P81 G

What links these images?

And these physicists? Aristotle Einstein Newton Galileo

Gravity is

The force that holds the particles in a nucleus together

The force you get when an object moves through air

The force that exists between all objects with mass

The force you get between two charged objects

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Lesson Aims

Appreciate that gravity is a force which acts between bodies even though they are not in contact.

Know that the Earth is a source of a gravitational field.

Appreciate the distinction between mass and weight.

Measure the acceleration due to gravity on Earth.

The force of gravity is

Attractive always

Repulsive always

Attractive and repulsive

Always with you

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Contact forces and action at a distance If two objects or materials need to be touching for the force to have an effect then it is a contact force. Examples: Friction Air resistance Is Gravity a contact force? No Gravity acts over a distance;- The force of gravity is said to be ACTION AT A DISTANCE

Mass and weight What is the difference between mass and weight ? Mass Weight Mass is a measure of the amount of “stuff” or number of particles which an object is made of. This remains constant wherever it is in the universe Weight is the force of gravity acting on an object. It changes depending on the strength of the gravitational field it is placed in. On Earth the weight of a 1 kg mass = 10 N 1kg Mass = 1kg Weight = 10 N

Gravitational field strength; g On Earth the weight of a 1 kg mass = 10 N g = gravitational field strength / N/kg W = weight / N m = mass / kg

How big is the value of g on Earth

What is the weight of a (i) 50 g (g for gram) eraser and (ii) a 67 Kg student on Earth.

What is the mass of a (i) pencil which weighs 0.2 N and (ii) and student who weighs 550 N on Earth.

Experiment P8(c) Measure either the mass or the weight of the following objects. then calculate the other one to complete the table. Two more things of your choice Your self Calculator Pencil case Eraser Pencil case Weight / N Mass / kg Object

On the moon Explain why an astronaut with a total mass > 120 kg can move so easily on the moon?

g on the moon = 1/6 g on Earth

How big is the value of g on the Moon

What is the weight of a (i) 50 g (g for gram) eraser and (ii) a 67 Kg student on the Moon?

What is the mass of a (i) pencil which weighs 0.2 N and (ii) and student who weighs 550 N on the Moon?

Galileo v Aristotle The larger the mass the quicker it accelerates. Who was correct? Nonsense; all masses accelerate at the same rate

The proof ……..finally

The size of ‘g’ on Earth by freefall Sheet P8(B) S Use S = ½ g t 2

Recording data Use S = ½ g t 2 So g = ? S = height g = acceleration of gravity t = time taken Mean value of g = Compare with g = 9.81 m/s 2 1.0 1.0 1.0 1.0 1.0 g /m/s 2 Time/s Height/m

Plenary