Last lesson
Hooke’s Law
Investigating forces and springs
You are going to investigate how much a spring stretches when a force is
applied to it.
The amount a spring stretches is called its extension. This is the
difference in length between the stretched spring and the length of the
spring when it was unstretched.
(Remember we are looking at the force on the spring. A mass of 100g will
have a weight (force of gravity pulling it down) of 1 Newton.
•
•
•
•
Add masses to your spring and carefully measure its extension. You can
do this until the spring breaks! (but you must wear goggles and be careful
during this investigation).
Your experimental report will include the following
A table of results
A graph of your results
A conclusion describing what you have discovered (think about this!)
Table of results
Force (N)
0
Length of spring Extension (cm)
(cm)
3.4
0
1
5.4
2
2
7.4
4
3
9.4
6
Graph
Force (N)
Complete investigation for Weds 21st
October
1. Table of results
2. Graph
3. Two sentences of conclusion
Extension (cm)
Hooke’s law
Elastic limit
Force
(N)
The extension of a spring is
proportional to the force
applied (until the elastic limit
is reached)
Extension (cm)
Steel, glass and wood
Force
Even though they don’t
stretch much, they obey
Hooke’s law for the first part
of the graph
Extention
Rubber
Force
Extension
Homework
• Set Friday 23rd October. Due Wednesday
3rd November.
• Complete Physics for IGCSE page 61
Questions 1 to 4
Today’s lesson
• Orbital motion and satellites
Satellites
How far could you kick a dog?
From a table, medium kick.
How far can you kick a dog?
Gravity
Harder kick?
Harder kick
Gravity
Small cannon?
Woof!
(help)
Small cannon
Woof!
(help)
Gravity
Bigger cannon?
Bigger cannon
Gravity
Gravity
Even bigger cannon?
Even bigger cannon
Gravity
Gravity
Gravity
VERY big cannon?
VERY big cannon
Gravity
Humungous cannon?
Dog in orbit!
The dog is
now in orbit!
(assuming no
air resistance
of course)
Dog in orbit!
The dog is
falling towards
the earth, but
never gets
there!
Dogs in orbit!
Gravity
The force that
keeps an
object moving
in a circle is
called the
centripetal
force (here
provided by
gravity)
Other examples
Earth’s
gravitational
attraction on moon
Uniform circular motion
The centripetal acceleration/force is always directed
towards the centre of the circle
Centripetal force/acceleration
velocity
Note!
• There is no such thing as centrifugal force!
(at least not until you get to university!)
CENTRIFUGAL
Types of orbits
Geostationary
The satellite orbits once every 24 hours, so appears to
stay above the same point on the earth’s surface
Looking from
above the North
pole
Geostationary
Useful for communications satellites. Also for weather
over one area.
Polar
Satellite orbits over the poles
Polar
Useful for mapping and remote sensing when the whole
of the earth’s surface needs to be studied
Some famous satellites
Sputnik
th
(4
October 1957)
Sputnik
•
•
•
•
•
•
th
(4
October 1957)
Russian
84 kg, 58cm diameter
800km above the earth
30 000km/h
Orbited every 90 mins
Fell after 92 days
Sputnik 2 (a month later)
Sputnik 2 (a month later)
• 450 kg
• Carried a dog! (Laika)
• Laika is believed to have died from
overheating in the cabin a few hours after
launch
It’s a dogs life.
Hubble Space telescope
Hubble Space telescope
• Launched in 1990
• 13.1m long
• Doesn’t have to see through the earth’s
atmosphere
That’s it!
Can you try some questions now?
Page 48 and 49 in
your book.
Questions 1 to 5