Every interaction involves two objects

1. An interaction always involves two objects, so it always produces two forces, one acting on each object.
2. If object A pushes or pulls on object B, then object B also pushes or pulls on object A at the same time.
3. These two forces together are called an interaction pair.
4. The two forces in an interaction pair have three features in common.
   1. They are the same type of force, so a gravitational force is always paired with a gravitational force and a normal contact force with a normal contact force.
   2. They act along the same line but point in opposite directions.
   3. They start and stop at the same instant, because they are produced by the same interaction.
5. The two forces act on different objects, which is the most important feature of an interaction pair.
6. To describe a pair fully, use the pattern "the force on object A by object B" for one force and "the force on object B by object A" for the other.

Pairs in contact interactions

1. In a contact interaction, the two surfaces press on each other or rub against each other, so each surface feels a force.
2. When your hand pushes a door, your hand exerts a force on the door and the door exerts a force back on your hand, which is why you can feel the door pressing on your palm.
3. When a mug rests on a desk, the mug pushes down on the desk and the desk pushes up on the mug.
   1. Both of these forces are normal contact forces, acting at right angles to the desk surface.
   2. One force acts on the desk and the other acts on the mug.
4. When you walk, your shoe pushes backwards on the pavement and the pavement pushes forwards on your shoe.
   1. Both of these forces are friction forces between the touching surfaces.
   2. The forward force on your shoe is the force that moves you along.
5. When a hammer hits a nail, the hammer pushes the nail into the wood and the nail pushes back on the hammer, which is why the hammer stops.

Pairs in non-contact interactions

1. Non-contact interactions also produce a force on each of the two objects, even though there is a gap between them.
2. In a gravitational interaction, the Earth pulls a falling apple downwards and the apple pulls the Earth upwards.
   1. The pull of the apple on the Earth has no noticeable effect, because the Earth has an enormous mass.
   2. The force on the Earth is still there, and it acts on the Earth, not on the apple.
3. In a magnetic interaction, a magnet pulls a steel paper clip towards it and the paper clip pulls the magnet towards itself.
   1. If the magnet is placed on a floating cork, the magnet visibly moves towards a paper clip held near it, which shows the paper clip really does exert a force on the magnet.
4. In an electrostatic interaction, two charged balloons hung side by side by threads push each other apart.
   1. Each balloon swings away from the other, which shows that both balloons experience a force.
5. Gravity between the Earth and the Moon is also an interaction pair, with the Earth pulling the Moon and the Moon pulling the Earth.
   1. The Moon's pull on the Earth's oceans helps to cause the tides.

Choosing the object you are studying

1. Because the two forces in a pair act on different objects, only one force from each pair acts on any object you choose to study.
2. To find the forces acting on an object, list every interaction that object is involved in and keep only the force that acts on it from each one.
3. For a mug resting on a desk, the mug takes part in two interactions.
   1. The interaction with the Earth gives the mug a downward gravitational force, its weight.
   2. The interaction with the desk gives the mug an upward normal contact force.
4. The partner forces, the mug pulling up on the Earth and the mug pushing down on the desk, act on other objects, so they are left out when you study the mug.