to run 100 metres if you run at 10m/s? Steve travels at 50m/s for 20s. How far does he go? Susan drives her car at 85mph (about 40m/s). How long does it take her to drive 20km?

Race Car Simulator http://lgfl.skoool.co.uk/content/keystage3/Physics/p c/learningSimulations/MESSC/launch.html

Read the directions in the box. Press Begin to start the simulation

AVERAGE SPEED VS. INSTANTANEOUS SPEED Using your equation v = d/t gives you average speed. – speed is distance covered over a certain time – typical units for measuring speed are m/s, km/h, mi/h (mph)

Your speed at any given instant is instantaneous speed. Your speedometer gives you instantaneous speed.

Why does instantaneous speed matter? Ask the policeman who stops you or a friend for speeding in the school zone. Your average speed may have been 25 mph, but the radar gun

Velocity is a vector quantity which means it includes both a magnitude and a direction. It is a rate of motion in a specific direction. “I'm going that-a-way at 30 kilometers per hour. My velocity is 30 kilometers per hour that-a-way…or… I am going 30 km/h east. “

Which of the following represent a velocity and which speed?

• • • •

5m/s 60 km/h 3 m/s, northeast 2 km/hr at 30o north of east • Mach 2 • 35 km/h south

1.Speed 2. speed 3.Velocit y 4.Velocit y 5.Speed 6.velocit

Velocity equation

Same as speed but …

v

Δd =

Δt

Δd = change in position (displacement) Δt = change in time

Velocity is a VECTOR!! A change in position means direction is

Speed vs. Velocity Speed is simply how fast you are travelling… This car is travelling at a speed of 20m/s

Velocity is “speed in a given direction”… This car is travelling at a velocity of 20m/s east

Distance-time graphs 2) Horizontal line = 40 40 stopped Distance Distance

4) Diagonal line downwards = Return to original position

30 30

(metres) (metres) 20 20

10 10 0 0

Time/s Time/s

20

1) Diagonal line = constant velocity

40

60

80

100

3) Steeper diagonal line = going faster, but at a constant velocity

DISTANCE/TIME GRAPHS A car takes 1.5 minutes to travel 500m down a busy road. It stops at lights for 30 seconds, then continues on for 1 minute as it goes another 1km. Plot this on a distance/time graph.

1.5

Distance

Using the distance/time graph:

1

Δd = 1km

(km)

2.What is the total distance 0.5 traveled? = 1.5 km 2. In what part of the trip is = part 3 the car going the fastest? 3. What is the fastest speed?

v = Δd / Δt v = 1km/1min v = 1000m/60s v = 16

Steepest section is fastest Δt = 1min

1

2 Time (min)

3

In a distance/time graph the slope of the line = the speed of the object.

SPEED QUESTIONS What would these look like on a distance/time graph? 1. stopped

2. slow

3. fast

4. accelerating

40 Distance (metres)

30 20 10 Time/s

0

20 20

40 40

60 60

80 80

100 100

1) What is the speed during the first 20 seconds? 2) How far is the object from the start after 60 seconds? 3) What is the speed during the last 40 seconds? 4) When was the object travelling the fastest?

READING MORE GRAPHS