04/08/2023 1

The Gravity FieldPhysics on the Road

Lesson 11

04/08/2023 2

LI…

Explain the difference between mass and weight

Know that the weight of an object is proportional to it’s mass

Describe a gravitational field

04/08/2023 3

Newton’s 3 Ideas

Mass and weight The gravity field Forces change velocities

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Mass and weight: on Earth, on the Moon

The force needed to push the sack is related to it’s mass and resistance to this is called inertia.

The force needed to lift the sack is related to it’s mass and called it’s weight.

Would there be difference on the moon?

04/08/2023 5

Mass and weight

The pull of gravity on an object is proportional to it’s mass and is called it’s weight.

weight = mass x gravity The force needed to

accelerate an object is proportional to it’s mass

force = mass x acceleration

The weight of an object is a force and has units of Newtons (N)

Mass is the amount of matter and has units of kilograms (kg)

W = mgF = ma

Gravity and acceleration are equivalent

04/08/2023 6

The Gravity Field In physics we use the idea of invisible regions or fields filling

space to explain forces which act at a distance (non-contact)

Gravity (or weight) is a non-contact force and is produced by a gravity field

Newton’s famous idea was that masses produce their own gravity fields. The bigger the mass the stronger the field.

planet

Planet surface

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Gravity and acceleration

W = mg and F = ma are equivalent formulas (because weight is a force)

So this means that for objects in a gravity field the field and the objects acceleration are equivalent. So gravity fields have two units

1. Newtons/kilogram - Nkg-1

2. Metres/second/second - ms-2

F = mg and a = F / m

So for either mass…

acceleration = mg / m = g

ie. they fall with same acceleration.

W w

gravity field

04/08/2023 8

Forces change velocities

Newton explained that forces change velocities. Sometimes this means a change of speed, but

not always.1. Use the ideas of vectors to explain that a change

of velocity can happen without a change of speed2. Give an astronomical example of this and use a

diagram3. Isaac Newtons ideas were slow to be accepted in

the 17th Century (particularly in France). Use the differences in transport between then and now to explain why.