Forces and Motion Fundamentals

Forces are simply pushes or pulls that make things move or change shape. You encounter them constantly - when you kick a football, gravity pulling you down, or friction slowing your bike.

There are two main types: contact forces (like friction) require objects to touch, while non-contact forces (like gravity or magnetism) work from a distance. Understanding this difference helps explain why magnets attract through air but you need to physically push a door.

Mass vs weight often confuses students, but it's straightforward. Mass is how much "stuff" is in an object and stays the same everywhere. Weight is the gravitational force acting on that mass, so it changes depending on where you are. The formula W = m × g shows this relationship clearly.

When springs stretch or compress, they follow Hooke's Law: F = k × e. This means the force needed doubles when you stretch twice as far. The elastic potential energy stored is ½ × k × e², which explains why trampolines work!

Quick Tip: Remember that displacement and velocity are vectors (have direction), while distance and speed are scalars (just magnitude). This distinction is essential for calculations.

Newton's Laws and Advanced Motion

Newton's three laws govern everything that moves around you. The first law says objects won't change motion unless a force acts on them - your body keeps moving forward in a braking car because of inertia.

The second law, F = ma, is your key equation for force problems. It shows that doubling the force doubles the acceleration, while doubling the mass halves it. This explains why sports cars accelerate faster than lorries with the same engine power.

Stopping distances combine thinking distance (reaction time) and braking distance. Your speed, reaction time, weather, and tyre condition all affect these distances. Remember that kinetic energy converts to heat energy through friction when braking.

Momentum $p = mv$ is conserved in collisions, making it perfect for analysing crashes or explosions. In any closed system, total momentum before equals total momentum after - this principle helps engineers design safer cars.

Newton's third law states that forces always come in pairs - when you walk, you push backwards on the ground, and it pushes forward on you with equal force.

Exam Focus: Practice drawing free body diagrams showing all forces acting on objects. The arrow sizes should represent relative force magnitudes, and remember that balanced forces mean zero acceleration.