1. Key Areas covered
Gravitational field strength of planets, natural satellites and stars.
Calculating the force exerted on objects placed in a gravity field.
Newton’s Universal Law of Gravitation.

2. What we will do today State the difference between mass and weight
Explain star and planet formation with reference to the force of gravity
Describe how space travel makes use of the ‘slingshot effect’
Carry out calculations using Newton’s Inverse Square Law of Gravitation

3. Gravity and mass
T. Ferns – 28/9/04 LO’s , ,

4. BBC Universe - Gravity
T. Ferns – 28/9/04 LO’s , ,

5. What’s the difference between mass and weight?
Mass is how much matter (material) is in an object and is measured in kg.
Weight is the force of gravity acting on an object (of known mass) and is measured in N kg-1.

6. Gravity demonstration
Place a large rubber sheet or bin bag etc on a table - this is space.
As you travel from one end to the other it takes time – this is the space-time continuum.
Place a tennis ball (a planet) and a marble (a moon) on the sheet and lift it up.
You will see the marble move towards the tennis ball.
Repeat again only this time add in a football (the Sun).
You will see that the marble and tennis ball are attracted to the Sun.
The bigger the mass, the bigger the gravitational pull.

7. Thinker???
Why don’t we constantly move towards the Sun if it has such a strong gravity?
More on this later

8. Where can you find gravity?
Everything that has a mass has its own gravitational field strength, the bigger the mass the bigger the field strength.
This explains why planets orbit the Sun and why the asteroid belt is close to Jupiter.
This is why different planets have different values of gravity.
BBC Universe - Gravity: See Brian Cox’s zero G flight

9. Gravity and star formation
Scientists believe that stars were formed by the gravitational attraction between hydrogen molecules in space.
This attraction built up over time, a large enough mass of gas such that the forces at the centre was so big that it caused the hydrogen molecules to fuse together, generating energy (the centre of the Sun).
This energy radiating outwards counteracts the gravitational force moving trying to compress the Sun inwards.
Eventually the Hydrogen will be used up and the Sun will collapse under its own gravity (in about 4 billion years)

10. Gravity and planet formation
Once a star is formed there will be a debris orbiting around it.
This debris may join together to form planets, again due to the gravitational attraction between the particles.
This is the most common explanation to explain the formation of our solar system.
We are all made of stars.

11. How can we make use of gravity?
Another application of the gravitational force is the ‘slingshot’ method used by space craft.
Basically an space craft is sent close to a planet, where it accelerates due to its gravitational field.
If the trajectory is right then the craft speeds past the planet with increased speed, if not, then it will crash into the planet.
The slingshot method was used by Apollo 13 when it used the field strength of the Moon to accelerate back towards the Earth.

12. What causes gravity? In short, we don’t know.
One popular theory is that there is a particle called a graviton that has an attractive force that draws things toward the centre of a mass.
If this particle was ever discovered and could be reproduced it could help with long distance space travel.
Zero gravity is bad for human health as it causes huge muscle wastage.

13. Newton’s Inverse Square Law of Gravitation
This law states that there is a force of attraction between any two objects in the universe.
The size of the force is proportional to the product of the masses of the two objects, and inversely proportional to the square of the distance between them.
m1 and m2 are the masses of the two objects, and r is the distance between them.
G = gravitational constant = x Nm2kg-2

14. Example 1
What is the force of attraction between two pupils of average mass (60 kg) sitting 1.5 metres apart?
F = Gm1m2 r2
= 6.67 x x 60 x
= 1.07 x 10-7 N

15. Value of r
It is important to realise that the value for r, the distance between two masses, is the distance between the centre of the two masses.
i.e. if we consider the force between the Earth and the Moon then the value of r is the distance from the centre of the Earth to the centre of the Moon.

16. Example 2
Taking the radius of the Earth to be 6.4 x 106 m, find the force of attraction on a 250kg satellite that is orbiting at a height of km above the Earth.
(mass of Earth =  1024 kg)
This question should be broken down into two parts.
First of all, find the distance, r, between the two objects.
Use Newton’s Universal Law

17. Example 2
1. r = radius of Earth + the height above the Earth
= 6.4 x x 103
= 4.24 x 107 m
2. F = Gm1m2 r2
= 6.67 x x 6.0  1024 x 250
(4.24 x 107)2
= 55.7N

18. 2012 Revised Higher

19. 2013 Revised Higher Open-ended question
Experiment:
Use bathroom scales and a metre stick to come up with an answer for above.