Contact and non-contact forces

A force is a push or pull that can change motion or shape. We split forces into contact (objects touching) and non-contact (act through space).

Contact forces

• Friction — opposes relative motion at a surface.
• Air resistance / drag — opposes motion through a fluid.
• Normal contact force (reaction) — perpendicular push of a surface on an object resting on it.
• Tension — pulls along a stretched rope or spring.
• Upthrust — buoyancy from a fluid (e.g. water on a swimmer).

Non-contact forces

• Gravity — between any two masses, attractive only.
• Magnetic — between magnets and magnetic materials.
• Electrostatic — between electric charges (attractive or repulsive).

Weight and gravity

The weight of an object is the gravitational force on it:

$W = mg$

• $W$ in N.
• $m$ in kg (its mass).
• $g$ is the gravitational field strength: 9.8 N/kg on Earth (often rounded to 10 N/kg).

Mass is constant; weight depends on $g$. On the Moon ($g \approx 1.6$ N/kg), a 60 kg astronaut still has mass 60 kg but weighs only ~96 N (vs ~600 N on Earth).

Free-body diagrams

A free-body diagram shows all forces acting on a single object as arrows from the centre. Lengths represent magnitudes.

Example — book on a table:

• Weight (mg) downward.
• Normal contact force from table upward.
• If at rest, these are equal and opposite (Newton's first law).