4. 1.To understand what we mean by gravity 2.To understand how we represent a gravitational field 3.To look at radial and uniform fields 4.To understand what we mean by the strength of a gravitational field 5.To revisit what we mean by “g” Book Reference : Pages 54-55

5. What can we remember from Y7, Y9 etc?.... What sort of objects cause gravity? What does an object need to cause gravity? If we consider gravity on a planet which direction does the force of gravity act?

6. There will be a force of gravity between any two objects if they have mass. The mass causes a force field around the object called the gravitational field However, unless one of the objects is large (the size of a moon etc) the attraction goes unnoticed The force of gravitational attraction between two bodies is equal & opposite. However, in the case of dropping a coin, we observe the coin falling towards the Earth & do not notice the large mass of the Earth being attracted towards the coin

7. If a small object is in free to fall due to gravity the path that it will follow is called a field line or line of force For a planet they are directed towards the centre at all points We call this a radial field & the strength varies with distance

8. We can consider two types of field pattern: Radial Field : Field lines are like the spokes of a wheel, act towards the centre of the mass & decrease with distance Uniform Field : The gravitational field strength is the same in direction and magnitude everywhere in the field. Field lines are therefore parallel and equally spaced What sort of field does the Earth have?

9. The force of gravity on a small object due to the Earth decreases with distance. Therefore the gravitational field strength decreases in the same way. The field pattern is therefore radial However, over small distances (small relative to the radius of the Earth), the change in gravitational field strength is insignificant In theory g is smaller at an altitude of 100m than it is on the surface but the difference is too small to generally worry about

10. The strength of a gravitational field g on a small mass m is the force per unit mass g = F/m Where F is the gravitational force acting on the mass m We call g the gravitational field strength. For ‘A’ level we consider the value to be 9.81 and the derived units from above will be Nkg -1 [ Exp Science animation 1]

11. The weight of an object is the force of gravity on it For an object of mass m in a gravitational field of strength g we can find the acceleration from (F=ma) a = F/m (and from before F = mg) a = mg/m a = g An object in freefall will fall with acceleration g

12. Thus g can have two descriptions : g : the gravitational field strength with units Nkg -1 g : the acceleration due to gravity with units ms -2 You will see both in the exam data sheet [ Exp Science animation 2]