Forces and motion — Newton's laws
Types of forces
Contact forces (require physical contact):
- Friction: opposes relative motion between surfaces.
- Normal contact force (reaction force): perpendicular to a surface, pushes objects apart.
- Tension: pulling force in a rope or string.
- Air resistance (drag): opposes motion through air.
Non-contact forces (act at a distance):
- Gravity (weight).
- Magnetic force.
- Electrostatic force.
Weight vs mass:
- Mass (m): amount of matter in an object. Measured in kilograms (kg). Constant everywhere.
- Weight (W): gravitational force on the object. Measured in newtons (N). W = m × g.
- g = gravitational field strength = 10 N/kg on Earth's surface (CCEA value).
Free-body diagrams
A free-body diagram shows ALL forces acting on a single object as arrows:
- Length of arrow = relative magnitude of force.
- Direction of arrow = direction of force.
- Label each force with its name and value.
For a stationary book on a table: weight downward (W = mg), normal reaction upward (N = W). Equal and opposite → balanced forces → no acceleration.
Newton's First Law
An object will remain at rest or continue at constant velocity unless acted on by a resultant force.
- If resultant force = 0: object is in equilibrium (stationary or constant velocity).
- A moving object with no resultant force continues at the same speed in the same direction.
Newton's Second Law
F = ma (resultant force = mass × acceleration)
Where F is in newtons (N), m in kg, a in m/s².
Rearranged: a = F/m; m = F/a.
Example: a 1000 kg car experiences a resultant force of 2000 N. a = F/m = 2000/1000 = 2 m/s².
Resultant force = vector sum of all forces on an object.
Newton's Third Law
Every action has an equal and opposite reaction.
When object A exerts a force on object B, object B exerts an equal and opposite force on object A.
Example: you push down on the floor (action); the floor pushes up on you with an equal force (reaction). These forces are equal in magnitude, opposite in direction, and act on DIFFERENT objects.
Terminal velocity
When an object falls through air:
- At first, weight > air resistance → resultant force downward → accelerates.
- As speed increases, air resistance increases.
- Eventually, air resistance = weight → resultant force = 0 → constant velocity = terminal velocity.
On a velocity-time graph: curve starts steep (accelerating), gradually flattens to a horizontal line (terminal velocity).
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