Gravity

Gravity is the attraction between objects due to their mass. The gravitation attraction between two objects is proportional to the product of the mass of the objects (the two masses multiplied by each other). It has unlimited range, and gets weaker with increasing distance according to the square of the distance between two objects.

Gravity is the force that keeps us on the planet, and that keeps Earth orbiting the Sun.

Gravity is always attractive, never repulsive.

Mass

Mass is a property of objects. It determines how resistant an object is to acceleration by a force. The more mass, the larger the force needed to accelerate the object and the greater its gravitational attraction to other objects is.

When someone jumps off a wall they accelerate downward. Newton's Third Law of Motion says that at the same time Earth experiences an upward acceleration from an equal and opposite force. In other words, a falling person and Earth accelerate towald each other. Because Earth has so much more mass than a person, Earth hardly moves at all.

The unit of mass, the kilogram (kg), is one of the basic units of measurement. Mass is difficult to measure directly, so weight is normally measured instead.

Weight

In everyday life, mass and weight are often used interchangeably, but they are different things. An astronaut on the Moon has the same mass as on Earth, but only weighs about one sixth because the gravity at the Moon's surface is only one sixth what it is on Earth.

Weight is the force taken to stop an object (with mass) from accelerating downward.

The direction "down" depends on gravity or an acceleration. Down is the direction in which we experience weight, or the direction in which we are accelerating. In other words, if we are standing on the ground, our weight tells us which way is down. If we are falling, the direction we are falling is down.

Freefall

An object in a gravitational field that is not restrained from falling will fall unrestricted. It is said to be in freefall. Objects in freefall are weightless.

Measurement

Gravity can easily be measured using a pendulum – the longer the better to allow measuring of the timing of the swing period to be as accurate and precise as possible. However, even a ~0.5 metre long pendulum can give good results: the last time I tried, I got 9.76 m·s^-2, which is less that 0.5% under the nearest local value of 9.79946 m·s^-2 shown on the GNS Science website.

GNS Science gravity measurements:

In the menu at the left, select the Gravity map layer (under Geophysics) and turn off everything else.
- Clicking on the + symbol will bring up the different types of data.
Zoom in on the area of New Zealand you're interested in.
Every black dot on the map is a gravity measurement; in the pop-up menu at the top left corner of the map select Gravity Survey Data.
Every other symbol, such as green or blue disc is a reference gravity measurement; in the pop-up menu at the top left corner of the map select Reference Gravity Stations.
Just to the right of that pop-up menu, click the area boundary drawing tool, next to the (i) icon.
Draw a box around one or more symbols by clicking where you want the corners of your selection area.
The gravity values are given as µN/kg; divide by a million to get N/kg, otherwise known as m·s^-2.

In general, the gravity measurements are higher in the south of the country. The difference in weight between Kaitaia in Northland and Oban on Rakiura/Stewart Island is just over 1 gram (force) per kilogram (mass). To use a nice round figure, a 100 kg man from Kaitaia will weigh 100.1 kg in Oban.

I've found it very interesting to see how the gravity value changes gradually moving along a line of measurements, such as those on 90 Mile Beach.