Mechanics: Newton's Laws – A Level Physics Edexcel Revision

Mechanics: Newton’s Laws

Newton’s First Law (Law of Inertia): If the sum of the forces acting on a body is zero, the body will stay at rest if it was at rest, or keep moving with a constant velocity if it was moving.
Inertial mass is a measure of how much a body resists changes in its velocity. It is associated with Newton’s First Law.
Newton’s Second Law (F=ma): The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. The direction of the acceleration is the direction of the net force.
Newton’s Third Law (Action-Reaction Law): For every action, there is an equal and opposite reaction. This means that any force exerted on a body will create a force of equal magnitude but opposite direction on the object that exerted the first force.

Calculating force: The formula derived from Newton’s second law is F = ma, where F is the force, m is the mass, and a is the acceleration.
Calculating acceleration: If you know the force applied on a body and its mass, you can rearrange the formula to a = F/m.
Calculating mass: If the force exerted and the resulting acceleration is known, the mass can be calculated as m = F/a.

Understanding Newton’s Laws can help explain a variety of everyday phenomena, like why you lurch forward in a car when it suddenly stops (inertia) or why you feel heavier or lighter on an elevator (net force and acceleration).
Understanding action-reaction pairs can help predict the motion of objects in a system, such as predicting the path of a billiard ball after it’s hit, or understanding how a rocket propels itself in space.

All of Newton’s laws contribute to our understanding of mechanics, the branch of physics that studies the motion of objects and the forces that cause this motion.
Remember that forces are vector quantities, meaning they have both magnitude and direction. When calculating net forces, it’s important to take into account the direction of each individual force.
Newton’s laws apply to bodies of any size, as long as we’re not at the atomic scale (where quantum mechanics is more appropriate) or moving near the speed of light (where relativistic effects come into play). Thus, these laws provide a good approximation for most everyday situations.
Newton’s laws are still very much relevant, but they have been superseded by Einstein’s Theory of Relativity in situations involving high speeds or strong gravitational fields.