2.7 - Contact & Non-Contact Forces

Forces can be contact or non-contact

• A force is a push or a pull on an object, caused by an interaction with another object.
• Every force has a size and a direction.

Contact forces

A contact force only acts when two objects are touching.

• Friction - Acts between two surfaces that are sliding, or trying to slide, past each other.
• Drag - Acts on an object moving through a fluid. Drag in air is called air resistance.
• Tension - The pull in a stretched rope, cable or string.
• Normal contact force - The push that two touching surfaces exert on each other, at right angles to the surfaces.
• Upthrust - The upward push a fluid exerts on an object placed in it.

Non-contact forces

A non-contact force can act even when the objects are not touching. These forces are caused by fields interacting.

• Gravitational force - The attraction between any two masses (such as the Earth and the Sun).
• Electrostatic force - The attraction or repulsion between charged objects (like charges repel, opposite charges attract).
• Magnetic force - The attraction or repulsion between magnetic objects.

Newton's third law

• Newton's third law states that when two objects interact, the forces they exert on each other are equal in size and opposite in direction.
• These two forces are called an interaction pair.

The two forces in an interaction pair:

• Are the same type of force
• Are equal in size
• Act in opposite directions
• Act on different objects (one force on each, so they don't cancel out)

Examples

• Chair on the ground - The chair pushes down on the ground, and the ground pushes up on the chair with an equal and opposite force.
• Sun and Earth - The gravitational pull of the Sun on the Earth is equal and opposite to the pull of the Earth on the Sun.