To understand the concept of speed, through formula and with units.
To make measurements that will allow us to calculate speed.
To appreciate that speed is not a constant, and that this change is called Acceleration.
Speed is a value of how far something goes in a certain time
To calculate speed we need to take 2 Measurements, how far an object goes and how long it took them.
Try these calculations before we take some measurements
1 The horse Alien Mechanics ran the last 65m of his race in 5 seconds, calculate his speed.
2 A man cycled 98m in 7s, calculate his speed
3 A cheetah is capable of covering 102 m in 6s calculate its speed.
4 When a car is travelling at 120km/hr it manages to do 1km in 30s, calculate its speed
We have some toys in the lab, What would we need to do to calculate their speed ?
Lets try this out, we should make out a table to record our results
In order to be good observers, we need to be careful with our measurements, to ensure they are
taking good care to start and stop timers when the object & to measure the distance from exactly where it started to exactly where it stopped
using the smallest degrees of measurement available (with a meter stick that means measuring to the mm)
Go to work,
Think about this .....
Do you think that the toys you tested traveled at this speed for the entire journey?
Discuss in your team
Velocity is a speed but we need to say its direction to define it fully
2 cars going on both sides of a motorway might both have a speed of 120 km/h,
but one has a velocity of 120 km/h East and the other 120 km/h West
In order to more accurately measure the motion of the objects we are going to use a Ticker Tape device.
The ticker tape makes a mark on the paper every 1/50th of a second. Or every 0.02 seconds. If we measure the distance between any 11 dots we can find out how far the object went in 0.2 seconds (much more precise than we could be with our timers)
Let us try this out with some ticker tape,
Lets get someone to count out 11 dots, cut the tape & number it 1, 2, 3, 4, etc
Each of you in your Double Groups should calculate the speed of the object on your ticker tape
Lets compare our findings
Did the object travel at the same speed at all times?
If not can you describe what happened with the speed of the object ?
Acceleration is a measure of how quickly an object changes its speed
So it is likely that your object changed speed, there are 2 options
To Accelerate means to increase speed, to Decelerate means to decrease speed.
To determine Acceleration we need to
- Calculate a speed at the beginning
- Calculate a speed later
- Find the time between the start speed and the end speed
- Complete the Formula
Acceleration occurs when a bodies speed Increases or Decreases
When the speed / velocity decreases the acceleration is given a negative value.
Check out the following Ticker tape samples, can you describe the motions that caused these patterns
Speed time graph
We can plot the speed of a object against time on a graph.
If we can put a value on the speed at different times we can plot points on a labelled grid, by joining these points we can create a speed time graph
A Car drives along at a constant speed of 12m/s for 20s, it then accelerates to 16m/s in 12s. It remains at this speed for 30s. When the driver hits the brakes and brings the car to a stop in 3s
A force is what changes the speed of an object