Last lesson

1
Last lesson
Hookes Law
2
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!)

3
Table of results
Force (N) Length of spring (cm) Extension (cm)
0 3.4 0
1 5.4 2
2 7.4 4
3 9.4 6
4
Graph
Force (N)

• Complete investigation for Weds 21st October
• Table of results
• Graph
• Two sentences of conclusion

Extension (cm)
5
Hookes law
Elastic limit
Force (N)
The extension of a spring is proportional to the
force applied (until the elastic limit is
reached)
Extension (cm)
6
Steel, glass and wood
Force
Even though they dont stretch much, they obey
Hookes law for the first part of the graph
Extention
7
Rubber
Force
Extension
8
Homework

• Set Friday 23rd October. Due Wednesday 3rd
  November.
• Complete Physics for IGCSE page 61 Questions 1 to
  4

9
Todays lesson

• Orbital motion and satellites

10
Satellites
11
How far could you kick a dog?

From a table, medium kick.
12
How far can you kick a dog?

Gravity
13
Harder kick?

14
Harder kick

Gravity
15
Small cannon?

Woof! (help)
16
Small cannon

Woof! (help)
Gravity
17
Bigger cannon?

18
Bigger cannon

Gravity
Gravity
19
Even bigger cannon?

20
Even bigger cannon

Gravity
Gravity
Gravity
21
VERY big cannon?
22
VERY big cannon
Gravity
23
Humungous cannon?
24
Dog in orbit!
The dog is now in orbit! (assuming no air
resistance of course)
25
Dog in orbit!
The dog is falling towards the earth, but never
gets there!
26
Dogs in orbit!
The force that keeps an object moving in a circle
is called the centripetal force (here provided
by gravity)
Gravity
27
Other examples
Earths gravitational attraction on moon
28
Uniform circular motion

• The centripetal acceleration/force is always
  directed towards the centre of the circle

Centripetal force/acceleration
velocity
29
Note!

• There is no such thing as centrifugal force! (at
  least not until you get to university!)

CENTRIFUGAL
30
Types of orbits
31
Geostationary

• The satellite orbits once every 24 hours, so
  appears to stay above the same point on the
  earths surface

Looking from above the North pole
32
Geostationary

• Useful for communications satellites. Also for
  weather over one area.

33
Polar

• Satellite orbits over the poles

34
Polar

• Useful for mapping and remote sensing when the
  whole of the earths surface needs to be studied

35
Some famous satellites
36
Sputnik (4th October 1957)
37
Sputnik (4th October 1957)

• Russian
• 84 kg, 58cm diameter
• 800km above the earth
• 30 000km/h
• Orbited every 90 mins
• Fell after 92 days

38
Sputnik 2 (a month later)
39
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

Its a dogs life.
40
Hubble Space telescope
41
Hubble Space telescope

• Launched in 1990
• 13.1m long
• Doesnt have to see through the earths atmosphere

42
Thats it!

•  

43
Can you try some questions now?
Page 48 and 49 in your book. Questions 1 to 5