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Newton's laws of motion
Newton's laws are the three fundamental rules that predict how an object's motion will change when forces act on it. They are the essential toolkit for solving almost every problem in mechanics.
Need to know
What you need to know
- Equilibrium means the resultant force is zero: $\sum F = 0$.
- An object in equilibrium is either stationary or moving at a constant velocity.
- This law is often used to solve for unknown forces by resolving forces in perpendicular directions (e.g., horizontally and vertically) and setting the sum in each direction to zero.
Explanation
The Rules of Motion
- First law: An object maintains constant velocity (including being at rest) unless a resultant force acts on it. This is the principle of equilibrium.
- Second law: The resultant force on an object is the product of its mass and acceleration (F=ma). Acceleration is always in the same direction as the resultant force.
- Third law: For every action, there is an equal and opposite reaction. These two forces act on different bodies and are of the same type.
- Weight is the force of gravity on an object. Near the Earth's surface, it is calculated as W = mg and always acts vertically downwards.