5.2.1+2 Average Speed

• Natuurkunde
• Middelbare school
• havo, vwo
• Leerjaar 2,3

Your speed will decide what distance 

you will travel every second.

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Two kinds of speed

Instantaneous and average speed

The instantaneous speed is the speed of an object at a particular moment (instant) in time.

The average speed on the other hand can be calculated by dividing the distance travelled by the total time it has taken to do so. 

The next two videos will explain 

to you how to calculate the instantaneous speed and the average speed.

Daphne Schippers can run very hard. But her speed is not constant. 

To calculate her average speed you can divide the total distance by the total time. 

To calculate the instantaneous speed at any one time, you will need to make use of a graph of her motion at every instance of her run.

Speed of sound

The speed of sound is 343 m/s (in air)

You can hear sounds with a frequency in between 20 Hz and 20 kHz

Sounds is a form of energy, like light, heat or electricity.

The speed of sound depends on the medium through which it travels 

(air = 343 m/s, water = 1400 m/s, steel = 6000 m/s). It needs matter though, so it will not travel through a vacuum.

Use the speed of sound to calculate the distance of a lightning strike from which you hear the thunder 8,5 seconds later

Speed of light

The speed of sound is 300.000 km/s

Because of this very high speed, light will travel enormous distances in the blink of an eye. This distance is expressed in light years.

A light year is not a measure of time. A lightyear is the distance light can travel in one year (s = v . t = 300.000 km/s . 365 dg . 24 h/dg .3600 s/h= 9.460.800.000.000 km= aproximately 9,5 trillion kilometres

Sunlight takes eight minutes to travel from the sun to the earth, how far does it need to travel?

Clara and Rob live 50 kilometers from each other.
They want to meet up and leave home at 8 o'clock in the morning.
Clara travels with a speed of 6 km/h, while Rob travels with a speed of 4 km/h. How far from home do they meet each other and at what time?

Quantities and units

A quantity is something you can measure

A unit is the measurement of the magnitude of the quantity

Quantities and their standard units

Scalars & vectors

Scalars are quantities which are described by magnitude only.

Vectors are quantities which are described  by both a magnitude and a direction

Distance 

the total distance travelled is 12 m, but in the end you end up in exactly the same spot where you have started so the final displacement is zero meter

Distance 

Distance is a scalar quantity that refers to

 "how much ground an object has covered"

without any regard for the direction of this motion.

(a scalar quantity)

Displacement

the total distance travelled is 12 m

But you end up at exactly the same spot from where you started.

Thereforce the total displacement will be zero meters in this case.

Displacement 

Displacement is a vector quantity that 

refers to "how far out of place an object is".

It is the object's overall change in position.

It refers to the distance moved in a particular direction.

(a vector quantity)

Speed 

Speed is the rate of change of distance of an object

Depends on the distance traveled and the times it takes to travel that distance

averagespeed=​totaltime​​totaldistancetravelled​​

(a scalar quantity)

Velocity 

Velocity of an object is the distance it has moved in a particular direction

So when talking about velocity, you should always mention a direction in your phrasing, like south, to the right etc..

velocity=​totaltime​​changeindisplacement​​

(a vector quantity)